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Editorial Board Maria José Garcia Borge, Madrid (Chair) Eugenio Nappi, Bari Rick Casten, Yale Klaus Peters, Darmstadt and EPS/NPB Ari Jokinen, Jyväskylä Hideyuki Sakai, Tokyo Reiner Krücken, Vancouver James Symons, Berkeley Yu-Gang Ma, Shanghai Marcel Toulemonde, Caen Douglas MacGregor, Glasgow and EPS/NPB

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2 Nuclear Physics News, Vol. 24, No. 1, 2014

NuclearPhysicsNews

Cover Illustration: Airlift of the tank for the new 5U accelerator to its final location in the Notre Dame Nuclear Science Laboratory. In the background the administration building of the university is the so called “Golden Dome” (see article on page 5).

Volume 24/No. 1

ContentsEditorialNuPECC: A 25-Year-Old Expert Board of the 40-Year-Old European Science Foundation

by Angela Bracco and Jean-Claude Worms .................................................................................................................. 3

Laboratory PortraitThe Nuclear Science Laboratory at the University of Notre Dame

by Ani Aprahamian, Philippe Collon, and Michael Wiescher ...................................................................................... 5

Feature ArticlesBeta-Delayed Fission: A Rare Decay Mode as Probe for Phenomena Near and Beyond the Fission Barrier

by Andrei N. Andreyev, Mark Huyse, and Piet Van Duppen ......................................................................................... 14Pairing Interaction and Two-Nucleon Transfer Reactions

by Gregory Potel, Andrea Idini, Francisco Barranco, Enrico Vigezzi, and Ricardo A. Broglia .................................. 19

Facilities and MethodsInvestigating the Structure of Neutron-Rich Nuclei with Neutrons

by Gary Simpson ........................................................................................................................................................... 26NSCL and the Facility for Rare Isotope Beams (FRIB) Project

by Alexandra Gade, C. Konrad Gelbke, and Thomas Glasmacher .............................................................................. 28

Meeting ReportsINPC 2013: Florence, 2–6 June 2013

by Angela Bracco, Pier Andrea Mandò, and Cosimo Signorini ................................................................................... 31Baryons 2013: International Conference on the Structure of Baryons, Glasgow, 24–28 June 2013

by I. J. Douglas MacGregor .......................................................................................................................................... 33XXXIII Mazurian Lakes Conference on Physics: Frontiers in Nuclear Physics, Piaski, Poland, 1–7 September 2013

by Chiara Mazzocchi, Krzysztof Rusek, and Krzysztof Rykaczewski ............................................................................ 35Latest News in Antiproton Physics Discussed at the LEAP 2013 Conference in Uppsala

by Tord Johansson ......................................................................................................................................................... 37

News and ViewsThe LUNA-MV Project at Gran Sasso Underground Laboratory

by Alessandra Guglielmetti ........................................................................................................................................... 40

Calendar.................................................................................................................................................. Inside Back Cover

editorial


The views expressed here do not represent the views and policies of NuPECC except where explicitly identified.

It has been decided by the editorial board to inform the readers in brief on the European Science Foundation (ESF). NuPECC is an expert board of this organization, which is largely and continuously benefitting from this po-sition.

As it is known, ESF is going through a drastic rearrangement of its structure and activities and a decision on whether to terminate this organiza-tion is expected to be taken at the end of 2014. However, at the same time, plans are being discussed to establish another organization that would deal with scientific services such as peer-review or evaluation, but could also continue to host the Expert Boards and Committees such as NuPECC. The rather recent birth of the new or-ganization, Science Europe, which is based in Brussels and has set up six scientific committees, has produced a consequent winding down of scientific activities of ESF.

As a consequence, since 2012 ESF has been actively engaged in reducing its traditional tasks (EUROCORES, Forward Looks, Conferences, Re-search Networking Programmes, Ex-ploratory Workshops) and in explor-ing a scenario for a potential successor organization.

The year 2014, however, is an im-portant year for ESF, which is cele-brating its 40th anniversary. This year marks indeed 40 years of achievement of collaboration in science policy and research management. This event will be celebrated in May in Strasbourg. On that occasion an archive of ESF

documentation will be also launched and publicized. In the 40 years of existence, ESF has accumulated a very large amount of documentation (books, publications, office archives) that represent an interesting and very valuable resource for various purposes (e.g., research policy studies, future policymaking). Thus the ESF Govern-ing Council has decided to properly preserve and archive this patrimony in a scientifically valid manner after the restructuring or termination of the or-ganization. The archiving project aims at referencing selected ESF documen-tation in a centralized database and depositing it in an appropriate hosting institution ensuring broad access to the documentation to research man-agement practitioners as well as to researchers studying research policies and practices. A very special collabo-ration with the University of Stras-bourg was set up to include ESF’s book collection in its own collections and make it available to a large audi-ence of students and academics. As re-gards publications and office archives there is a proposal for the ESF docu-mentation to be hosted by the Histori-cal Archives of the European Union (HAEU) at the European University Institute in Florence. Other proposals and projects related to it are possibly forthcoming.

Turning now to NuPECC we would like to stress some points that express well the importance of being recog-nized as an expert board of ESF:

• The expert boards were mostly created in ESF to respond to spe-

cific scientific needs. They pro-vide scientific and policy advice and initiate strategic develop-ments and thus their scientific ser-vices in Europe, or even in a more global framework, are indispens-able for Europe’s scientific land-scape. Being in ESF NuPECC was facilitated in providing to EU its responses to framework pro-gram consultations.

• Many NuPECC publications, in-cluding the forthcoming report “Nuclear Physics for Medicine,” are published with ESF and this opens opportunities for the com-munication of our activities to a broader audience. In addition, the information on facilities re-cently collected and organized by NuPECC are also used for the MERIL database (the Mapping of the European Research In-frastructure Landscape, created with support of the ESF Member Organizations). This database represents a comprehensive in-ventory of research infrastruc-tures of “more-than-national” relevance across all scientific domains and makes the informa-tion publicly available through an interactive online portal with analytical capabilities.

• ESF has always recognized the independence of its expert boards. Indeed the qualified in-dependence of NuPECC has been a key point to assure ap-propriate performance of its mis-sion.

NuPECC: A 25-Year-Old Expert Board of the 40-Year-Old European Science Foundation

editorial


• For NuPECC to have rather regular reviews made by special panels and to have annual status reports to be presented at the ESF Governing Council meet-ings is stimulating and it helps to define and realize its activity plans.

These are some of the reasons why NuPECC would like to be hosted in a platform with other ESF expert boards after 2015 and possibly within the po-tential successor organization. In gen-eral NuPECC feels that direct transfer of expert advice and information to top-level management and gover-nance is important.

Contacts with Science Europe have also been established to have construc-tive interactions on issues pertaining

to the building of the European Re-search Area, and for which NuPECC should be consulted regarding nuclear physics.

It is important to underline that ESF is really dedicating particular effort to help creating a common platform for the expert boards.

However, the most important point is that there seems to be a real oppor-tunity to fund a downsized successor of ESF. The possibility of a potential successor relies on the fact that, if the winding down of ESF left gaps in services—such as peer review, evalu-ation, and so on—then those gaps should be filled in order to help Eu-ropean Research Area quality and re-search.

We would like in conclusion to recommend making a good use of the

ESF archive when it becomes avail-able, to celebrate its 40th anniversary and wait for more information about this organization, hopefully positive, in a few months!

AngelA BrAcco

NuPECC Chair

JeAn-clAude Worms

European Science Foundation

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laboratory portrait


The Nuclear Science Laboratory (NSL) at the University of Notre Dame (ND) is a mid-sized accelera-tor laboratory, situated 100 miles east of Chicago, serving the local nuclear physics group and a large growing na-tional and international user commu-nity [1]. The research focus of the ND nuclear physics group is on nuclear astrophysics, nuclear structure, and re-action physics. This is complemented by new initiatives in the development of accelerator applications and in questions associated with nuclear re-actions in plasma environments. The NSL faculty has also continued to be very successful in the development of new instrumentation, and substantial improvements to the laboratory in-frastructure. Presently the laboratory supports seven tenure track faculty members, five research faculty, six postdoctoral associates, seven techni-cal and administrative personal, and thirty-three graduate students.

The NSL accelerator facilities (Figure 1) have substantially improved over the last three years with the pur-chase and installation of a new 5MV single ended (5U) Pelletron accelera-tor complementing the research activi-ties of the 11-MV-tandem-(FN)-Pelle-tron accelerator. The installation and testing of the 5U has been completed and first measurements for its experi-mental have started. The 5U accelera-tor provides intense light and heavy ion beams and is primarily dedicated to the nuclear astrophysics program. The FN accelerator is used for mul-tiple scientific initiatives; it is a key instrument for the NSL nuclear struc-ture and astrophysics programs, for the production of radioactive beams

at the TwinSol facility, which operates as a radioactive beam facility for light isotopes since 1995, and the Accel-erator Mass Spectrometry (AMS) pro-gram for nuclear astrophysics. Over the last few years the tandem is also increasingly used for a broad range of applications ranging from isotope pro-duction and radiation chemistry for medical and biological application, to nuclear reaction studies for homeland security and nuclear forensics applica-tions, AMS for climate and geologi-cal applications, and finally PIXE and XRF for the analysis of historical, ar-chaeological, and other materials [2]. Complementing these two machines, the NSL operates a 1MV JN Van de Graaff accelerator as well as the 200 kV Cockcroft Walton accelerator—in the past used as an implanter—for ex-panding nuclear astrophysics experi-ments to lower energies.

Besides the installation of the new accelerator there have been a num-ber of new instrumental development initiatives at both the NSL and other facilities to support the scientific pro-gram of the laboratory’s faculty. The key instrument is the St. GEORGE separator (Strong Gradient Electro-magnetic Online Recoil separator for capture Gamma ray Experiments) for low energy inverse kinematics experi-ments with intense heavy ion beams, the GEORGINA gamma detector ar-ray (GE-detector Online aRray for Gammas In Nuclear Astrophysics), and the NERINA neutron counter (NEutron detector aRay In Nuclear Astrophysics). These detection sys-tems provide a wide range of oppor-tunities for low energy nuclear astro-physics experiments of interest for the different phases of stellar burning that drive stellar evolution. For the nuclear

The Nuclear Science Laboratory at the University of Notre Dame

Figure 1. Present layout of the NSL showing the location of the accelerators and other major research.

laboratory portrait


structure program the Internal Con-version Electron Ball Array (ICeBall) was recently moved from Yale Univer-sity’s Wright Nuclear Structure Labo-ratory and has been re-commissioned at the NSL. ICeBall is a mini-orange spectrometer arrangement with an ab-solute efficiency of 15% over 4π. The use of ICEBall with the GEORGINA Ge detector array is a very promising new capability for the NSL. Further developments at outside facilities in-clude the design for an underground accelerator complex, DIANA to be located at the 4850 ft level of the San-ford Underground Research Facility (SURF) and the recoil separator sys-tem, SECAR (SEparator for Capture Reactions), for nuclear astrophysics experiments with radioactive beams at FRIB (Facility for Rare Ion Beams) at Michigan State University.

The NSL has an ever-growing num-ber of outside users. Since May 2011, the lab had more than 100 visitors us-ing the local facilities. They came from 15 foreign countries, 14 U.S. univer-sities and colleges, four U.S. national laboratories, and two industrial labo-ratories. Besides U.S. user groups this also includes a substantial number of users from European countries, such as Austria, France, Germany, Hun-gary, Italy, and the United Kingdom, followed by user groups from South America, the Middle East, India, and China. While the NSL is not a user facility we try to accommodate these requests to the best of our capabilities.

NSL Equipment and Instrumentation

The FN tandem accelerator is the workhorse of the laboratory. Installed in 1967, the FN tandem was up-graded in 1995 with two Pelletron chains, and routinely reaches an operating volt-age of 10.5 MV at the terminal with a CO2/N2 tank gas mixture [1]. The tandem serves for a number of nuclear

structure and astrophysics experi-ments, it is the backbone for the AMS program it serves as the driver for the TwinSol dual superconducting sole-noid separator. In addition the tandem serves three target stations for basic nuclear physics experiments, includ-ing a large scale scattering chamber, a neutron time of flight beam-line, a station for the measurement of con-version electrons. Finally the tandem serves two dedicated target stations for applied physics experiments, one for radiochemistry measurements op-erated independently by the ND Radi-ation Laboratory, and one PIXE mate-rial analysis station. The FN is served by two ion sources, a duoplasmatron source, which is primarily used for the production of 3He and 4He beams, and a 40 cathode Multi-SNICS sput-ter source. The laboratory is in the process of up-grading the ion source area and replacing the injection mag-net for improving the mass resolution for heavy ion beams for the AMS pro-gram. The FN provides opportunities for a broad and diverse NSL user com-munity including researchers from small local research institutions like Hope College and Indiana University South Bend to users from Princeton and Fermilab.

The TwinSol radioactive nuclear beam facility, has been developed as a collaboration between groups from the University of Notre Dame and the University of Michigan [3]. TwinSol is a dual superconducting solenoid separator system which selects radio-active reaction products up to mass A = 20 from nuclear reactions trig-gered by a heavy ion primary beam on different target materials, and focuses the intense radioactive beam species on a secondary target for low energy radioactive beam reaction studies. TwinSol has found a wide range of ap-plications in the study of radioactive beam processes for nuclear astrophys-

ics, the investigation of neutron halos, and the impact of halos on low energy fusion processes.

The Tandem is also central for the Accelerator Mass Spectrometry (AMS) program at the NSL [4]. The separation is improved by an additional velocity filter positioned after the second ana-lyzing magnet in the AMS bean-line. The gas-filled Browne-Buechner Spec-trometer serves as a final for isobar separation station. The AMS system provides an additional technique to the nuclear astrophysics research efforts at the NSL, but is also increasingly used for a wider range of applications in the analysis of geological, paleoclimate, and forensic samples. It uniquely com-bines the high sensitivity provided by the gas-filled magnet AMS analysis technique with the energies and beams made available by the FN accelerator. Presently the facility focuses on the analysis of medium mass long-lived ra-dioactive species from 36Cl, 44Ti, 56Ne, 60Fe, and 93Zr improvements in the ion source injection system will expand the mass range well above A = 100.

The 5U single-ended Pelletron was installed in spring 2013 to re-place the 3.5 MV KN Van de Graaff that had served for operating the low energy nuclear astrophysics program for the last 20 years. The new ma-chine reaches 5MV terminal voltage. With four Pelletron chains it achieves the high beam intensities necessary for the nuclear astrophysics program of the laboratory. It is a vertical ma-chine with a compact Nanogan ECR ion source mounted in the terminal (Figure 2). The ECR source allows the production of intense heavy ion beams in 2+ or 3+ charge states, a feature that is important for reaching higher en-ergy beams for inverse kinematics experiments. A number of beams have been developed, proton and alpha beams are produced with intensities of up to 200 mA, but higher currents

laboratory portrait


are expected for the future due to im-provements. Heavy ion beams like 14N, 16O, 38Ar have been developed and tested for different charge states. Beam intensities in excess of 100 mA have been obtained. The 5U Pelle-tron serves three beam-lines, two are equipped with high beam power target stations, one a solid beam-stop target for radiative capture experiments, and one a recirculating gas target system, which can be operated both in the extended as well as jet target mode. These two beam-lines will be primar-ily used for intense light ion proton and alpha beams, while a third beam-line is dedicated to heavy ion beam experiments with the recoil separator St. GEORGE.

St. GEORGE is a state of the art recoil separator for separating heavy recoil reaction products from low en-ergy inverse kinematics experiments (Figure 3). The device is in the tra-dition of the DRAGON separator at

TRIUMF (Canada) and the ERNA separator at Ruhr-University Bochum (Germany), now at CIRCE in Caserta (Italy), and is designed and dedicated

to the study of critical nuclear reac-tions in stellar helium burning. The reactions take place in a high density helium jet gas target HIPPO, with the heavy ion beam being delivered by the 5U accelerator. St. GEORGE con-sists of 14 quadrupole and six dipole magnets to ensure proper charge sepa-ration and mass separation between primary beam particles and reaction recoils. The main separation is based on a velocity filter positioned after the first separation units. St. GEORGE has a calculated rejection power of 1015 and a predicted mass resolving power of 100. Further background reduction of multiple scattered beam particles will be achieved in the detection sys-tem itself, which takes advantage of energy and timing measurements to differentiate recoiling reaction prod-ucts from the remaining primary beam particles. The timing detection system requires beam and recoil products to pass through two thin foils before en-ergy measurements are made with a silicon detector, and energy straggling in these foils will degrade the mass resolution of the detection system.

Figure 3. The St. GEORGE recoil separator with the HIPP gas jet target at the bottom part of the picture.

Figure 2. Column and ECR of the 5U accelerator at the NSL.

laboratory portrait


The NSL’s 1 MV JN Van de Graff was decommissioned during the time of the 5U installation, it covered the very low energy range between 150 keV and 900 keV, and tradition-ally reached fairly high proton and al-pha beam currents up to 150 mA. The machine is presently being rebuilt and up-graded to be moved to the 4,850-ft level of SURF as a first step toward the development of an underground accelerator program in the United States.

