Science and Production of Exotic Nuclei

NAS - Board of Physics and Astronomy

R. F. CastenYale