Research at the NSLThe nuclear astrophysics program

of the NSL is recognized worldwide and has centered on the study of low energy nuclear reactions in stellar hy-drogen, helium, and carbon burning [5]. This is of particular importance for the understanding of nucleosyn-thesis in early stars, the origin of seed materials in explosive stellar environ-ments, and the source of neutrinos in our sun and other main sequence stars. A number of key reactions for neu-trino and neutron production in qui-escent and explosive stellar environ-ments have been investigated. Also new sources of neutron production during the helium and carbon burning phases in stellar evolution have been addressed. Furthermore, the group has worked on important aspects associ-ated with the thermonuclear runaways in cataclysmic binary star systems as well as with the study of the reac-tions in shock-front driven explosive nucleosynthesis environments associ-ated with the origin of long-lived ga-lactic radioactivity. New theoretical tools have been developed for deriv-ing reliable reaction rates and for in-vestigating the impact of the nuclear reactions on nucleosynthesis, energy production, and time scale of dynamic stellar environments. Studies of criti-cal nuclear structure parameters (e.g., masses, deformation, and incompress-

ibility) have been performed to ex-tract information on critical input for understanding the nucleosynthesis as-pects of core collapse supernovae as-sociated with the p- and r-process and the origin of long-lived galactic γ-ray sources. Beyond interests in nuclear astrophysics, the nuclear structure ef-fort has focused on the investigation of collective modes and on novel modes of quantal rotation using techniques of γ-ray spectroscopy. The experimental program is guided and complemented by theoretical efforts aimed at reliable predictions of nuclear structure and stellar reaction rates and on possible observable signatures associated with these rates. Similarly, r-process related measurements are guided by simula-tion studies to identify the most sensi-tive isotopes for mass and decay mea-surements. Strong efforts have been made to develop a program in nuclear physics and accelerator applications, often in collaboration with other uni-versity groups and institutions. This includes the development of AMS techniques using new long-lived iso-topes as well as the application of PIXE and X-Ray Fluorescence (XRF) as analytical tools on archaeological, forensic, and biological samples [2].

The following sections give a brief overview of some of the scien-tific highlights of the NSL research program in nuclear astrophysics, Ac-celerator Mass Spectrometry, and ra-dioactive beam physics as well as an overview of our nascent applied pro-gram.

Nucleosynthesis in Quiescent and Explosive Stellar Burning

The measurements of critical low energy reaction cross-sections for stellar burning are one of the major research initiatives in nuclear astro-physics. Several fundamental prob-lems are associated with the lack of reliable low energy data of nuclear

reactions during the various stellar burning phases. The so-called neu-trino problem has been solved but improvements in the associated reac-tion rates for the production of solar neutrino emitters remains important as a new tool to probe directly the condi-tions in the solar interior. At the NSL we have studied experimentally and theoretically a number of reactions that impact the solar neutrino flux. The 3He(α,γ)7Be reaction is important for neutrino production by the pp-chains in our sun, new measurements as well as an extensive new R-matrix analy-sis reduces previous uncertainties in the extrapolation of the low energy cross-section to less than 4% [6]. De-tailed measurements of proton capture on 12C, 14N, 15N, and 17O isotopes that influence the neutrino produc-tion by the CNO cycles, have been performed over a wide energy range. The results directly address the role of the CNO cycles in solar neutrino pro-duction and serve as important input for using the measurement of CNO neutrinos with BOREXINO as inde-pendent tool for determining the solar metallicity. This program on radiative capture measurements will continue at the 5U accelerator using the new GEORGINA gamma detection system to improve the efficiency of the mea-surements. It will be expanded toward radiative capture reactions in higher mass regions to address nucleosynthe-sis patterns in nova explosions.

Stellar helium burning is of particular relevance since the reaction chain 4He(2α,γ)12C(α,γ)16O(α,γ)20Ne determines the 12C/16O in our uni-verse. Particular attention was given to the study of the 12C(α,γ)16O analysis, probing the reaction components by indirect techniques like elastic scatter-ing on 12C and beta-delayed alpha de-cay analysis of 16N. This information puts considerable constraints on the R-matrix analysis of the 12C(α,γ)16O

laboratory portrait


radiative capture cross-section and re-duces considerably the uncertainty in the S-factor extrapolation of the low energy data as shown in Figure 4. The 16O(α,γ)20Ne reaction has been mea-sured successfully and low energy extrapolation has been pursued by R-matrix techniques. The installation of the St. GEORGE separator opens new possibility by studying the alpha radiative capture processes in inverse kinematics using intense heavy ion beams on a high density helium gas jet target. This new experimental pro-gram has just started with calibrating the separator with well-known alpha capture reactions such as 3He(α,γ)7Be and 14N(α,γ)18F in inverse kinemat-ics. Besides the 12C(α,γ)16O and the 16O(α,γ)20Ne reactions, other alpha capture studies such as 15N(α,γ)19F of relevance for helium shell burning patterns and the origin of 19F through the AGB phase of low mass stars will be investigated.

The question of the stellar neutron sources for the s-process in stellar he-lium and carbon burning has been one of the important missions for the NSL research program. We investigated the impact of (α,n) reaction on 17O and 18O that play a crucial role for the overall neutron budget in stellar burn-ing environments. In particular one of the dominant neutron source of the s-process is the 22Ne(α,n)25Mg reaction. Direct measurements of it are handi-capped by the large cosmic ray in-duced neutron background and there-fore indirect techniques have been developed to probe the alpha cluster structure of the compound nucleus 26Mg. Alpha scattering and alpha transfer reactions have been utilized for this purpose to achieve a consistent picture of the alpha cluster structure in 26Mg near the alpha and neutron threshold. For carbon burning, new initiatives were developed to study the low energy cross sections of the

various reaction channels of 12C + 12C fusion and explore the possibility of the 12C(12C,n)23Mg reaction as an s-process neutron source in shell carbon burning during late stellar evolution. This effort resulted in a full simulation of nucleosynthesis patterns during carbon burning.

The NSL has developed an exten-sive program for investigating the ori-gin of p-nuclei, the rarest stable neu-tron deficient isotopes above A = 100. The p-nuclei are produced by photo-dissociation of heavy mass materials in supernova environments. The cross sections of critical photo-excitation re-actions along the predicted p-process path are derived by studying the in-verse radiative capture reactions using the activation method by monitoring the characteristic emission of the re-action product. Using this technique, a number of (p,g) and (α,g) reactions have been studied in the Z = 50 closed shell range. These measurements have been complemented by systematic analysis of the alpha scattering poten-tial to simulate the data with improved statistical model calculations.

Reactions that are important in ex-plosive nucleosynthesis environments often involve radioactive nuclei since the reaction path is far from the line of stability. For that reason the NSL has been a driving force for large-scale facilities like FRIB that promise the delivery of high intensity radioactive beams and participates on new devel-opments such as the recoil separator SECAR (SEparator for Capture Reac-tions) for nuclear astrophysics experi-ments at FRIB. However, there is also considerable effort in using transfer and capture reactions to probe indi-rectly the various reaction components that determine the stellar reaction and decay rates for critical processes along the rp-process path and the r-process path. These studies are being per-formed at the FN tandem accelerator

Figure 4. Experimental data and R-matrix analysis of all reaction channels 12C(α,α), 16N(b-α), 12C(α,γ) feeding the 16O compound nucleus. This is an ex-ample of a comprehensive approach toward extrapolating the reaction cross-section of 12C(α,γ) to the stellar energy range around 300 keV.

laboratory portrait


but more recently take advantage of other laboratories such as the RCNP in Osaka, Japan, or GSI near Darmstadt, Germany.

The experimental program in nu-clear astrophysics is complemented by theoretical studies that aim at identify-ing the most important experiments, and most challenging questions to pursue. Research is focused on the implementation of new reaction rates, masses, and lifetimes into nucleo-synthesis simulations for evolution-ary and explosive stellar conditions. This includes the development of the powerful multi-channel, multi-level R-matrix code AZURE [6] that has set new standards in the field of low energy nuclear reactions and has been applied for the analysis of numerous reaction studies.

A new formalism was developed for calculating fusion rates at extreme densities as anticipated for the core of white dwarf or the crust of neutron star crusts. The formalism is suitable for applications ranging across extremes of temperature and density, from ther-monuclear to pycnonuclear regimes. While initial studies concentrated on interactions between even–even nu-clei, new initiatives have also included odd–even and odd–odd nuclei. Recent experimental work at ANL on fusion reactions between stable carbon and radioactive carbon isotopes showed remarkable good agreement with the predictions (Ernst Rehm, private com-munication). This work primarily ad-dressed nucleosynthesis patterns in high density neutron crusts by follow-ing the fate of x-ray burst ashes in ac-creting neutron star environments.

Extensive simulations have been performed for the prediction of r-pro-cess abundance patterns. We have con-ducted a sensitivity study of the light r-process path to identify the nuclei that have the greatest impact on the final abundance distributions (Figure 5).

Using the FRDM mass model as a standard, the study was performed by varying the binding energy of each individual nucleus by ±25% and then evaluating the impact of that change on the entire range of the r-process production [7]. The work is currently being expanded to analyze the sen-sitivity of the r-process abundance distribution to half-lives and neutron capture reaction rates. The goal of these r-process sensitivity studies is to pinpoint the key nuclei that have the most crucial impact on the abun-dance distribution. These studies have become the basis on which several proposals to Californium Rare Isotope Breeder Upgrade (CARIBU), TRI-UMF, NSCL, and RIKEN have been submitted and will become the basis

for some of the future experimental program of the NSL group.

AMS Development and ApplicationsThe AMS program has focused on

measurements related to galactic ra-dioactivity and main stellar burning as well as the production of Short-Lived Radionuclides (SLRs) in the Early Solar System (ESS). The Notre Dame AMS system, unique in North Amer-ica in this configuration, provides the nuclear astrophysics research efforts of the laboratory with an additional, highly sensitive detection technique. It combines the high sensitivity pro-vided by the gas-filled magnet separa-tion technique with the energies and beams made available by the FN ac-celerator, as well as the accessibility

Figure 5. The most impactful nuclei as determined from binding energies for a classical hot r-process trajectory using three different mass models (FRDM, Duflo-Zuker, HFB-21). The black line shows the potential experimental reach of CARIBU at ANL while the gray line is the potential reach of FRIB in construc-tion at the NSCL in Michigan State University.

laboratory portrait


to beam time [4]. Recently the AMS astrophysics program has focused on 44Ti, 60Fe, 36Cl, and 93Zr detection and is currently developing 55Mn as well as a 14C capability for the growing NSL applied program. Below some details on some of these programs.

One isotope, 44Ti, offers a unique synergy between observational, theo-retical, and experimental nuclear as-trophysics associated with explosive silicon burning. It is typically assumed to be mainly produced by the alpha capture reaction 40Ca(α,γ)44Ti in the alpha-rich freeze-out phase of an ex-panding supernova shock front. The concurrent studies of this reaction at the NSL via AMS and via direct radia-tive capture in-beam γ measurements have placed very precise restrictions on the reaction rate in the burning window of astrophysical interest.

60Fe is predominantly produced in the galaxy by type II supernovae and plays an important role in astrophysical investigation of a possible supernova near the solar system about three mil-lion years ago. Recent measurements of the NSL group of the 60Fe(n,γ)61Fe at the FZ Karlsruhe, Germany and the 59Fe(γ,n)60Fe depend critically on the 60Fe half-life, which is currently open for debate as a recent t1/2 = 2.6 × 106y has been reported by the Munich AMS group. The NSL group now pursues AMS detection of 60Fe based on the gas-filled magnet technique to sepa-rate the 60Fe-60Ni isobars. Our pres-ent 60Fe/Fe sensitivity of ~1 × 10–13 has allowed us to make important 60Fe concentration measurements on a number of samples at ND in an effort to re-determine its astrophysically im-portant half-life. Recently a new 60Fe sample was obtained from the VERA group in Vienna for a direct decay ac-tivity measurement of 60Fe that will complement the measurements of the 60Fe material produced by our group at MSU during the previous funding cycle.

Radionuclides with lifetimes τ < 100 My (e.g., 146Sm, 129I, 60Fe, 53Mn, 41Ca, and 36Cl) are often referred to as SLRs known to have been extant when the Solar System formed 4.568 Gy ago. Identifying the origins of SLRs can provide insight into the origins of our solar system and the processes that shaped it. Now extinct 36Cl in the ESS is thought to have been produced by local particle irradiation. However, the models that attempt to recreate the production of 36Cl in the ESS lack ex-perimental data for the nuclear reac-tions considered. In particular, data for the 33S(α,p)36Cl reaction, which is an important reaction in the production of 36Cl, could help reduce the uncertain-ties in the models and better constrain the environment where the SLRs were produced. A very successful AMS project to measure this reaction cross-section was recently performed at the NSL and the results and comparisons to model predictions have been pub-lished [8].

AMS detection of the rare isotope 93Zr (t1/2 = 1.6 Ma) has application potential in two fields of research: the improved astrophysical modeling

of nucleosynthesis processes through an independent determination of the 92Zr(n,γ)93Zr cross-section at stellar energies, currently only measurable using the TOF technique, and using this radionuclide as a tracer in hydro-logical and radioactive waste studies. A combination of chemical reduction of the Nb interfering isobar in combi-nation with the gas-filled magnet tech-nique and a newly developed Bragg peak detector show promise to allow the detection of 93Zr in the ~10–8 iso-topic ratio range.

Radioactive Beam Studies at the NSL

The Radioactive Nuclear Beam (RNB) group at ND has maintained an extensive research program at the TwinSol facility (Figure 6) using the FN tandem accelerator as a driver. It involves the study of nuclear reactions with beams of short-lived radioactive nuclei together with the development of instrumentation and techniques to facilitate these investigations. TwinSol provides intense radioactive beams up to A = 20 with the mass range limited by energy range of the tandem driver.

Figure 6. The TwinSol radioactive beam facility Notre Dame.

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Secondary beams of 6He, 7Be, 8Li, 8B, 10Be, 12B, 12N, and 18Ne have been used in experiments with maximum intensities from 104 to 107 s–1, de-pending on the primary beam.

More recently, produced beams of 6He, 10Be, and 12B were used simul-taneously to study the cluster struc-ture of the composite systems 10Be and 14C, respectively, using an active target-time projection chamber (AT-TPC). The NSL presently plans to ex-pand the range of TwinSol application by using it as a HELIOS (HELIcal Orbit Spectrometer) type of separator system, with the first solenoid acting as a focusing element to enhance the acceptance and the second solenoid acting as helical orbit spectrometer.

Beams such as 11,14C, 15O, and 17,18F have been developed with a similar range of intensities but have not been used in experiments to date.

Detector Development and Testing

Considerable effort was dedicated for the support of the SCENE (Scin-tillation Efficiency of Nuclear recoils in Noble Elements) experiment for the development of a liquid argon detector for direct dark matter search experi-ments. The SCENE experiment was devised as a collaboration between Princeton University, Fermi National Laboratory, and several other institu-tions to measure the energy- and elec-tric-field dependencies of the ioniza-tion and light yields from low-energy nuclear recoils in liquid argon. These data are of interest for understanding the mechanism of liquid argon scin-tillation at moderate to high linear energy transfer. For SCENE, a small, dedicated LAr-TPC is used in coinci-dence with liquid scintillator neutron detectors to collect elastic neutron scattering events. With a pulsed neu-tron beam, clean data has been ob-

tained for recoil energies as low as 3.0 keV. Future runs are also planned to improve and expand the dataset, in particular for measurements of the ionization yield.

The prototype for the AT-TPC of MSU for ReA3 experiments was tested with different gases and beams. Beams of 6He, 10Be, and 12B pro-duced by the TwinSol facility were used simultaneously to study the cluster structure of the composite system, 10Be and 14C, respectively. The products of complete and incom-plete fusion from the beam with 4He or 36Ar were detected for the study of the fusion process with halo nuclei. The three body break up of 14C is be-ing analyzed. The detector tests have been very successful and presently we are discussing a number of scientific studies associated with cluster-break-up of near threshold states in even-even nuclei using the ND accelerator facilities.

Applications for Cultural Heritage and Societal Impact at the NSL

PIXE at the NSL is a very useful tool for determining the elemental composition of a wide range of speci-mens. We have developed a small test set-up to initiate a broader applied pro-gram in cultural heritage at the NSL.

Pottery samples from the U.S. Southwest Anasazi sites have been analyzed using PIXE techniques. Of particular interest was the pigment composition of the painted pottery surface of Mesa-Verde Black on white pottery. The goal of the archaeolo-gists involved was to differentiate the source materials used to make the pig-ments on various samples taken from different locations that may provide insight into the production methods and possible geographical origins of the ceramics.

Roman Silver Denarii from the period of 240 BC to 250 AD were

analyzed using high energy PIXE techniques at the FN tandem accel-erator. After mapping the composi-tion of the surface of the coin with a 3 MeV beam, the composition of the inner coin layers were probed with 7 to 9 MeV proton energy. The silver/copper ratio of the coins was deter-mined and showed a gradual decline in silver between 240 BC and 200 AD, followed by a rapid devaluation in the subsequent decades of the Se-veri emperors.

To complement the PIXE approach methods like XRF and Raman spec-trometry are being developed for probing medieval manuscripts from the ND rare book collection. We are in the progress of developing a program for scanning and analyzing selected samples from the Ancient and Medi-eval Manuscript collection of the ND libraries. The main purpose is to study the pigment structure and composition on medieval document ornaments and paintings on selected library samples using XRF and Raman spectrometry techniques. This initiative is being broadened by the development of an art analysis program. The analysis of forfeited painting materials will lead to a broader effort in art forensics for the identification of forgeries.

Radiation chemistry studies at the NSL examine the fundamental pro-cesses that occur due to the passage of ionizing radiation in condensed matter. Energy deposition, medium decomposition, and subsequent chem-istry are being investigated in a vari-ety of materials of importance to the nuclear power industry. Processes in-duced in matter by ionizing radiation are strongly dependent on the type and energy of the incident radiation because of the geometry of the local energy deposition. The FN tandem accelerator in the NSL is ideal for performing these types of studies be-cause of the variety of light ions and

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energies available. Systematic stud-ies have been performed on water and a variety of aqueous solutions that mimic the coolants in nuclear power reactors.

Heterogeneous radiation chemis-try does not only address corrosion issues of reactor components but is also a crucial aspect for the long-term storage of waste materials. Water radi-olysis has been shown to be substan-tially altered at or near to interfaces as compared to bulk water. The transport of energy, charge, and matter through interfaces is now receiving renewed interest because of the fundamental studies performed at the NSL.

Future energy needs involving nu-clear power will necessarily involve the treatment and storage of waste ma-terials. Many nuclear waste materials are alpha particle emitters and studies at the NSL have often included 5 MeV helium ions to mimic this radiation. Comparison of the results with con-ventional gamma radiolysis indicates that several polymers and resins that are thought to be radiation resistant can readily decompose with alpha particle radiolysis. Fundamental stud-ies with various components of resins are still in progress, but the results in-dicate that reactions of highly excited states within the incident radiation track are responsible. Ionic liquids are the new designer solvents and work currently underway is helping design more radiation robust liquids to be used in electrochemical waste separa-tion systems.

Advanced fast neutron detec-tion systems are being developed for Homeland Security and related appli-cations based on organic scintillation detectors. Such detectors have proven

capabilities of neutron and γ-ray de-tection; however, more research is needed to optimize their γ-ray dis-crimination capability before they can be widely deployed. The proposed systems are designed to provide detec-tion capability equivalent to or better than existing 3He-based systems, and, in many cases, for a fraction of the cost, given the current shortage of 3He material. In addition, the advanced digital data-acquisition capabilities developed for the proposed systems provide real-time particle classifica-tion with a simplified data stream for quick and reliable identification of special nuclear material.

A second project is to develop neu-tron detection capability for precision Non-Destructive Assays (NDAs) of actinide-fluoride samples. Fluorine is commonly used in actinide com-pounds (mostly UF6) in the nuclear fuel cycle. The method will signifi-cantly advance safeguards verification at existing declared facilities, nuclear materials accounting, process control, nuclear criticality safety monitoring, and a variety of other nonprolifera-tion applications. INL in partnership with Oak Ridge National Labora-tory, Rutgers University, and ND are undertaking a precision (better than 10%) determination of the absolute cross section of the 19F(α,n)22Na reac-tion as a function of energy. The spe-cific goal is to identify the neutron and γ-ray energy spectrum emitted from 19F(α,n)22Na at alpha energies per-tinent to NDA. The project uses the Versatile Array of Neutron Detectors at Low Energy (VANDLE) detector array in a two-part experiment, with both fluorine and α beams, to com-pletely characterize the 19F(α,n)22Na

reaction and overcome difficulties with previous measurements that seri-ously limited the precision of neutron-based assays of UF6 samples.

References1. A. Aprahamian and M. Wiescher, Nu-

clear Physics News 12 (4) (2002) 5. 2. P. Collon and M. Wiescher, Physics To-

day 65 (2012) 58.3. F. D. Becchetti, M. Y. Lee, T. W.

O’Donnell, D. A. Roberts, J. J. Kolata, L. O. Lamm, G. Rogachev, V. Gui-marães, P. A. DeYoung, and S. Vincent, Nucl. Instr. Meth. A505 (2003) 377.

4. D. Robertson, C. Schmitt, P. Collon, D. Henderson, B. Shumard, L. Lamm, E. Stech, T. Butterfield, P. Engel, G. Hsu, G. Konecki, S. Kurtz, R. Meharchand, A. Signoracci, and J. Wittenbach, Nucl. Instr. and Meth. B 259 (2007) 669.

5. M. Wiescher, F. Käppeler, K. Lan-ganke, Ann. Rev. Astr. Astrophys. 50 (2012) 165.

6. R. E. Azuma, E. Uberseder, E. C. Simp-son, C. R. Brune, H. Costantini, R. J. de Boer, J. Görres, M. Heil, P. J. LeBlanc, C. Ugalde, and M. Wiescher. Phys. Rev. C 81 (2010), 045805.

7. S. Brett, I. Bentley, N. Paul, R. Surman, and A. Aprahamian, Eur. Phys. J. 48 (2012) 184.

8. M. Bowers, Y. Kashiv, W. Bauder, M. Beard, P. Collon, W. Lu, K. Ostdiek, and D. Robertson, Phys. Rev. 88 (2013) 065802.

Ani AprAhAmiAn,philippe Collon,

And miChAel WiesCher

Nuclear Science Laboratory, University of Notre Dame,Notre Dame, Indiana, USA

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Recently, with the advent of advanced radioactive beam facilities and novel experimental techniques, unexplored regions of exotic nuclei and new phenomena became ac-cessible for detailed spectroscopic studies [1]. Examples are the discovery of neutron halos, the appearance of new magic numbers, exotic types of particle decays (e.g., two proton radioactivity), shape coexistence, and exotic shapes of nuclei [2–4] to name a few. Along the same lines, it is relevant to ask what happens to the interesting, but at the same time, complex phenomenon of nuclear fission, when one moves further away from the classical and relatively well-studied region of fission in the trans-uranium nuclei?

Fission of atomic nuclei, discovered in 1938, represents one of the most dramatic examples of a nuclear metamor-phosis, whereby the nucleus splits preferentially into two smaller fragments releasing a large amount of energy. His-torically, several distinctive types of fission were identified, such as particle-induced fission (e.g., neutron-induced fis-sion), spontaneous fission (SF), and spontaneously fission-ing isomers, beta-delayed fission (bDF, being the subject of this article) [5], electromagnetically induced (Coulomb ex-citation, or Coulex) fission of radioactive nuclei at relativis-tic energies [6], photofission, and surrogate-type of fission.

The fission process is often broadly classified as high-energy fission, in which the excitation energy E* of the fis-sioning nucleus strongly exceeds the fission barrier height Bf , or, as low-energy fission (E* ≤ Bf). Figure 1 schemati-cally shows the present status of experimental low-energy fission studies. Low-energy fission is a unique tool to probe the nuclear potential energy landscape and its dynamical evolution, as a complex function of elongation, mass-asym-metry, spin and excitation energy, from the single “com-pound nucleus” system over the top of the fission barrier and further to the scission point, culminating in the forma-tion of fission fragments. This evolution involves a subtle interplay of collective (macroscopic or mean field) and single-particle (microscopic) effects, such as shell-effects and pairing, all of which are considered both for the initial

nucleus and for the final fission fragments and at large de-formations. Fission enables the study of nuclear-structure effects in the heaviest nuclei and has direct consequences on their creation in nuclear explosions and in the astrophys-ical r-process [6], which is terminated by fission, and on the abundance of medium-mass elements in the universe through so-called “fission recycling.” Three fission pro-cesses are thought to be important for r-process termination by fission: spontaneous fission, neutron-induced fission, and beta-delayed fission.

In contrast to high-energy fission, in which the micro-scopic effects are washed out, the interplay between macro-

Figure 1. The nuclei for which fission fragments mass or nuclear-charge distributions have been measured by low-energy fission. The distributions are shown for selected systems. Blue open circles: distributions, measured in con-ventional particle-induced fission experiments and sponta-neous fission. Green crosses: nuclear-charge distributions, measured by Coulomb-excitation [6]. Open and closed dia-monds show 26 known bDF cases in the three regions of bDF (see text), the fissioning daughter is indicated. Filled diamonds mark 11 daughter nuclides for which mass dis-tribution was measured, two of them—180Hg and 242Cf are shown in the plot. The references to all data in the plot can be found in Ref. [5].

Beta-Delayed Fission: A Rare Decay Mode as Probe for Phenomena Near and Beyond the Fission Barrier

Andrei n. Andreyev1, MArk Huyse2, And Piet vAn duPPen21 University of York, UK, and Advanced Science Research Center, Japan Atomic Energy Agency, Tokai-mura, Japan

2KU Leuven, Belgium

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scopic and microscopic effects in fission can be sensitively explored at low excitation energy. In particular, in SF from the ground state, the excitation energy is E* = 0 MeV, while in SF from isomeric states or in thermal neutron-induced fission it does not exceed a few MeV. However, SF stud-ies are limited to heavy actinides and trans-actinides. By using Coulomb-excited fission of relativistic radioactive beams [6], fission studies became available in new regions of the Nuclidic Chart with exotic N/Z ratios, see the nuclei marked by crosses in Figure 1. In this case, the excitation energy is centered around E* ~ 11 MeV.

In terms of the excitation energy, bDF is intermediate between SF and Coulomb-induced fission. Beta-delayed fission, discovered in 1965–1966 in Dubna [5], is a two-step nuclear decay process that couples beta decay and fission, see Figure 2. Similar to other beta-delayed decay processes, in bDF, a parent nucleus (a precursor) first un-dergoes b decay, populating excited state(s) in the daugh-ter nuclide. In the case of neutron-deficient nuclei, electron capture (EC) or b+ decay is considered, while b– decay hap-pens on the neutron-rich side of the Nuclidic Chart. We will use throughout this text the term bDF for both the neutron-rich and neutron-deficient nuclei. In bDF, the maximum excitation energy of the daughter nucleus is limited by the QEC (Qb– in case of neutron-rich nuclei) of the parent. The typical QEC values are in the range of 3–6 MeV and 9–12

MeV for the known bDF nuclei in the trans-uranium and lead regions, respectively. If the excitation energy of these states, E*, is comparable to or greater than the fission bar-rier height, Bf, of the daughter nucleus (E* ~ Bf) then fission may happen in competition with other decay modes (e.g., g decay and/or particle emission). Therefore, the special fea-ture of bDF is that fission proceeds from excited state(s) of the daughter nuclide. As these states are populated in the beta decay, the time scale of the bDF events is determined by the half-life of the parent nucleus. As in most cases the b-decay half-lives are longer than tens of ms, it makes bDF more easily accessible for experimental studies.

In summary, the importance of bDF is highlighted by its ability to provide low-energy fission data for very exotic nuclei that do not decay by SF and that are difficult to ac-cess by other techniques.

Presently, 26 bDF cases are known experimentally, see Figure 1 and review [5], where all relevant references are given. This can be compared to the number of ~220 cases of beta-delayed charged particle emission on the neutron-defi-cient side and ~200 cases of beta-delayed neutron emission on the neutron-rich side [2]. Globally speaking, the bDF nuclei are situated in three distinct but extended regions of the Nuclidic Chart: in the neutron-rich Ac and Pa isotopes, and the neutron-deficient isotopes in the trans-uranium and lead regions.

Historically, the first cases of bDF were discovered in Dubna in the precursors 232,234Am (thus, 232,234Pu are fis-sioning daughter nuclides). This region is relatively easily accessible by complete fusion reactions with heavy ions but the initial experiments used quite unselective produc-tion and detection techniques (e.g., the fission track mica foils). In many cases, the identification of the bDF precur-sors and the assignment of fission to a specific nuclide was done based on half-life and extensive cross-irradiation with projectile energy and different projectile-target combina-tions. The Berkeley group significantly contributed to bDF studies of 228Np, 232,234Am, 238Bk, and 242,244,246,248Es iso-topes by using radiochemical separation techniques, which allowed the Z value of the bDF precursor to be established. The use of the silicon detectors to measure the fission frag-ment energies allowed to deduce the total kinetic energy release and the fragment’s mass distribution. The latter was shown to be asymmetric, as for most of neighbouring trans-uranium nuclei, studied by spontaneous fission. In Figure 1, an example of the mass distribution for bDF of 242Es (242Cf is the fissioning nuclide) is shown. Later experiments in-cluded the use of a recoil separator like (e.g., the velocity filter SHIP at GSI) where the bDF of 246Md was measured, but still no direct Z or A could be performed.

Figure 2. Simplified diagram for the b+/EC delayed fission in the neutron-deficient nuclei. Shown are the ground states of the parent (A,Z) and daughter (A,Z-1) nuclei, and as a function of elongation, the potential energy and associated shapes of the daughter nucleus. QEC value of the parent and fission barrier Bf of the daughter nuclei are indicated by vertical arrows. The bDF of excited states with E* ~ Bf in the daughter nucleus is shown by horizontal arrows. The color code on the right-hand side represents the fission probabilities; the darker colors correspond to higher prob-abilities.

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The second region of bDF nuclei includes six neutron-rich nuclei of Ac-Pa (e.g., 228,230Ac) mostly produced by transfer reactions on the heaviest stable targets. However, although these measurements are more relevant for the r-process, due to the extreme difficulties to produce neutron-rich nuclei, often uncertain data exist in the literature.

The first cases of bDF in the neutron-deficient nuclei in the lead region (being the third region of bDF occurrence on the Chart of Nuclides) were discovered in the very neu-tron-deficient nuclei 180Tl (N/Z = 1.22), 188Bi and 196At in 1987 in Dubna [8]. They were produced in complete fusion reactions with heavy ions, only half-life information could be deduced for fission fragments. Overall, in the pre-2008 bDF studies a broad variety of different techniques was used with different degrees of selectivity in production and detection. In most cases however, no direct A and Z of the bDF precursor was deduced.

Recently, extensive bDF studies in the very neutron-deficient nuclei between Tl and Fr have been performed by our collaboration, which constitute the core of this com-munication. As shown in Figure 1, the respective bDF nuclei lie very close to the border of known nuclides and possess very unusual neutron-to-proton ratios, for example, N/Z = 1.23–1.25 for 178,180Hg in contrast to a typical ratio of N/Z = 1.55–1.59 in the uranium region, where numerous SF and bDF cases are known. This allows to investigate

potential differences in the bDF process and its observables in the two regions, which differ in many nuclear-structure properties. It is important to stress that while the bDF of the most neutron-rich nuclides, relevant for the r-process, cannot be presently studied experimentally, the bDF prop-erties deduced for the neutron-deficient isotopes, will help to determine the N/Z ratio dependence.

In a series experiments, performed by our collaboration at the velocity filter SHIP (GSI), the occurrence of bDF in 186,188Bi and in 192,194At was firmly established [9, 10].

Since 2008, our collaboration initiated dedicated bDF studies at the ISOLDE mass separator at CERN (Geneva) [11]. The coupling of the Resonance Ionization Laser Ion Source (RILIS) [12] to ISOLDE opened up new possibili-ties for bDF studies. The RILIS allows unique selective ionization of the element of interest, thus, Z-identification. Figure 3 shows the simplified operational principles of this technique, as was first used at ISOLDE for detailed bDF studies of 178,180Tl [5, 13]. The use of the laser ionization technique also allows unique isomer separation, which is especially important for the odd-odd bDF precursors, many of which have more than one nuclear state capable of bDF. As an example, the case of bDF of 180Tl is discussed here. After selective ionization, acceleration up to 30 keV, and mass separation, a pure 180Tl beam of ~150 atoms/s was analysed by the Windmill detection system, which included several silicon detectors. The use of two silicon detectors in a compact geometry allowed both singles a/fission de-cays and double-fold fission fragment coincidences to be efficiently measured. In a ~50-hour long experiment, 1111 singles and 356 coincidence fission events were observed and attributed to the bDF of 180Tl, see Figure 4.

The uniqueness of this technique is the unambiguous A and Z identification of the precursor, via the combination of the mass-selection by ISOLDE and Z-selection by the RILIS. Other advantages include a point-like source, the implantation in a very thin foil whereby both fission frag-ments can be efficiently measured with little deterioration of their kinetic energies, and the proximity of germanium detectors for g-ray spectroscopy. Simultaneous measure-ment of fission and a decays in the same detectors reduces the systematic errors for branching ratio determination sub-stantially.

The mass distribution for fission fragments of 180Hg is clearly asymmetric; the most abundantly produced frag-ments are 100Ru and 80Kr and their neighbors. No com-monly expected symmetric split in two semi-magic 90Zr nuclei was observed, and the authors claimed observation of “new type of asymmetric fission in proton-rich nuclei,” which differs from asymmetric fission in the trans-uranium region [13].

Figure 3. Schematic view of the ISOLDE and RILIS opera-tion as applied in the bDF studies of 180Tl. The 1.4-GeV 2 mA proton beam impinges on the thick 50 g/cm2 238U tar-get, producing a variety of reaction products via the spall-ation, fragmentation, and fission reactions. The neutral reaction products diffuse towards the hot cavity where the thallium atoms are selectively ionized to 1+ charge state by two overlapping synchronized laser beams precisely tuned to provide thallium ionization in a two-color excitation and ionization scheme. The ionized thallium ions are extract-ed by the high-voltage potential of 30 kV, followed by the A = 180 mass separation with the ISOLDE dipole magnet. The mass-separated 180Tl ions are finally implanted in the carbon foils of the Windmill system, for subsequent mea-surements of their decays.

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Several different theoretical approaches were applied to understand the asymmetric mass split, such as macroscopic-microscopic model by Möller et al. [14, 15], the self-con-sistent nuclear density functional theory employing Skyrme SKM* and Gogny D1S energy density functionals by Warda et al. [16] and different versions of scission-point models by Andreev et al. [17] and Panebianco et al. [18]. Figure 5 shows the contrasting behavior of the potential energy sur-face in the fission of a traditional fissioning nucleus 236U and of 180Hg. A robust description of the asymmetric mass split of 180Hg was obtained in all models despite the fact that quite different underlying mechanisms are proposed by the differ-ent studies.

In the recent experiments at ISOLDE, the bDF of 178,180Tl, 194,196At and of 200,202Fr was firmly established [5] and in most cases the mass distribution of the fission frag-ments was established. The clearly mass-asymmetric mass distribution of fission fragments of 178,180Hg (being the fis-

sioning daughters of 178,180Tl) established a new region of asymmetric fission in addition to the previously known one in the trans-actinides. An extended region of predominantly symmetric fission is situated between the two regions of asymmetric fission around 208Pb (Figure 1).

To date, 26 bDF isotopes are known in three regions of the Chart of Nuclides. However, in many cases only scarce in-formation is presently available. Substantial progress can be expected in all three regions, due to developments of novel and/or improved production and detection methods. Below we highlight some of the interesting bDF studies feasible in the near future.

The main efforts in all three bDF regions should concen-trate on detailed experiments to reliably measure bDF prob-abilities, partial half-lives, and energy/mass distributions of fission fragments, similar to those, performed for (e.g., 180Hg in Ref. [13]). A direct measurement of the Z values is also needed to firmly establish the A and Z distributions of the fission fragments. The experiments with the laser-ionized isomerically pure beams of 192,194At, 186,188Bi, and 202Fr should determine whether both isomers of each isotope un-dergo bDF and whether any difference exists in the bDF pro-cess of different isomers. The importance of these isotopes is further highlighted by the fact that their fissioning daughters 186,188Pb, 192,194Po, and 202Rn lie in the transitional region between 178,180Hg, exhibiting asymmetric low-energy fis-sion, and 204Rn, which fissions symmetrically at similar ex-citation energies, see Figure 1.

The search for new bDF cases is another important task. For example, with the presently available beam intensities dedicated searches for bDF of the neutron-rich 228,230,232Fr and of 228,230,232Ac are possible at ISOL facilities (such as ISOLDE or ISAC [TRIUMF]). In contrast to the earlier ra-diochemical studies of 228,230Ac at Lanzhou [19], a unique Z and A identification of the parent isotope could be obtained, along with the measurements of fission fragments energy and mass distributions.

Furthermore, in the past, by using the multinucleon-transfer reactions of 11.4 MeV/u 238U ions with W/Ta tar-gets at the GSI ISOL mass-separator, new isotopes 232Ra and 232,234Ac were produced [20]. The use of this method to search for bDF of 232,234Ac could be an interesting extension of the bDF studies of 230Ac, also produced in the transfer reaction. In the lead region, a search for bDF of the odd-A precursors should be performed.

The aforementioned goals require improved production and detection techniques. The new in-flight recoil separators, such as S3 (SPIRAL2 at GANIL) [21] will provide unprec-edented opportunities to reach the neutron-deficient nuclei in the trans-uranium region, which is not accessible using the high-energy proton-induced reactions and based on ISOL

Figure 4. Top panel: a coincidence energy spectrum for bDF of 180Tl measured by two silicon detectors. The two-peaked structure originates because the two fission fragments have different energies, a direct result of the asymmetric mass distribution. Bottom panel: the derived fission-fragment distribution of the daughter isotope 180Hg as a function of the fragment mass and the total kinetic energy.

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techniques. Due to a substantial beam intensity increase, a gain by at least an order of magnitude in production rates can be expected. Combined with better separation capabili-ties and improved detection systems, these facilities will cer-tainly open a new era in bDF studies in the trans-uranium region. The same technique can also be used to study the shortest-lived bDF nuclides in the lead region, such as 192At, which is not yet accessible at ISOL facilities due to its short half-life compared to the relatively long release time from the target-ion source.

Laser-based techniques, such as RILIS@ISOLDE [11, 12], CRIS@ISOLDE [22], and the recently developed IG-LIS method [23], coupled to the S3 separator, will further increase the sensitivity of the experiments and allow us to address the problem of existence of two isomers in a bDF precursor.

More generally, as far as low-energy fission studies are concerned, several promising projects are presently being developed. As a continuation of the Coulomb-induced fis-sion experiments with relativistic secondary beams the next generation of such studies has recently been initiated by the SOFIA collaboration at GSI [24]. These experiments will benefit from the improved beam intensity of the initial 238U beam and from detector developments that should enable the unique mass and charge identification of fission fragments with a precision of one unit.

In another recent approach, the VAMOS spectrometer (GANIL) was used to study fission initiated by multi-nu-cleon transfer reactions in inverse kinematics between a 238U beam and a 12C target. The first experiments produced dif-ferent minor actinides, within a range of excitation energies below 30 MeV [25].

A new ambitious method to study low-energy fission exploits the inelastic electron scattering off exotic radioac-tive beams in a colliding beam kinematics. Two such proj-

ects are currently underway: ELISe (FAIR) [26] and SCRIT (RIKEN) [27]. All these new developments and efforts show that substantial progress in bDF and low-energy fission stud-ies and in the understanding of the fission process is expected in the near future.

References 1. C. Fahlander and B. Jonson, Nobel Symposium 152: Physics

with Radioactive Beams, Physica Scripta I152 (2013) 010301. 2. M. Pfützner et al., Rev. Mod. Phys. 84 (2012) 567. 3. K. Heyde and J. Wood, Rev. Mod. Phys. 83 (2011) 1467. 4. L. P. Gaffney et al., Nature, 497 (2013) 199. 5. A. N. Andreyev, M. Huyse, and P. Van Duppen, Rev. Mod.

Phys., 85 (2013) 1541. 6. K.-H. Schmidt et al., Nucl. Phys. A665 (2000) 221. 7. I. Petermann et al., Eur. Phys. J. A48 (2012) 1. 8. Y. A. Lazarev et al., Europhys. Letts. 4 (1987) 893. 9. A. N. Andreyev et al., Phys. Rev. C 87 (2013) 014317.10. J. F. Lane et al., Phys. Rev. C 87 (2013) 014318.11. E. Kugler, Hyp. Int. 129 (2000) 23.12. V. N. Fedosseev et al., Rev. Sci. Instr. 83 (2012) 02A903.13. A. N. Andreyev et al., Phys. Rev. Lett. 105 (2011) 252502.14. P. Möller et al., Nature 409 (6822) 785.15. T. Ichikawa et al., Phys. Rev. C 86 (2012) 024610.16. M. Warda, A. Staszczak, and W. Nazarewicz, Phys. Rev. C 86

(2012) 024601.17. A. V. Andreev, G. G. Adamian, and N. V. Antonenko, Phys.

Rev. C 86 (2012) 044315.18. S. Panebianco et al., Phys. Rev. C 86 (2012) 064601.19. X. Yanbing et al., Phys. Rev. C 74 (2006) 047303.20. K. L. Gippert et al., Nucl. Phys. A 453 (1986) 1.21. A. Drouart et al., EPJ Web of Conferences 17 (2011) 14004.22. K. M. Lynch et al., J. Phys.: Conf. Ser. 381 (2012) 012128.23. R. Ferrer et al., Nucl. Instrum. Methods B291 (2012) 29.24. J. Taieb, Private Communication. 25. X. Derkx et al., EPJ Web of Conferences 2 (2010) 07001.26. H. Simon, Nucl. Phys. A 787 (2007) 102.27. T. Suda et al., Phys. Rev. Lett. 102, (2009) 102501.

Figure 5. Calculated PES surfaces for 180Hg and 236U taken from Ref. [15], as a function of dimensionless quadrupole moment and the mass asymmetry. The shapes of the nuclei at several key locations as they proceed to fission are drawn, connected to the points on the surface by arrows.

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IntroductionSoon after the formulation of BCS theory [1], it was rec-

ognized by Bohr, Mottelson, and Pines that the existence of an energy gap in the intrinsic excitation spectrum of deformed nuclei displayed a suggestive analogy with that observed in the electronic spectra of metallic superconduc-tors and could, like this one, be described at profit in terms of correlated pairs [2]. Their paper represented the starting point of more than fifty years of experimental and theoreti-cal BCS flavored studies of pairing in nuclei [3].

One of the important results that has emerged from this quest is that pairing has not one (bare nucleon-nucleon (NN) interaction plus eventual 3N corrections, e.g., Refs. [4–6] and refs. therein) but two origins, the second one resulting from the exchange of collective nuclear vibrations between pairs of nucleons moving in time reversal states lying close to the Fermi energy (Refs. [7–9] and refs. therein; see also Ref. [10] and A. Idini, Renormalization Effects in Nuclei, http://air.unimi.it/handle/24341216315). This is why in dis-cussing the pairing phenomenon one is simply forced to “complicate” the force through many-body correlations, a reflection of the retardation effects displayed by the nuclear pairing dielectric function.

In keeping with the fact that the building blocks of pair-ing correlations are Cooper pairs, two-nucleon transfer is specific to probe them, the associated absolute differential cross-sections being the main, model independent observ-ables relating theory with experiment.

In the first part of the present contribution we report on recent progress made within this context [11–13], progress that has allowed light to be shed into the interplay of bare

and induced pairing interactions and to obtain, inter alia, quantitative evidence of phonon mediated pairing in halo exotic nuclei ([14–17] and refs. therein). This is the subject of the second part of the article.

Pair Transfer and Pairing Correlations in NucleiAt the basis of BCS theory of superconductivity one finds

the condensation of strongly overlapping Cooper pairs, a model that has been applied with success to the description of pairing correlations in atomic nuclei. There is however a main difference, as compared with the case of the con-densed matter scenario in which BCS theory originated. In the nuclear case, fluctuations of the pairing field as well as of the normal density are very important and renormalize in a conspicuous way the different quantities entering the theory. In particular, around closed shell nuclei, systematic evidence exists of the correlation and stability of the pair addition (Figure 1) and pair subtraction modes, which are strongly excited in two-particle transfer reactions. Pairing vibrations (Refs. [18–20] and refs. therein), the nuclear em-bodiment of single Cooper pairs, smooth out through zero-point fluctuations (ZPF) the sharp change of the occupancy of levels around the Fermi energy (Figure 1a, bottom), tak-ing place in mean field, thus paving the way for an eventual phase transition from normal to superfluid phases.

A number of pairing vibrational bands have been ob-served throughout the mass table, containing up to three phonon states [21]. Because of the strong correlations dis-played by these vibrational modes, their microscopic prop-erties can be accurately described in terms of RPA and of a constant pairing strength, leading to reliable values of the

Pairing Interaction and Two-Nucleon Transfer Reactions

GreGory Potel1, AndreA IdInI2, FrAncIsco BArrAnco3, enrIco VIGezzI4, And rIcArdo A. BroGlIA4,5,6,7 1CEA-Saclay, IRFU/Service de Physique Nucléaire, Gif-sur-Yvette, France2Institüt für Kernphysik, Technische Universität Darmstadt, Darmstadt, Germany3Applied Physics Department III, University of Seville, Seville, Spain4INFN Milan, Milan, Italy5Department of Physics, University of Milan and INFN Milan, Milan, Italy 6The Niels Bohr Institute, Copenhagen, Denmark7FoldLESs S.r.l., Monza, Italy

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X- and Y-amplitudes (Figures 1b and c), and thus of two-nucleon spectroscopic amplitudes (Ref. [21], Tables XVI–XVIII). The study of pairing vibrations provides, among other things, insight into the mechanism by which a nuclear superfluid phase eventually emerges from the condensation of pairing vibrational modes, as the system under study moves progressively away from closed shell nuclei. The condensation of these extended and thus strongly overlap-ping, bosonic objects gives rise to a highly correlated super-fluid state, displaying overall phase coherence. Superfluid-ity is tantamount to the existence of a finite ground-state average value of the pair addition and removal operators, P+, P in the ground state, that is, to a finite value of the order parameter

a10 = |〈BCS|P+|BCS〉| = |〈BCS|P|BCS〉|

a quantity that provides an estimate of the number of cor-related pairs in the BCS ground state (≈4–8).

It also gives a measure of deformation in gauge space, the counterpart of deformation in ordinary space (e.g., Refs. [10, 19] and refs. therein). Just as adjacent (I, I ± 2) states lying along a, for example, quadrupole ro-tational band are connected by strongly enhanced values of the quadrupole operator, the adjacent 0+ ground states (N, N ± 2) of, for example, a chain of superfluid isotopes

are connected by strongly enhanced values of the pair trans-fer operator, measured in terms of single- (particle-hole) and of two-particle units, respectively [19, 22]. This result testifies to the fact that these 0+ states are members of a pairing rotational band. Within this scenario, pairing vibra-tions and rotations together with single-particle motion and vibrations and rotations in “normal” (three-dimensional) space constitute elementary modes of excitation.

The suggestive analogy concerning the nuclear phenom-ena associated with spontaneous symmetry breaking in 3-D and in gauge space (Table XI, Ref. [21]) although extend-ing also to the reaction (decay) processes in which these rotational modes are specifically probed, is not operative as far as the calculational details are concerned. In fact, Coulomb excitation (electromagnetic decay) and Cooper pair transfer display very different levels of calculational challenges (complexity) concerning their implementa-tion. This is keeping with the fact that in Coulomb excita-tion, let alone electromagnetic decay, one has to deal with a single mass partition, a fact which makes it possible to treat structure and reaction, to a large extent, separately. This is not the case for two-nucleon transfer reaction, in which case mass partition is different between entrance and exit channels, a fact that leads to recoil effects and thus to an important coupling between relative motion (reaction)

Figure 1. RPA, Nuclear Field Theory (NFT) diagramatic representation of the structure [33, 34] and reactions [26] of, and with pair addition modes. This pairing vibration is mainly a correlated superposition of two-particle states with (c) forwardsgoing amplitudes Xk on the different orbitals above the Fermi energy (εk > εF ). The possibility of creating this state by populating hole states below the Fermi energy (b), with backwardsgoing amplitudes Yi(εi εF ) arises from the presence of two-particle, two-hole configurations in the ground state of the closed shell system (a, top), ZPF which smooth out the discontinuity in level occupancy at εF (a, bottom). The solid dot represents the strength and form factor with which particles, moving in time reversed states, couple to the collective, quasi-boson pairing degree of freedom. It results from the combined effect of a four-point vertex (bare interaction), see graph (d), and of vertex correction (induced interaction) processes, an example of which is given in diagram (f ). Diagram (e) is representative of processes which dress the single-particle states. By intervening processes (e) and (f ) with an external field which picks up two nucleons from the system, one can force the virtual phonon to become a real final state. Assuming that the pair addition mode is the two-neutron halo of 11Li, the wavy line representing the quadrupole vibration of the 8He core, coupled to a p3/2 (π) proton state, the process g) describes the population of the first excited state of 9Li in the reaction 1H( 11Li, 9Li (1/2-;2.69 MeV)) 3H [14, 15].

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and intrinsic motion (structure). In fact, the situation is even richer, in keeping with the fact that nucleons may be transferred not only simultaneously but also successively. Thus, one is confronted, in the calculation of the abso-lute value of two-nucleon transfer cross-sections, with the opening of a new channel and thus of a new mass partition (e.g., (N + 2) + p → (N + 1) + d → (N) + t). It is then not surprising that the theory of Coulomb excitation and electromagnetic decay was quantitatively operative already few years after the first observation of rotational bands [19, 23], while it took decades (Figure 10 of Ref. [11] for an overview of the groups and the practitioners involved in the quest) after the first observation of a pairing rotational band [20, 25] before one was able to calculate absolute Cooper pair transfer cross-sections that account for the observa-tions within experimental errors (Figure 2).

The fact that, as a rule, successive transfer dominates over simultaneous transfer, and that, in both processes, the transferred nucleons display equivalent pairing cor-relations, is a consequence of the fact that Cooper pairs are weakly bound (<<εF), highly extended (>>R) objects. Consequently, the minimum theory of two-nucleon transfer corresponds to second order DWBA, where the two above mentioned processes are taken into account properly cor-rected by non-orthogonality effects (Refs. [26] and [11] and refs. therein. It is only recently that these well known elements were implemented into a versatile software (G. Potel, Cooper, private communication) with which, mak-ing use of well tested, state of the art spectroscopic am-plitudes, and global optical potentials, one can calculate absolute two-particle transfer differential cross-sections that account for the experimental findings within experi-mental errors throughout the mass table [19–22, 24, 25]. Examples of these quantitative results are displayed in Figure 2 (Figures 5 and 6, Table 3 of Ref. [11]).

It is worth pointing out the difference existing in the physics which is at the basis of the agreement between theory and experiment displayed in the upper and middle panels (“upper”) of Figure 2, as compared to the two lowest panels, in particular the lowest right panel (“lowest”). In fact, the results displayed in “upper” depend little on the de-tails of the pairing interaction employed, or the exact value of the energies and Z-values (see below) of the single-par-ticle levels used in the calculations, a fact intimately con-nected with the constancy of the lowest quadrupole mode through the Sn-isotopes (evidence of the validity of gener-alized seniority), and of the large two-neutron separation energy associated with 208Pb. This is the reason why simple models like BCS or RPA which embody the physics of co-herent pairing modes, i.e., pairing rotations and vibrations,

provide essentially “exact” two-nucleon spectroscopic am-plitudes (i.e., UνVν and (Xj ,Yj) factors). On the other hand, the results displayed in “lower” are very sensitive to the details of the single-particle energies and associated Z-values, as well as to components in the 11Li ground-state

Figure 2. Absolute cross-sections associated with two–neutron transfer reactions involving superfluid (Sn-iso-topes) and pair vibrational (208Pb and 9Li) nuclei. Making use of the “exact” two-nucleon spectroscopic amplitudes (see text), of global optical potentials, and of two–nucleon transfer software developed within the framework of sec-ond–order DWBA (Potel, private communication), the cor-responding absolute differential cross-sections associated with these reactions were calculated and are displayed (continuous curves) in comparison with the experimental data [11, 12, 14, 15, 20, 27]. It is of notice that the abso-lute differential cross-section associated with the 9Li(1/2–, 2.69 MeV) state provides a realization of the NFT process depicted in Figure 1g in direct comparison with the data (right bottom panel).

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wavefunction displaying a 1% probability, a result of rather refined NFT calculations.

In keeping with the above parlance, the type of results displayed in “upper” and “lower” provide confidence in the fact that one now knows how to accurately calculate ab-solute two-nucleon differential cross-sections in (nuclear structure) simple cases as well as to quantitatively predict new mechanisms to dynamically violate gauge invariance.

Let us now elaborate on the fact that the coherent char-acter of pairing correlations manifests itself equally well in simultaneous than in successive transfer processes. In fact, for superfluid nuclei the quantity (a0) is given, in the case of simultaneous transfer, by the relation

a0 = ∑v>0〈BCS|av+av

+– |BCS〉

and by the expression

a0 = ∑i,v>0〈BCS|av+|i〉〈i|av

+– |BCS〉

≈ ∑n,v>0〈BCS|av+av

+|BCS〉〈BCS|avav

+– |BCS〉

in the case of successive transfer. Making use of the qua-siparticle transformations both relations lead to ∑v>0UvVv. This result is intimately connected with the large distance (correlation length ξ ≈ ħ vF /D ≈ 36 fm) over which Cooper pair partners correlate.

The Pairing Interaction and Medium Polarization Effects

The nature of the attractive pairing force acting between electrons represented a central question in the development of the theory of superconductivity in metals (screened Cou-lomb field plus electron-phonon mediated interaction, Ref. [38] and refs therein). In the nuclear case the bare NN-in-teraction is strongly attractive in the 1S0 channel for a wide range of relative momenta, and mean field calculations lead to neutron or proton pairing gaps of the order of those de-rived from experimental data. It is of notice, however, the latest developments concerning 3N (mainly repulsive) cor-rections to the bare NN-interaction [4].

Before dealing with the question of how two-nucleon transfer reactions can shed light on the question of the in-terplay (relative importance) of bare and induced pairing interaction, let us remind the basics of medium polarization effects. The relevance of these effects in connection with one-nucleon transfer reactions has been recognized since a long time (Ref. [28] and refs. therein). In such reactions, as well as in (e,ep) processes, one often observes that the single-particle strength associated with levels lying close to the Fermi energy is fragmented over a number of peaks, and the single-particle content of the main peak varies typi-

cally from 60% to 80% of the value expected in the inde-pendent particle limit.

Part of the reduction of single-particle strength can be ascribed to the short-range part of the NN−interaction (short wavelength mechanism) which shifts single-particle strength away from the Fermi energy (high momentum processes). Another part of the reduction is associated with a long wavelength mechanism resulting from the inter-weaving of single-particle and low-lying collective vibra-tions (low-k processes). Examples of such processes are displayed in Figure 3. A nucleon can bounce inelastically off the nuclear surface, setting it into vibration, changing its state of motion and, at a later time, by reabsorbing the vibration return to its original state as shown in (I, upper diagram). Important effects are also connected with the pro-cess depicted in (I, lower diagram), obtained from a time ordering from process (I, upper diagram). It leads to a par-tial blocking of the ground-state correlations (oyster-like diagram), process giving rise to an effect known in atomic physics as the Lamb shift.

Through the processes displayed in Figure 3, a nucleon moving in a single-particle configuration is forced into more complicated configurations. In other words, the single-par-ticle strength becomes fragmented, and the discontinuity of the occupation numbers at the Fermi energy, Z = 1 in the case of the non-interacting system, is reduced (Z < 1) .

The probability with which the associated components of the ground-state wavefunction containing phonon degrees of freedom are present in the dressed single-particle states can, in principle, be experimentally determined in one-par-ticle transfer processes populating the excited states of the A-1 system. As an example, we refer to the p(11Be,10Be(2+))d reaction [29]. The presence of such components has been shown to be relevant also in break-up reactions [30]. We note that the importance of contributions of multi-step pro-cesses, which can populate the final states in question, even in absence of correlations in the initial state, must be quanti-tatively assessed. In fact, the possibility of observing the ex-citation of states associated with the “complex” components of the single-particle wavefunction of the initial ground state is connected with situations in which multistep processes are hindered by structure and/or Q−value effects.

Renormalization effects of the nuclear pairing gaps have been discussed for quite some time in connection with in-finite matter (Ref. [31] and refs. therein). Work started at the end of the 1990s provided evidence through the result of detailed calculations that, in finite nuclei, the exchange of virtual phonons—in particular quadrupole and octu-pole surface vibrations—between two neutrons coupled to J p = 0+ gives rise to an energy dependent attractive force

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leading to state dependent pairing gaps that, in average, account for a conspicuous fraction of the empirical values [7–10] (within this context, see Ref. [19], p. 432). The pro-cesses at the basis of the induced pairing interaction are de-picted in Figure 3 (II, upper diagram): a vibration excited by a nucleon is reabsorbed by a second nucleon. Such a process leads to an induced interaction among nucleons, as-sociated with the polarization of the nuclear medium.

The superposition of the bare and the phonon induced interactions (Veff = V bare + V ind) increases in nuclei, the value of the Cooper pair binding energy as compared to the

Vbare result, while the coupling of surface phonons to sin-gle-particle states leads to a depopulation of the pure single-particle states through self-energy processes (Figure 3(I)). As a consequence, the BCS gap equation is modified by the presence of Z-factors [9, 38], leading to:

Q�1 D �Z1

X

2

V eff .1; 2/Z2Q�2

2 QE2

:

OHECQF D !

�

.1 � �/ Ond ��

4NOQ�

� OQ�

�

;

˛ D4Csym

T

"

�

Z

A

�2

1

�

�

Z

A

�2

2

#

D4Csym

T�;

E�D

MtotX

iD1

Ekini .CP/ C

3

2MnT C Q;

One can then identify two contributions to the gap, D~ = D

~bare + D~ind. The effects of the basic renormalization diagrams can be taken into account up to infinite order, by

Figure 3. The NFT scheme synthetized in Figures 1a–f becomes operative concerning the structure of 10Li and 11Li: (I) self-energy processes, giving rise to parity inversion in 10Li; (II) bare (boxed inset) and induced pairing interac-tion binding the halo neutron pair to the 9Li core, through a bootstrap mechanism, in which the neutrons exchange the pigmy dipole resonance of 11Li, as well as the quadrupole vibration of the core, as testified by the wavefunc-tion b. In other words, the color snapshots displayed in (a) and (b) attempt at describing the becoming of the neu-tron halo Cooper pair of 11Li, from an uncorrelated s2

1/2(0) configuration to a strongly correlated, (weakly) bound two-neutron state. It is of notice that the bare interaction (boxed inset in (II)), corresponding to the process depict-ed in Figure 1d (NFT four point vertex, rule (II) of NFT, Ref. [34], p. 314) lowers the s2

1/2(0) (as well as the p21/2(0))

pure configurations by only 100 keV, and is not able, by itself, to bind the pair. The color plots display the modulus square of the two-neutron wavefunction as a function of the coordinates of the two nucleons (left) and the probabil-ity distribution of one neutron with respect to the second one held fixed on the x-axis (at a radius of 5 fm, solid dot). The red circle schematically represents the core. After Ref. [37].

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solving the Nambu-Gor’kov equations, leading to a consis-tent theoretical picture, that accounts for these effects both on the single-particle motion and on the pairing interac-tion. The resulting total gap D~ is considerably larger than the value DBCS obtained solving the usual BCS equations with the bare (Argonne) pairing force without taking into account renormalization of single-particle motion. The con-tributions coming from the bare and from the induced inter-actions to D~ are comparable. At the Fermi energy the value of D~ is larger than experiment by ≈20%. It is of notice that coupling to spin modes will somewhat reduce the value of D~. However, at present no complete microscopic calculation of the pairing gap including both the bare interaction and medium polarization effects exists. Main open problems re-main the determination of the initial mean field, the role of three-body forces and the coupling to spin modes.

Theory indicates that the induced interaction is con-centrated around the Fermi energy and is strongly surface peaked. It is, however, not straightforward to have direct information of these properties: its effects can, in many cases, be simulated by adjusting the strength of the bare interaction. In fact, the spatial dependence of the Cooper pair, at least in well bound nuclei, depends only weakly on the details of the pairing interaction. Within this context one can posit that the pairing gap, although intimately con-nected with pairing in nuclei, is not the specific quantity to probe the corresponding correlations, at least as far as the nature of the interaction that generates them is concerned. This is also in keeping with the fact that the pairing gap is a derived quantity (e.g., 3-point empirical value, requiring the knowledge of three different nuclear masses). On the other hand, with the help of two-nucleon transfer reactions, one can force the virtual processes displayed in Figures 1e and f to become final, observable states. In fact, being able to accurately calculate absolute differential cross-sections, information about the phonon admixture in the Cooper pair wavefunction can be obtained by studying pair transfer to excited collective vibrational states of the core (Figure 1g).

Let us conclude this section with a technical note. At vari-ance with infinite systems in general, and condensed mat-ter in particular, in which case particle number fluctuations are negligible, in the nuclear case they play an important role. This is the reason why much work has been dedicated to this question (projection methods, RPA techniques, etc.; Refs. [3, 10, 35, 36] and refs. therein). Within this context, the pairing gap becomes D = (D

~2 + G2 S0 (RPA)/2)1/2, where S0(RPA) contains the (particle-conserving) matrix elements of P+ and P (Ref. [10], p. 151). While projection effects are dominant at the phase transition, they lead to corrections of the order of 10–20% for the ground state pairing gap.

The Case of Halo NucleiRenormalization effects can have particularly striking

consequences in halo nuclei like 11Li, systems which are weakly bound and easily polarizable. In particular, it was proposed [32] that the coupling of single-particle levels to quadrupole vibrations of these systems plays an important role to explain the positive parity of the ground state of N = 7 isotones, a dynamical effect going beyond mean field theory. The particle-vibration matrix elements associated with quadrupole vibrations are, in these nuclei, very large (Figure 3(I)). In fact, the neutron 2s1/2 orbital is shifted downwards by several MeV by virtue of its coupling to configurations of the type [d5/2 ⊗ 2+]1/2+ (polarization dia-gram (I, upper) in Figure 3). Furthermore, the neutron 1p1/2 orbital is shifted upwards as a result of the suppression of ground state correlations (Pauli principle processes) mostly associated with the configuration [p1/2 ⊗ p3/2

–1]2+ ⊗ 2+]0+ (correlation diagram (I, lower diagram) in Figure 3).

A dynamical Nuclear Field Theory (NFT, Refs. [33, 34] and refs. therein) description of the two-neutron halo nuclei 12Be and 11Li, based on the coupling to the vibrations of these systems and of their cores, provides an overall ac-count of their nuclear structure properties [17, 37]. Deal-ing with a single, dressed Cooper pair, the corresponding wavefunction can be obtained by summing the processes shown in Figures 1e and f to infinite order with the help of Dyson’s equation. Such a treatment of the variety of cou-plings is tantamount to a full diagonalization, including the (discretized) continuum. In fact, and as is well known, the continuum plays an essential role in the case of 11Li, for which all the relevant single-particle orbitals are resonant or virtual states, in keeping with the fact that 10Li is un-bound. Furthermore in 11Li, an important role is played by the low-lying dipole state (pigmy resonance ≈1 MeV), responsible of much of the glue binding the neutron halo Cooper pair to the 9Li core. The resulting wavefunction of the dressed neutron halo can be written as shown in Figures 3b and (II).

It turns out that the (short range) bare 1S0 neutron-neu-tron pairing interaction leads, in the present case, to a small contribution. This is in keeping with the very low angular momenta available to the neutrons (essentially s,p states being involved in the very extended and diffuse 11Li halo). The wavefunction of the 3/2– ground state of 11Li is then obtained by coupling the p3/2(p) proton, treated as a specta-tor, to the neutron halo.

A detailed analysis of the reaction 1H(11Li,9Li)3H reac-tion performed at TRIUMF [14] with a 11Li beam (inverse kinematics) has been carried out [15]. Two states were ob-served: the 9Li ground state and the first 9Li(1/2–) excited

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state, which is interpreted as the lowest member of the p3/2(p) ⊗ 2+ multiplet. It is of notice that the angular dis-tribution associated with the ground state is very sensitive to the relative weight of the s2 and p2 configurations in the wavefunction displayed in Figures 3b and (II), wavefunc-tion which reproduces quite accurately the experimental findings (Figure 2).

The renormalization processes in which the neutrons of the halo Cooper pair of 11Li either emit and reabsorb a col-lective (p-h)-like quadrupole vibration (effective mass pro-cesses, Figure 1e) or exchange a phonon (vertex correction, Figure 1f) can, in a two-particle pick-up reaction (Figure 1g), populate the first excited state 1/2– of 9Li. The absolute value of the corresponding two-nucleon transfer cross- section provides an accurate measure of the probabil-ity with which the |(s1/2,d5/2)2+ ⊗ 2+;0〉 component ap-pears in the 11Li ground state (Figure 3b) and thus of the role the quadrupole vibration plays in binding the neutron halo Cooper pair. This is also in keeping with the fact that alternative channels, like final-state inelas-tic excitation and neutron break-up, lead to negligible contributions [15].

The fact that theory reproduces the observed absolute differential cross-sections, testifies to the fact that NFT of structure and reactions [26, 33, 34] is able to accurately pre-dict [37] and describe [15] the consequences of the induced nuclear pairing interaction.

While this result can, arguably, be considered a milestone in the understanding of the origin of pairing in nuclei, we feel equally important and timely the developments taking place at a breathtaking pace, concerning the connection of the NN-bare interaction and the quark degrees of freedom, and of its regularization in terms of renormalization group methods or similar techniques (Vlow–k), to work out a pairing interaction (taking also 3N terms into account), which can be used in nuclear structure calculations. It is likely that these develop-ments will contribute together with the ones presented above, in an important and hopefully conclusive way to the quest of assessing the relative role of bare and medium polarization effects in the nuclear pairing interaction.

AcknowledgmentWe thank Luisa Zetta and Paolo Guazzoni as well as

Ritu Kanungo and Isao Tanihata for discussions and clari-fications concerning their state of the art A+2Sn(p,t)ASn and 1H(11Li, 9Li)3H data, respectively. Collaboration with Ben Bayman is gratefully acknowledged. RAB acknowledges his debt towards Daniel R. Bès for many discussions and clarifications concerning the physics that is at the basis of the subjects treated in the present contribution.

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jamin, Reading, Mass., 1975). 20. J. H. Bjerregaard et al., Nucl. Phys. 89 (1966) 337.21. R. A. Broglia, O. Hansen, and C. Riedel, Adv. Nucl. Phys.

6 (1973) 287; http://www.mi.infn.it/~vigezzi/BHR/Broglia HansenRiedel.pdf

22. R. A. Broglia, C. Riedel, and T. Udagawa, Nucl. Phys. 184A (1972) 23.

23. K. Alder et al., Rev. Mod. Phys. 28 (1956) 43224. H. Flynn et al., Nucl. Phys. 154A (1970) 225.25. G. Bassani et al., Phys. Rev. 139 (1965) B830.26. R. A. Broglia and A. Winther, Heavy Ion Reactions (Addison-

Wesley, New York, 1991). 27. P. Guazzoni et al., Phys. Rev. C60 (1999) 054603; ibid

C69(2004) 024619; C78(2008) 064608; C83(2011) 044614; C85(2012) 054609.

28. C. Mahaux et al., Phys. Rep. 120 (1985) 1.29. J. S. Winfield et al., Nucl. Phys. A 683 (2001) 48.30. A. M. Moro and R. Crespo, Phys. Rev. C85 (2012) 054613.31. H.-J. Schulze et al., Phys. Lett. B 375 (1996) 1.32. H. Sagawa, B. A. Brown, and H. Esbensen, Phys. Lett. B309

(1993) 1. 33. D. R. Bes and R. A. Broglia in Proceedings of the Interna-

tional School of Physics “Enrico Fermi,” Course LXIX, eds. A. Bohr and R. A. Broglia, North Holland, Amsterdam (1977), 55.

34. P. F. Bortignon et al., Phys. Rep. 30 (1977) 305.35. M. Anguiano et al., Nucl. Phys. A 696 (2001) 467.36. Y. R. Shimizu et al., Phys. Rev. Lett. 85 (2000) 2260.37. F. Barranco et al., Eur. Phys. J. 11 (2001) 385.38. J. R. Schrieffer, Theory of Superconductivity (Benjamin, New

York, 1964).

facilities and methods


Studies of neutron-rich nuclei close to 78Ni or 132Sn give a rare opportunity to test the interactions used in shell-model calculations in a neutron-rich heavy-mass region. Furthermore, the proximity of the r-process path to these nuclei makes an understanding of their structure important. In the neutron-rich A = 100 region the ground states of some of the nuclei here undergo a rapid change in shape from a spherical one at N = 58 to a deformed one at N = 60 with the addition of just two neutrons. This shape-change process is still not fully understood and allows insights into the appearance of collective behavior in medium-heavy nuclei to be gained.

It is difficult to investigate excited states of neutron-rich nuclei with masses A = 80–160 using standard nuclear-physics accelerators, such as tandems. One particularly successful technique for studying the intermedi-ate-to-high spin excited states in these nuclei is to place spontaneous-fission sources at the centre of large arrays of germanium (Ge) g-ray detectors [1]. The division in two of a 252Cf, or 248Cm, nucleus generally results in two fission fragments, one with mass A ~ 100 and the other A ~ 145. As the par-ent nucleus has about 60% more neu-trons than protons, and this process is cold (only ~4–5 neutrons evaporated on average), then the two resulting fragments are nearly always neutron rich [1]. Decays of the excited states in these isotopes can then be detected in a Ge detector array and triple-g coinci-dences used to select the cascade, or fis-sion split, of interest [1]. The limitation of this technique is however that only two spontaneous-fission sources are realistically available. An almost-iden-tical reaction to spontaneous fission is to induce fission in trans-actinde targets

using neutrons with thermal or cold (<0.025 eV), energies. Several targets are available for such studies with fis-sion cross-sections of a few hundred barns. Changing the target specie can significantly vary the fission-product mass distribution and the population of the fragments of interest can be optimized by careful target selection. Hence A ~ 80–95 or A ~ 125–135 neu-tron-rich nuclei, where little is known, can be populated at intermediate spins in such reactions. With this in mind, the EXILL (EXOGAM at ILL) col-laboration was formed to install the ef-ficient EXOGAM array (Figure 1) [2] of Ge g-ray detectors, from GANIL, at the PF1B cold neutron guide of the Institut Laue-Langevin (ILL) reactor in Grenoble. This in effect temporar-ily “EXILed” EXOGAM to the ILL. The array was operational for a total of two 50-day reactor cycles. This was the first time that a modern, high-efficiency array of Ge detectors had been used with an intense, cold-neutron beam. The main aims of this novel project are to study prompt g rays emitted by neutron-rich fission fragments close to

132Sn, north-east of 78Ni, in the A = 100 region and to detect prompt g decays following (n,g) reactions on other tar-gets.

In low-energy fission, the excited states in more than 100 nuclei can be studied concurrently via prompt g-g-g spectroscopy with an efficient high-granularity Ge array. After neu-tron evaporation, the secondary fis-sion fragments have average spins of ~6–8 ħ, allowing the study of interme-diate-spin states. Nuclei with spins as high as 20 ħ have been studied in such reactions too. Compared to the fission yields from a 252Cf source, inducing fission with thermal neutrons in targets such as 235U or 241Pu populates some of the nuclei in in the A = 80–95 and A = 130 regions with an order-of-mag-nitude higher production, for the same fission rate. Both these targets have been used in the EXILL campaign and allow excited states with intermediate spins in nuclei with just a few particles or holes outside the doubly magic 78Ni and 132Sn to be studied. The simple structure of these neutron-rich A ~ 80 or A ~ 130 nuclei allows sensitive tests of the predictions of shell-model calcu-lations to be performed. Such informa-tion is useful not only for testing theo-retical model predictions but has an astrophysical interest too, as the rapid neutron-capture (r-process) path passes close to, or even through, the mass distributions produced. Nuclear struc-ture information is an important input to such calculations. The neutron-rich nuclei of the A ~ 100 region are also well produced with these two targets and these isotopes have the interesting property that a rapid change in shape of their ground state, from a spheri-cal one to a strongly prolate-deformed one, has been observed when increas-

Investigating the Structure of Neutron-Rich Nuclei with Neutrons

Figure 1. The EXOGAM array in op-eration at the PF1 beam line.



ing the number of neutrons from 58 to 60. Their study allows insights into the onset of collective behavior in nuclei.

As more than 100 neutron-rich nuclei are available for study in the EXILL campaign then this contributes to the worldwide effort of studying nuclei with high neutron-to-proton ratios. The ongoing construction, or recent commissioning, of second-gen-eration, intense radioactive ion-beam facilities at several locations world-wide aims to study the properties of the most neutron-rich nuclei. Although it will not be possible to study nuclei as far from stability as those produced at these new radioactive ion-beam facilities, the EXILL campaign con-tributes to these efforts by allowing detailed spectroscopic information to be obtained across a large area on the neutron-rich side of the nuclear chart. This allows the detailed evolution of trends in nuclear-structure properties to be followed, such as level energies, nuclear shapes, and fission yields.

The (n,g) reaction has been used for studying excited states in near-stability nuclei for many years. All states within a certain energy and spin range can be populated in the resulting nucleus al-lowing “complete” spectroscopy to be performed. The EXILL campaign was unique in that it was the first use of a large, high-efficiency, high-granularity Ge array for (n,g) experiments allowing greatly increased sensitivity for coinci-dence measurements. Thus even targets could be studied that have extremely low neutron capture cross-sections of tens of millibarns or where only small milligram quantities of enriched mate-rial are available.

The EXILL campaign has been possible due to the low-background, high cold-neutron flux available from the ILL reactor. At the end of the PF1B cold neutron guide a thermal-equiva-lent neutron flux of 2 × 1010 n/cm2/s is available [3]. In order to perform prompt g-ray spectroscopy, a carefully

constructed collimation system was used which produced a pencil-like beam with dimensions of ~1 cm2 at the target position. This reduced the flux by two orders of magnitude but re-sulted in a beam with low divergence, almost free of g-ray background. A va-riety of stable targets were placed in the beam for (n, g) studies and two fis-sile targets, 235U and 239Pu.

The EXOGAM array was used in two configurations during the EXILL campaign. The first consisted of 10 Clover Ge detectors, supplemented with 6 GASP single-crystal coaxial Ge detectors from INFN, Legnaro. This configuration was optimized for g-ray spectroscopy experiments and the ar-ray had a total photopeak efficiency of about 6% at 1.3 MeV. Both the 235U and 241Pu fission targets were used with this setup, along with a selection of stable targets for (n,g) studies. In the second configuration 8 EXOGAM Clo-ver detectors were used in combination with 16 LaBr3 g-ray scintillation detec-tors from the FATIMA collaboration. These scintillation detectors had time resolutions each of 140–180 ps and al-lowed the lifetimes of excited nuclear states to be measured via a direct timing coincidence measurement. Knowledge of the lifetimes of excited nuclear states can give information on the collective or single-particle nature of an excited nuclear state. A mixed Ge-LaBr3 array was necessary as the moderate energy resolution of the LaBr3 detectors (~3%) does not allow a clean selection of an individual decay cascade. The Ge de-tectors provide this selection, but have inferior timing performance (resolution ~10 ns). The same targets were used with this configuration as in the first reactor cycle and this is the first time that such direct timing measurements have been attempted using a mixed Ge-LaBr3 array with a fission target.

The signals from the preamplifiers of all detectors were fed into a data acquisition system consisting of 100

MHz digitizer modules which allowed both energy and time information to be recorded [4]. The time signals from the LaBr3 scintillator detectors were processed using analog electronics. All data were recorded in a triggerless mode and some 60 TB of data were collected over the two reactor cycles. This amount of data is the equivalent to the total amount collected by the 40 instruments of the ILL in its previous 40 years of operation! Some 41 exper-imental proposals were submitted for the first reactor cycle with EXILL in the spectroscopy configuration and 32 in the combined EXILL-FATIMA one. Around 120 scientists and students participated in the data taking and sev-eral years work of data analysis now lie ahead for this collaboration.

AcknowledgmentsThe EXILL campaign would not

have been possible without the sup-port of several services at the ILL and the LPSC. We are grateful to the EX-OGAM collaboration for the loan of the detectors, to GANIL for assistance during installation and dismantling, and to the INFN Legnaro laboratory for the loan of the GASP detectors.

References1. I. Ahmad and W. R. Phillips, Rep. Prog.

Phys. 58 (1995) 1415.2. J. Simpson and the EXOGAM collab-

oration, Acta Physica Hungarica, New Series, Heavy Ion Physics 11 (2000) 159.

3. H. Abele et al., Nucl. Instr. and Meth. A 562 (2006) 407.

4. P. Mutti et al., Proc. of the ANNIMA Conference (2013).

Gary SimpSon

on behalf of the EXILL core team and collaboration

LPSC, Grenobleand University of the

West of Scotland



The National Superconducting Cy-clotron Laboratory (NSCL) at Michi-gan State University (MSU), shown in Figure 1, is the largest campus-based nuclear science facility in the United States. NSCL is funded by the National Science Foundation (NSF) for oper-ating its Coupled Cyclotron Facility (CCF) as a national user facility and for conducting research in nuclear physics, nuclear astrophysics, and accelerator physics.

The Facility for Rare Isotope Beams (FRIB) will be a new U.S. Department of Energy Office of Science (DOE-SC) national user facility supporting the mission of the Office of Nuclear Phys-ics. The FRIB Project designs and es-tablishes FRIB. FRIB will make effec-tive use of NSCL’s infrastructure when it becomes operational. The FRIB Proj-ect is funded by the DOE-SC, MSU, and the State of Michigan. When FRIB construction is complete, NSCL will cease operations and merge into FRIB, and FRIB operations will be funded by the DOE-SC. The NSCL user group has already merged into the FRIB User Organization (FRIBUO) that has over

1,250 members and approximately 20 working groups.

NSCL and the FRIB project have over 500 employees, including more than 35 faculty members with joint appointments in MSU’s Departments of Physics and Astronomy, Chemis-try, and Electrical and Computer En-gineering. Presently, more than 140 students—approximately half of them doctoral students—are employed and educated at the laboratory.

NSCL maintains and operates two coupled superconducting cyclotrons, a high-acceptance superconducting fragment separator, a superconducting linear reaccelerator, and a diverse set of experimental apparatus. The CCF is capable of delivering a broad range of primary beams from hydrogen to uranium that are used for the in-flight production of secondary, rare isotope beams with energies up to nearly 170 MeV/nucleon. The in-flight technique allows for sub-microsecond isotope separation in a chemistry-independent way with short beam development times of a few hours to one day. The high beam energies provide efficient access to nuclei very close to the drip-

lines, both because thick targets can be used and because ions in mixed beams (“cocktail beams”) can be identified on an event-by-event basis. Rare isotopes produced with the in-flight technique can be stopped in and extracted from a He gas cell and subsequently used for precision ion trap or laser spectroscopy experiments at very low energy or for charge breeding and reacceleration with a state of the art superconducting linac dubbed ReAx where x denotes the maximum energy per nucleon of uranium ions that can be delivered by a particular linac section. A first experi-ment with reaccelerated beams from ReA1.5 has been conducted in August 2013. Beams from ReA3 will be avail-able for research in late 2014 and an upgrade to ReA6 is in the advanced design stage and may come on-line in 2015–2016 depending on funding.

The current layout of NSCL’s ex-perimental areas is shown in Figure 2. Major experimental apparatus includes the large-acceptance high-resolution S800 Spectrograph and the high-field Sweeper Magnet, the high-resolution array HiRA for charged-particle de-tection, high-resolution and high- efficiency γ-ray detection systems (the Segmented Germanium Array SeGA and the Caesium Iodide Array CAE-SAR, respectively), neutron detec-tion arrays suited for various energies (Modular Neutron Array—MoNA and its extension LISA, Neutron Emission Ration Observer—NERO, Low-Energy Neutron Detector Array—LENDA and the Neutron Walls), the Beta Counting System (BCS) as well as beta NMR/NQR setups, diamond timing detec-tors, the low-energy beam and ion (Penning) trap facility LEBIT, and the Figure 1. NSCL building complex on the campus of Michigan State University.

NSCL and the Facility for Rare Isotope Beams (FRIB) Project



beam-cooler and laser spectroscopy facility BECOLA. A superconducting linear re-accelerator (ReA3) and its dedicated experimental area, beam-lines, and apparatus are nearing com-pletion. For research with reaccelerated beams, an active-target time projection chamber (AT-TPC), a multi-purpose beam-line, a gas-jet target and a SEpa-rator for CApture Reactions (SECAR) are currently under construction or in the advanced planning stages. In the past year, NSCL hosted the advanced γ-ray tracking array GRETINA. After successful completion of an extended scientific campaign, GRETINA was moved to the ATLAS facility at ANL in the summer of 2013. The first ex-periment with reaccelerated rare iso-topes in the new ReA3 experimental hall at NSCL was performed in August 2013 with the Array for Nuclear As-trophysics Studies with Exotic Nuclei (ANASEN) built by Florida State Uni-versity and Louisiana State University.

Research at NSCL addresses impor-tant questions in basic nuclear physics, nuclear astrophysics, accelerator phys-ics, and associated instrumentation re-search and development. About 5−10%

of the beam time is allocated to cross-disciplinary and applied research. Beam time is approved by the NSCL director who is advised by a Program Advisory Committee (PAC) consisting of several internationally accomplished experts from other institutions.

DOE-SC and MSU signed the Co-operative Agreement to design and establish FRIB on 8 June 2009 (Fig-

ure 3). In September 2010, the project received Critical Decision 1 approval from the DOE-SC acquisition ex-ecutive. In August 2013, DOE-SC ap-proved a performance baseline of $730 M with an associated completion date in 2022 (Critical Decision 2). The Proj-ect is managed to an early completion in December 2020. Also approved was Critical Decision 3a, which allows the project to proceed with long-lead pro-curements. Commencement of the start of civil construction is subject to a Fis-cal Year 2014 appropriation.

FRIB will provide researchers op-portunities to study the properties of rare isotopes and to put this knowledge to use in various applications, including in materials science, nuclear medicine, and nuclear weapons stockpile stew-ardship. The research areas include:

• Nuclear Structure—What is the nature of the nuclear force that binds protons and neutrons into stable nuclei and rare isotopes? What are the limits of nuclear ex-istence?

• Nuclear Astrophysics—What is the nature of neutron stars and dense nuclear matter? What is the

Figure 2. Schematic layout of the NSCL facility. The experimental area utilizing reaccelerated beams from ReA6 is schematic with the final layout driven by user demand (see text for the status of equipment).

Figure 3. Architect’s rendering of the baselined Facility for Rare Isotope Beams (FRIB).



origin of elements heavier than iron in the Cosmos? What are the nuclear reactions that drive stars and stellar explosions?

• Tests of Fundamental Symme-tries—Why is there now more matter than antimatter in the Uni-verse?

• Application of Isotopes to Soci-ety—What are the potential uses in medicine, energy, material sci-ences, and national security?

The FRIB design provides for fast, stopped, and reaccelerated beams of rare isotopes. Features of the FRIB de-sign include:

• A state-of-the-art superconduct-ing-RF driver linear accelerator provides 400 kW for all beams with uranium accelerated to 200 MeV/nucleon and lighter ions to higher energies (protons up to 600 MeV).

• Space in the linac tunnel and shielding in the production area allows upgrading the driver linac energy to 400 MeV/nucleon for uranium and 1 GeV for protons without significant interruption of the future science program.

• A high-power in-flight produc-tion target and a three-stage high-acceptance, high-resolution fragment separator produce and deliver rare isotopes with high rates and high purity.

• Provisions have been made in the fragment separator to allow future implementation of isotope harvesting and the addition of (limited) multi-user capability.

• Space is available and provisions have been made in the facility de-sign to allow the addition of a sec-ond target facility, for example for ISOL beam production with pro-tons or light ions up to 400 kW.

• Three beam stopping stations—two gas stopping stations and

one solid stopper—will provide “stopped” beams with highest efficiency for precision experi-ments and for reacceleration.

• A superconducting-RF reaccel-erator will be able to provide beams up to 12 MeV/nucleon (uranium) and higher energies for lighter beams (e.g., 21 MeV/nucleon for 48Cr).

• Large experimental areas (47,000 sq. ft.) can accommodate new ex-perimental apparatus for science with stopped beams, reacceler-ated beams, and fast beams. The site as space available to double the size of experimental areas or for housing additional rare-iso-tope research facilities.

• A full set of well-tested experi-mental equipment is already in place for research in all FRIB sci-ence areas.

• Opportunity for a pre-FRIB sci-ence program using the existing in-flight separated beams from the Coupled Cyclotron Facility and the ReA3 reaccelerator. Us-ers will be able to mount and test equipment and techniques and do science with beams at all energies in-situ so that they are immedi-ately ready for experiments when FRIB is complete; this will allow for a continually evolving science program during the time FRIB is under construction, which will seamlessly merge into the re-search program at FRIB.

• A User Relations Office sup-ports ongoing research with the CCF and the development of new user programs and experimental equipment.

The final design of the FRIB con-ventional facilities—the tunnel and support buildings—is complete. Pre-construction site preparation is com-plete and pilings for the earth-retention

system have been placed. Research and development activities have been suc-cessfully completed, with much of the R&D work accomplished in collabora-tion with national laboratories. Final design of the technical systems—accel-erator and experimental equipment—is underway and anticipated to be sub-stantially complete in 2014.

For more information on the FRIB Project, see http://www.frib.msu.edu. The independent FRIB Users Organi-zation website is http://www.fribusers.org.

AlexAndrA GAde

C. KonrAd GelbKe

NSCL/FRIB Laboratory, Michigan State University, Michigan, USA

ThomAs GlAsmACher



meeting reports


The 26th International Nuclear Physics Conference (INPC) was held in Florence on 2–6 June 2013 (Figure 1), three years after the previous edi-tion of 2010 in Vancouver. The INPC is the main conference in the field of Nuclear Physics taking place every 3 years and is supported by IUPAP (International Union for Pure and Applied Physics), which also selects the venue among the proposed ones. For the organization of INPC2013 a substantial contribution was given by INFN (the Italian Institute of Nuclear Physics) and support was also pro-vided by three Universities—Univer-sità degli Studi di Firenze, Milano, and Padova—and several sponsors.

In the evening preceding the start-ing of the conference, a public lecture, on the activities of the LABEC labo-ratory in Florence concerning applica-tions of nuclear physics techniques for societal purposes, was given by Pier Andrea Mandò.

As in most past editions this confer-ence covered a wide spectrum of top-ics: (i) Nuclear structure and Nuclear reactions; (ii) Hot and dense nuclear matter; (iii) Fundamental symmetries and interactions in nuclei; (iv) Had-ron structure and Hadron in Nuclei; (v) Nuclear astrophysics; (vi) Neu-

trinos and nuclei; (vii) Nuclear phys-ics–based applications; and (viii) New facilities and instrumentation.

The conference was well attended with 730 participants (out of which 200 were women) from 54 different countries. Thanks to the work of the international advisory and program committees it was decided to invite several young colleagues. The main task of the plenary invited speakers was to illustrate the best theoretical and experimental achievements in the different nuclear physics topics that are impacting worldwide the present research programs in the field. This goal was well accomplished by the 33 speakers (out of which 8 were women). Their presentations were well orga-nized in terms of content and clarity and thus transmitted the main physics messages to a very mixed audience. This has triggered many unexpected links, discussions, or collaborations among participants and contributed to make the attendance of the paral-lel sessions very lively. Indeed, the afternoon parallel sessions, with 88 invited and 218 contributed talks, had an impressive participation. The talks of students and young researchers were to a large extent outstanding and in general manifested the interest and

the determination of the new genera-tion to contribute to the progress with experiments, theory, and associated technology and applications.

It is difficult to transmit in a few lines the richness of the program of this conference, but we would like to stress that the participation and the enthusiasm in discussing the physics results were remarkable. Therefore, the short description below is far from being exhaustive.

It was really impressive to see the rather large number of scientific high-lights, presented in the context of topi-cal overviews in plenary sessions and more in detail in parallel sessions as invited and selected contributions. In the sector of nuclear structure there are several highlights concerning the quest of shell evolution and new magic numbers (in particular the re-cent evidence in Ca isotope), of super-heavy elements, of collective modes and new shapes (including pear type), and of loosely bound light nuclei. Many new interesting experimental results benefited from the progress on traps and laser techniques, and on reaction and spectroscopy techniques. For the latter, gamma-ray detection resulted a key tool (e.g., the AGATA array). From the interpretation of the

INPC 2013: Florence, 2–6 June 2013

Figure 1. The conference group photo.

meeting reports


results, it is clear that theory is mak-ing particular efforts also to provide new predictions for few and many bodies in terms of ab initio models, three body forces, and energy density Functional methods and in particular cases also from lattice simulations. Some talks emphasized that nuclear reactions are essential tools to address nuclear structure questions but also to learn on more global properties (e.g., barriers and potentials) and on the nuclear equation of state, which are relevant for astrophysics problems. Nuclear astrophysics is presently blooming because of the availability now and more in the future of new radioactive beams, key tools to ad-dress extensively several basic ques-tions concerning nucleosynthesis. In addition, measurements at dedicated low energy accelerators on the stellar energy production will continue (e.g., LUNA). The discussed plans for major facilities for radioactive ions (GANIL, FAIR, RIKEN, TRUMF and FRIBS) and for smaller facilities (e.g., ALTO, ISOLDE, JYFL, SPES-LNL, and oth-ers) promise a very exciting future.

The progress on the structure of nucleons, its tomography, and on me-son productions is remarkable. It was pointed out that the present measure-ments provide the needed stringent tests to QCD in the non perturbative regime and that it will be important to continue the effort in this direction to learn on the strong force with the necessary detail. JLAB with 12 GeV beams, FAIR under construction, and the Electron-Ion collider project rep-resent this endeavor. The presented results on the properties of hadrons in nuclei mainly concerned strange-ness as produced with different probes including heavy ion collisions. Soon JPARC will make a major step in this direction. The highlights in the field of Hot and Dense QCD are related to the recent results of ALICE at LHC on

the production and decay of different particles characterizing the very hot hadronic matter, the quark degrees of freedom and deconfinement.

The comparison with RHIC results was discussed together with the major theoretical developments.

For the study of weak interactions the conference concentrated mainly on selected topics concerning neu-trino physics, particularly at nuclear reactors, neutrino interaction, and beta decay related to the testing of the CKM Unitarity matrix. Among the works on fundamental interactions it is worth mentioning the measurement, using nuclear physics methods, of the neutron electric dipole moment pro-viding a stringent test of the Standard Model.

The talks on applications and new instrumentation were in general very attractive. There was a particular curi-osity about the work made in Japan to monitor radioactivity after Fukushima because it was presented as a scientific report of nuclear physicists. A new fa-cility that, in the future, in addition to basic research with intense high reso-lution gamma beams, will play an im-portant role in applications is ELI_NP (ESFRI facility in Bucarest).

Excellent talks were given, in a dedicated plenary session, by the three winners of the IUPAP prizes, Rabia Burcu Cakirli (from MPI-Heidelberg) on mass measurements of exotic nu-clei, Stefano Gandolfi (from LANL) on ab initio calculations, and Bjorn Peter Schenke (BNL) on relativistic heavy ions (Figure 2).

We were very proud that EPS de-cided to deliver the IBA prize 2013 during INPC2013. It was a great plea-sure for all of us to applaud warmly the winner, Prof. Marco Durante from GSI.

INPC 2013 made a special effort to attract many graduate students and thus we had 106 presenting posters, plus 33 were selected for oral contri-butions. As in the previous edition, INPC 2013 teamed up with Nuclear Physics A to provide awards to the two best student oral presentations and five top poster presentations at the confer-ence. An international panel of judges together with members from the edito-rial board of Nuclear Physics A finally decided on the following award win-ners: (i) Ulrika Forsberg (University of Lund, Sweden) for the oral presen-tation “Spectroscopy of Element 115 decay chains”; (ii) Tadashi Hashimoto

Figure 2. The three winners of the IUPAP prize and the IUPAP chair and scien-tific secretary.

meeting reports


(University of Tokyo, Japan) for the oral presentation “A search for the K-pp bound state in the 3He(inflight-K-,n) reaction at J-PARC”; (iii) Esther Sabine Bönig (TU Darmstadt, Ger-many) for a poster “Quadrupole col-lectivity in neutron-rich Cd isotopes”; (iv) Michele Gelain (University of Padova, Italy) for a poster “Character-ization of a highly-segmented Silicon detector for the TRACE prototype”; (v) Andrej Herzan (University of Jyvaskyla, Finland) for “Spectroscopy of 193Bi”; (vi) Timothy John Hobbs (Indiana University, USA) for the poster “The nonperturbative charm

content of the nucleon”; (vii) Andrea Tsinganis (CERN and NTUA, Athens, Greece) for the poster “Measurement of the 242Pu(n,f) cross section at the CERN n_TOF facility.”

As a conclusive remark we would like to stress that by attending this conference one had a very positive im-pression on the field being very vital, healthy, and dynamic. Many young people are eagerly and enthusiastically working and are important actors in new experiments, theory, and facilities.

At the end of the conference IUPAP announced the selection of the host of the next INPC conference: it will be

held in 2016 in Adelaide, Australia. We look forward to discussing excit-ing progress there as well!

AngelA BrAcco University and INFN Milano

Pier AndreA MAndò University and INFN Firenze

cosiMo signorini University and INFN Padova

Baryons 2013, the thirteenth Inter-national Conference on the Structure of Baryons, was hosted by the School of Physics and Astronomy at the Uni-versity of Glasgow, Scotland. The conference continued a long series of triennial meetings, which started at

Duke University in 1970, to discuss ex-perimental and theoretical advances in our understanding of the properties of baryons, the essential building blocks of the atomic nucleus. The 2013 con-ference attracted 149 participants from 73 different institutions in 22 countries, spanning 6 continents (Figure 1). There were 26 plenary talks and 8 parallel session keynote talks given by experts in the field as well as 81 contributed talks and 4 poster presentations.

The conference took place immedi-ately following the 3rd CLAS12 Euro-pean Workshop, which was also held in Glasgow. Many delegates from the CLAS12 meeting stayed on to attend Baryons 2013 and this increased par-ticipation and interaction.

The conference opened with a plenary talk given by Volker Crede, Florida State University, on progress toward understanding baryon reso-nances. This introduced the theme of baryon spectroscopy and included reports on experimental photo- and

electro-pion production at CLAS (Jef-ferson Lab), CBELSA/TAPS (Bonn). The conference also heard of hadron-induced experiments with proton and deuteron beams at COSY (Jülich). Complementary studies using pion beams at J-PARC (Tokai) are expected to provide new information which will dramatically extend the precision and quantity of (p,2p) reaction data and lead to new constraints on coupled-channel effects.

Major progress in our theoretical understanding of nucleon resonances was reported through improvements in partial wave analysis treatments. Accounts of recent work using the MAID, SAID, Bonn-Gatchina, and Jülich codes were presented and com-pared. Also discussed were the effects of dynamic coupling of resonances to unbound continuum states.

Another major topic discussed was the spin and flavor structure of the nu-cleon, with accounts of current experi-mental and theoretical investigations

Baryons 2013: International Conference on the Structure of Baryons, Glasgow, 24–28 June 2013

meeting reports


to shed light on the so-called spin puzzle and plans to extend DIS and SIDIS measurements to higher values of Bjorken x at the upgraded 12-GeV electron beam at Jefferson Lab.

The advantages of using the AdS/QCD approach in a light-front wave-function formalism to describe baryon spectroscopy and hadronic form fac-tors was described by several speak-ers. Other theoretical talks explored the physics of dynamical chiral sym-metry breaking, sea quarks and how these relate to non-perturbative QCD. A great deal of work on lattice QCD has been carried out recently to de-termine the ground and excited state properties of baryons. New techniques and the availability of computing power are revolutionizing calculations and many baryon masses can now be described with reasonable precision.

Other topics discussed included discrepancies in recent proton radius studies discovered between recent

high-precision electron scattering measurements and data from muonic hydrogen Lamb-shift measurements, measurements to determine the Pro-ton’s weak charge and the search for dark photons at Jefferson Lab. Look-ing to the future the conference heard of plans for the EIC, a future electron-ion collider that will shed light on the role of sea-quarks and gluons in the structure and properties of baryons.

A public lecture on the “Isotopic Legacy of Frederick Soddy,” given by David Sanderson of the Scottish Universities Environmental Research Centre, provided a historical account of the first nuclear physics work at Glasgow University. Soddy worked in Glasgow on the chemical proper-ties of radioactive materials from 1904 to 1914 and introduced the term “isotope” in a Nature paper in Decem-ber 1913 [1]. It is said that the term “isotope” actually originated in the course of a dinner party discussion at

Glasgow University. Soddy was subse-quently awarded the 1921 Nobel Prize in Chemistry for his “contributions to our knowledge of the chemistry of radioactive substances, and his in-vestigations into the origin and nature of isotopes” (see http://www.nobel prize.org/nobel_prizes/chemistry/ laureates/). A modern analysis of sev-eral surviving radioactive samples stored in the “Soddy box” provided new insight into Soddy’s work at Glasgow. Investigations are currently being carried out to find safe ways of curating this historical material for fu-ture educational purposes.

The social program included a Whisky Tasting for those who arrived early and a Civic Reception hosted by Glasgow City Council at the City Chambers. The conference dinner, at the nearby Oran Mor restaurant, had a traditional Scottish theme, including Scotch broth and haggis. After-dinner entertainment was provided by the Hillhead High School Ceilidh Band.

Two conference prizes were awarded to Igor Senderovich, Arizona State University, and Karin Schoen-ning, Uppsala University, for the out-standing quality of their contributed talks.

Copies of presentations made at the conference are available at the confer-ence website: http://nuclear.gla.ac.uk/Baryons2013/.

AcknowledgmentsThe organizers thank the Scottish

Universities Physics Alliance (SUPA), Hamamatsu, Glasgow City Coun-cil, and Grant’s Whisky for generous sponsorship. The next Baryons con-ference will be hosted by Florida State University.

References1. F. Soddy, Nature 92 (1913).

i. J. douglAs MAcgregor

University of Glasgow, UK

Figure 1. Baryons 2013 delegates in front of the Glasgow conference venue.

meeting reports


The 33rd Mazurian Lakes Confer-ence on Physics was held at Piaski, Poland, on 1–7 September 2013. The history of Mazurian meetings dates back to 1968. This traditional confer-ence is now organized every two years by the University of Warsaw, the Na-tional Centre for Nuclear Research, and the Pro-Physica Foundation. Its goal is to bring together scientists to discuss the hottest topics in nuclear physics in an environment facilitating contacts between the participants stay-ing at a remote location (Figure 1).

Over 140 physicists from 17 coun-tries all over the world enjoyed lively discussions on the latest develop-ments in the fields of low-energy nu-clear physics, both experimental and theoretical. The scientific program included about 80 oral presentations. Each conference day started with a keynote lecture, followed by the in-vited and contributed talks. The num-ber of excellent contributed abstracts

was so large that a crowded poster session included the presentation and discussions of almost 50 posters.

The conference began with a talk given by M. Targowski (Mikołaj Ko-pernik University of Toruń, Poland) presenting mostly unknown aspects of Mikołaj Kopernik’s life. It included well-documented stories about Koper-nik’s investigations and conclusions, the search for Kopernik’s birthplace, the topic of his national origin and even some letters referring to his pri-vate life, somewhat more rich than might be expected for a 15th-century canon (church official). Evidently, rare cloudy nights in the Mazurian Region prevented the continuous ob-servation of shining stars by this great astronomer.

Scientific topics discussed at the conference ranged from nuclear structure to nuclear reactions, from nuclear astrophysics to the synthesis of new elements, from results from

just commissioned powerful detec-tor arrays to new facilities under construction and the applications of nuclear physics research. An intro-duction to the frontiers in low-energy nuclear physics made by W. Nazare-wicz was followed by a presentation of nuclear spectroscopy results dis-cussed with respect to the Standard Model (P. Butler and B. Blank). An impressive harvest of new isotopes and isomers discovered at RIKEN (Wako, Japan) was shared with the audience by T. Kubo, G. Lorusso, and P. Boutachkov, while GSI share of new isotopes identification was men-tioned by H. Geissel. First results of two gamma tracking “demonstrator” arrays, GRETINA and AGATA, were presented by C. Campbell and S. Le-oni. Several talks addressed the phys-ics of loosely bound and unbound states (M. Płoszajczak, I. Mukha, K. Kemper, M. Pfützner, and S. Or-rigo). Modern theoretical approaches

XXXIII Mazurian Lakes Conference on Physics: Frontiers in Nuclear Physics, Piaski, Poland, 1–7 September 2013

Figure 1. Conference participants at the shore of Lake Bełdany (photo: M. Zielinska).

meeting reports


to nuclear structure and reactions in-cluded the application of the energy density functional (J. Dobaczewski), three-body forces (S. Kistryn and G. Hagen) and isospin-mixing phe-nomena (W. Satuła). State-of-the-art nuclear astrophysics studies were presented by M. Wiescher, P. Woods, C. Deibel, K. Czerski, A. Caciolli, M. Mumpower, A. Tumino, and P. Descouvemont. A number of decay spectroscopy results relevant to our understanding of the structure of ex-otic nuclei contributed to the analy-sis of astrophysical processes in hot stars (C. Mazzocchi, R. Grzywacz, A. Jokinen, G. Lorusso, and S. Bottoni). The importance of today’s nuclear re-actions studies was explained by A. Bonnacorso, I. Martel, S. Kistryn, M. Mazzocco, K. Wimmer, A. Diaz-Tor-res, V. Goldberg, V. Pesudo-Fortes, J. Johansen, M. Bondi, and T. Cap. V. Zagrebayev’s talk offered a link be-tween reaction studies and the pro-duction of super-heavy elements. His talk was followed by the presenta-tions of K. Siwek-Wilczyńska and Z. Majka analyzing particular reactions leading to new super heavy nuclei. Theoretical aspects of the structure of the heaviest nuclei were discussed by M. Bender, A. Baran, and by J. Skalski. All facilities contributing to the discovery and spectroscopic studies of new chemical elements and super heavy isotopes beyond the recently named Z = 112 Coperni-cium were presented at the Mazurian Conference. Yu. Oganessian (Dubna) summarized recent campaigns us-ing radioactive actinide target ma-terials like the Oak Ridge made Z = 97 249Bk and intense 48Ca beams to observe the properties of the yet-unnamed elements 118, 117, 115, and 113, and their longer-lived daughter activities at the “Hot Fusion Island.” Plans for discoveries of even heavier atomic nuclei using a Z = 98 mixed-

Cf target from Oak Ridge, and the construction status of the new “SHE Factory” were shared with confer-ence participants. C. Düllmann, D. Ackermann, and M. Block (Darm-stadt) presented, among other results, the confirmation of the discovery of element 117, the search for new ele-ments 119 and 120, longer-term per-spectives and developments needed for the discoveries of new elements beyond 118, and the relevance of direct mass measurements and laser spectroscopy of the heaviest nuclei. K. Morimoto summarized nearly 600 days of an experimental campaign leading to the firm observation of the 278(113) isotope linked by several alpha decays to well known heavy nuclei on the nuclear mainland. K. Gregorich and D. Rudolph presented new detection setups for decay spec-troscopy of super heavy nuclei, now complemented by efficient gamma detector arrays. The spectroscopy of Z = 115 isotopes confirmed earlier reported discoveries and led to the observation of first g-ray and KX-ray signals in prompt coincidence with alpha decays—a first hint for Z-fin-gerprinting of nuclei at the “Hot Fu-sion Island” (D. Rudolph). In-beam gamma and electron spectroscopy of Z > 100 nuclei yielding the excited levels and allowing the deformation of these heavy isotopes to be deduced were summarized by R. D. Herzberg and T.-L. Khoo (the latter talk given partially in Polish). Coulomb excita-tion studies of not-as-heavy nuclei, using stable and radioactive beams, were presented by K. Hadyńska-Klęk, A. Trzcińska, M. Zielińska, and K. Wrzosek-Lipska as well as S. Leoni and P. Butler. A new tech-nique to investigate nano- and micro-second gamma-decaying isomers in neutron-rich nuclei was explained by W. Królas. Single-hole states in the doubly-magic 132Sn, namely the neu-

tron levels in 131Sn populated using a radioactive 130Sn beam at Oak Ridge were presented by A. Bey.

The long-term future of the Ma-zurian Conferences is justified by the number of talks presenting the status of new laboratories and powerful de-tector arrays. FAIR (H. Simon and H. Geissel), SPIRAL-2 (M. Lewitowicz), FRIB (G. Bollen), ARIEL (G. Hack-man), CARIBU (G. Savard), SPES (F. Gramena), SHE Factory (Yu. Oganes-sian), National Cyclotron Laboratory (B. Fornal), and ELI-NP (D. Balaban-ski) equipped with efficient spectrom-eters like S3 (A. Drouart) and detector arrays like the full versions of GRETA (C. Campbell) and AGATA (S. Leoni), VANDLE (R. Grzywacz), MTAS (A. Fijałkowska), RIB isomer-scope (W. Królas), traps and lasers (A. Jokinen, M. Block, and J. Papuga) will allow us to study even more exotic isotopes and processes involving these nuclei. These new and expensive construc-tions will continue to benefit the phys-ics community and our society in gen-eral. The latter may be deduced from several talks at the Mazurian Con-ference presenting the applications of nuclear physics. Hadron therapy (P. Olko and A. Biegun), radioiso-topes for diagnostics and therapy (R. Mikołajczak), medical imaging (L. Królicki) and biological response to radiation (U. Kaźmierczak) are evident outcomes of earlier nuclear physics developments and are help-ing thousands of patients every day. Nuclear power has a substantial and CO2-free contribution to the world en-ergy budget. The decay heat, not well studied as yet, plays an important, sometimes dramatic, role during the nuclear fuel cycle (A. Fijałkowska). New detector techniques may keep us safer from terrorist attacks and prevent smuggling of nuclear materials (K. Peräjärvi). Cultural Heritage research profits from non-destructive inspec-

meeting reports


tion of art objects offered by nuclear techniques (H.-E. Mahnke).

We should not forget the social as-pect of the conference. With the aid of

excellent weather, in between the lec-tures, the participants could continue scientific discussions while enjoying outdoor activities offered by the Piaski venue: kayaking, canoeing, cycling, and of course sailing with the tradi-tional Regatta, this year won by Juha Äystö and his “Team Finland” (Figure 2). Everybody enjoyed the vocal skills of several national teams at the camp-fire, in particular an unforgettable solo performances by Kosuke Morita and Magda Zielińska.

The conference was closed by the announcement of the best poster (A. Korgul) and by a comprehensive sum-mary made by A. Maj, which provided also a natural bridge to the next Pol-ish traditional conference to be held

in Zakopane in 2014. The TASCA’13 workshop chaired by Ch. Düllmann and A. Yakushev immediately fol-lowed the Mazurian Conference on 7 September at Piaski.

The 34th Mazurian Lakes Confer-ence on Physics will be held in Sep-tember 2015, not surprisingly in the Mazurian Lakes Region.

Chiara MazzoCChi and Krzysztof ruseK

University of Warsaw

Krzysztof ryKaCzewsKi

Oak Ridge National Laboratory

Figure 2. “Team Finland,” led by Juha Äystö—regatta winners (photo: M. Zielinska).

Latest News in Antiproton Physics Discussed at the LEAP 2013 Conference in Uppsala

Low-energy antiproton physics is an interdisciplinary field centered around the antimatter partner of the proton that ranges from particle, nu-clear, atomic and astrophysics to ap-plied physics. It confronts directly the symmetry between matter and anti-

matter and addresses many key ques-tions of contemporary research: What are the fundamental symmetries of nature and in which way are they vio-lated? Why is there basically no anti-matter in the universe? How does the strong interaction and its symmetries

shape the structure of hadrons? Does matter and antimatter respond in the same way to gravity?

The highly acclaimed synthesis and trapping of antihydrogen atoms at CERN’s Antiproton Decelerator (AD) provides unique opportunities to

Figure 1. Participants of the LEAP 2013 conference in Uppsala.

meeting reports


probe the fundamental laws and sym-metries. Satellite and balloon experi-ments are searching for cosmic anti-matter, the results of which could have profound implications on cosmology. Antiprotons will be used to study the properties and structures of atoms, nu-clei and hadrons at the upcoming Fa-cility for Antiproton and Ion Research (FAIR) in Darmstadt. These items were discussed at LEAP 2013, the 11th International Conference on Low Energy Antiproton Physics, that took place at the Angstrom Laboratory, Up-psala University, Sweden, during 11–15 June. The conference was jointly hosted by the Department of Physics and Astronomy and the Department of Chemistry of Uppsala University with Tord Johansson as chair and Pi-otr Froelich as co-chair. The confer-ence attracted nearly 100 participants (Figure 1) and featured more than 20 invited plenary speakers, more than 20 contributed talks and a dozen posters with an emphasis on promoting young researchers. Some of the highlights of the packed program are presented in the following.

Symmetries and AntihydrogenThe conference began with a ses-

sion on symmetries covering theo-retical and experimental overviews of testing fundamental symmetries. At the center of this is the CPT sym-metry, the combined action of chang-ing particles and antiparticles (charge conjugation, C), parity transformation (P), and time reversal (T). A violation of this symmetry would have severe consequences for our understanding of the laws of nature since invari-ance under CPT emerges directly from quantum field theory and Lo-rentz invariance. Several approaches to test this symmetry at the AD were presented during the conference. Es-pecially encouraging were the reports on the rapid progress in trapping and cooling antihydrogen atoms that al-lowed first experiments on the road

toward tests of the CPT-symmetry in the domain of (anti)atomic physics. The ALPHA collaboration reported on the first measurement of hyperfine transition in ground state antihydro-gen and reviewed their future plans. The ATRAP collaboration reported on a large improvement of the measured value of the antiproton magnetic mo-ment, as well as their latest progress in antihydrogen formation using a double charge exchange process. The ASACUSA collaboration reported on the two-photon laser spectroscopy measurements on antiprotonic helium that has yielded the to date the most precise measurement of the antipro-ton-electron mass ratio to date. They also reviewed their approach toward the hyperfine spectroscopy of antihy-drogen. The Baryon Antibaryon Sym-metry Experiment (BASE) presented their intent to make a test of CPT in-variance by a precise comparison be-tween the proton and antiproton mag-netic moment.

Antimatter in the UniverseThe constraints on the presence

of antimatter in the Universe were reviewed. An excess of antiparticles, compared to expectations in the cos-mic radiation could be a signal from dark matter particles. The PAMELA satellite-borne experiment presented their results on the positron and an-tiproton flux in the cosmic radiation. The Alpha Magnetic Spectrometer Ex-periment (AMS) at the International Space Station presented high statistics result on positron and electron fluxes up to 350 GeV. The positron fraction in the data shows no fine structure or anisotropy but it seems as a large por-tion of the high-energy electrons and positrons originate from an unknown common source.

Gravity and AntimatterThe gravitational interaction of

antimatter with matter was discussed

both from the theoretical and experi-mental sides. An antiapple is expected to fall down on Earth exactly as an ordinary apple according to Einstein´s Weak Equivalence Principle (WEP). This principle has, however, never been tested experimentally for anti-matter and, as pointed out at the con-ference, there is room for a difference between matter-matter and antimat-ter-matter gravitational interaction within certain theoretical frameworks. The result from the TRAP collabora-tion on a gravitational redshift limit from simultaneously trapped protons and antiprotons was presented as a stringent test of the difference in the gravitational constant for matter and antimatter. The AEgIS and GBAR col-laborations presented their approaches toward a measurement of the gravi-tational acceleration of antihydrogen at the AD. The ALPHA collaboration presented the first measurement on the effect of gravity on an ensemble of antihydrogen atoms in a trap. The error bars, in this pioneering test, are too large to draw any definite conclu-sions, but the first experimental step has been taken.

Hadron PhysicsAntiprotons are also excellent tools

for hadron physics, as already proven at the LEAR facility at CERN. The gluon rich antiproton-proton annihi-lation makes these reactions ideal to search for exotic hadrons with gluon content, i.e., glueballs (hadrons con-sisting only of gluons) and hybrids (hadrons having a gluon component). The FAIR facility will open up a new era in this field. The hadron physics programme of the PANDA collabo-ration at FAIR, that will address this topic, was presented. Charmed me-son spectroscopy is also on its pro-gram. This is a hot topic, which was reviewed during the conference, due to recently discovered narrow X,Y,Z charmonium-like mesons, some of which cannot be explained as being


meeting reports

ordinary quark-antiquark configura-tions. Other topics that were covered both from theoretical and experimen-tal points of view were double lambda hypernuclei and nucleon structure.

Facilities and InstrumentationThe near-future upgrades for anti-

atomic physics were discussed, such as the construction of the Extra Low Energy Antiproton ring (ELENA) at the AD that will lower the energy of the antiprotons and thereby increase the number of useful antiprotons for stop experiments by up to two orders of magnitude.

The transfer of the storage ring CRYRING from Stockholm will al-low for an early start of physics with low energy antiptrotons at FAIR. Many developments toward a more efficient production of antihydrogen as well as the instrumentation of the PANDA detector were discussed.

ApplicationsTalks on applications and new

techniques with antiprotons included the ACE experiment at the AD, which is studying the possible use of antipro-tons for cancer therapy, and develop-ments toward spin-polarized antipro-tons and antihydrogen.

The conference included a boat trip to the 17th century renaissance castle Skokloster. At the conference dinner Prof. em. Gösta Ekspong gave a cel-ebrated exposé of his firsthand memo-ries of the discovery of the antiproton at the Bevatron in Berkley. LEAP 2013 ended with two well-attended public lectures that presented the role of symmetries in physics, both from the experimental and theoretical per-spective. Gerard Gabrielse, Harvard University, talked about “Cold mat-ter and antimatter—how similar are they?” and Ulf Danielsson, Uppsala University, “Mirror, mirror on the

wall--the beauty of the universe and its symmetries.” The next LEAP meet-ing is planned for Kanazawa, Japan, in 2016, and will be chaired by Yasunori Yamazaki.

For full details of the speakers and the presentations, see http://www.physics.uu.se/leap2013. The proceed-ings will be published in Hyperfine Interactions.

tord Johansson

Uppsala University

In the Laboratory Portrait “The Radioactive Ion Beams in Brazil (RIBRAS) Facility” by A. Lépine-Szily, R. Lichtenhäler, and V. Guimarães in Nuclear Physics News 23(3), pages 5–11, Figure 1 was printed incorrectly. The figure is reprinted below.

Figure 1. The experimental set-up, from the left, with the production target, the W beam stopper, the first solenoid followed by the intermediate scattering chambers, with the secondary target and detectors installed in it, followed by the second solenoid and the large scattering chamber.

Correction

news and views


Since 1991, in the Gran Sasso underground Laboratory [1], in be-tween the three main experimental halls where large collaborations run experiments on neutrinos and dark matter, a relatively small collabora-tion has been performing the LUNA experiment [2]. LUNA is an acronym for “Laboratory for Underground Nuclear Astrophysics”: in this labo-ratory, the LUNA collaboration was able to reproduce a few thermonu-clear reactions usually taking place inside stars [3, 4]. But why go un-derground to perform these measure-ments? For a nuclear reaction be-tween charged particles to occur in a star, a large number of interacting nu-clei has to be available and the Cou-lomb repulsion has to be overcome. The convolution of these two effects make most of the reactions occuring in the so-called Gamow peak, an en-ergetic region located well below the Coulomb barrier. The probability of the reaction, its cross-section, expo-nentially decreases with the interac-tion energy due to the tunneling of the Coulomb barrier and therefore, in stellar environments, turns out to be extremely low. Of course, when trying to reproduce the reaction in a laboratory, with the typical beam cur-rents and target densities achievable, the low cross-section translates into a low reaction rate that could be of the order of a few events per month or even lower. Therefore, it is man-datory to reduce as much as possible any background component in order to be able to detect such feeble sig-nal. The rock overburden above the Gran Sasso Laboratory decreases by more than six orders of magnitude

the natural muon flux and by three orders of magnitude the neutron one, making the Lab an excellent loca-tion to measure such nuclear reac-tion cross-sections at typical stellar energies, maximizing the signal to background ratio. So far, the LUNA collaboration has been engaged in measuring reactions belonging to the Hydrogen burning, which has the very important task of transforming four protons into Helium with a net energy release, and of the Big Bang Nucleosynthesis, responsible for the formation of the lightest elements in the early Universe. Two different ac-celerators have been used: a 50 kV “homemade” machine and a 400 kV commercial one, both able to deliver intense beams, with long-term stabil-ity and precise energy determination. The cross-sections already measured have important consequences for neutrino physics, element nucleosyn-thesis and cosmology, and constitute important ingredients in stellar mod-els. In order to make a step forward and be able to measure reactions be-longing to the Helium burning that are important at higher temperatures in stars that ultimately means larger interaction energies, a higher voltage accelerator is necessary. For this rea-son, the LUNA collaboration is now involved also in a new adventure, the LUNA-MV project, which foresees the installation of a 3.5 MV machine in the Gran Sasso Underground Labo-ratory. In particular, the experimental program foresees the measurement of the 12C(a,g)16O, the 13C(a,n)16O, and the 22Ne(a,n)25Mg reactions. The first determines the time scale of Helium burning and the abundances

of Carbon and Oxygen at its end, with important consequences on stel-lar evolution and further nucleosyn-thesis processes. The (a,n) reactions on 13C and 22Ne instead provide the neutron flux necessary for the slow neutron capture process (s-process) responsible for the formation of the heavy elements. All three of these reactions have already been deeply investigated in laboratories at the Earth’s surface, but the experimental status of the art is still not satisfac-tory and a significant step forward could only come by a measurement performed in a deep underground lab-oratory. The LUNA-MV project has been funded with about 5.5 million Euros by the Italian Research Minis-try in the framework of the so-called “Progetti Premiali” funds. With such money, we will be able to buy a com-mercial 3.5 MV machine equipped with two beam-lines and to develop proper neutron and gamma detec-tion systems. The enterprise has now started and a new, larger collabora-tion is growing up to perform this ex-citing and long-lasting experimental program. Stay tuned!

References1. http://www.lngs.infn.it2. http://luna.lngs.infn.it3. H. Costantini et al., Rep. on Prog. in

Phys. 72 (2009) 086301. 4. C. Broggini et al., Ann. Rev. of Nucl.

and Part. Sci. 60 (2009) 53.

AlessAndrA GuGlielmetti

Università degli Studi di Milano and INFN Milano

The LUNA-MV Project at Gran Sasso Underground Laboratory

calendar

2014May 12–16

Ischia, Italy. 11th International Spring Seminar on Nuclear Physics

http://ischia2014.na.infn.it/index.php

May 25–30San Antonio, TX, USA. CAARI

2014 http://www.caari.com/

May 27–30Nis, Serbia. Second International

Conference on Radiation and Do-simetry in Various Fields of Re-search (RAD 2014)

http://www.rad2014.elfak.rs/welcome.php

May 29–June 3Cracow, Poland. MESON2014http://meson.if.uj.edu.pl/

June 1–6Tokyo, Japan. ARIS2014http://ribf.riken.jp/ARIS2014/

June 9–13Kyiv, Ukraine. 5th International

Conference on Current Problems in Nuclear Physics and Atomic Energy (NPAE-Kyiv2014)

http://www.kinr.kiev.ua/ NPAE-Kyiv2014/

June 30–July 4Darmstadt, Germany. Direct

Reactions with Exotic Beams DREB2014

https://indico.gsi.de/conferenceDisplay.py?confId=2347

July 7–11Debrecen, Hungary. Nuclei in the

Cosmos NIC14http://www.nic2014.org/

July 20–25Vancouver, Canada. Nuclear

Structure 2014http://ns2014.triumf.ca/

August 31–September 7Zakopane, Poland. Zakopane

Conference on Nuclear Physics “Ex-tremes of the Nuclear Landscape”

http://zakopane2014.ifj.edu.pl/

September 8–13Kaliningrad, Russia. VII Inter-

national Symposium on Exotic Nu-clei (EXON-2014)

http://exon2014.jinr.ru/

September 15–19Wien, Austria. EXA2014 Inter-

national Conference on Exotic At-oms and Related Topics

http://www.oeaw.ac.at/smi/research/talks-events/exotic-atoms/exa-14/

September 15–October 10Stockholm, Sweden. Computa-

tional Challenges in Nuclear and Many-Body Physics

http://agenda.albanova.se/conferenceDisplay.py?confId=3987

September 16–24Erice, Italy. Nuclei in the Labo-

ratory and in the Cosmoshttp://crunch.ikp.physik.tu-

darmstadt.de/erice/2014/index.php

September 21–26Canberra, Australia. 5th Joint

International Conference on Hyper-fine Interactions and Symposium on Nuclear Quadrupole Interactions (HFI/NQI 2014)

http://www.hfinqi.consec.com.au/

September 29–October 3St. Goar, Germany. 9th Interna-

tional Conference on Nuclear Phys-ics at Storage Rings STORI’14

http://web-docs.gsi.de/~stori14/

October 13–17Worms, Germany. EPS Nuclear

Physics Divisional Conference “Sci-ence and Technology for FAIR 2014”

https://indico.gsi.de/conferenceDisplay.py?confId=2443

November 3–8Ho Chi Minh City, Vietnam. In-

ternational Symposium on Physics of Unstable Nuclei 2014 (ISPUN14)

http://www.inst.gov.vn/ispun14/

2015June 21–26

Catania, Italy. 12th International Conference on Nucleus-Nucleus Collisions (NN2015)

http://www.lns.infn.it/link/nn2015

September 14–19Kraków, Poland. 5th Interna-

tional Conference on “Collective Motion in Nuclei under Extreme Conditions” (COMEX5)

http://comex5.ifj.edu.pl/

More information available in the Calendar of Events on the NuPECC website: http://www.nupecc.org/

Vol. 24 No. 1

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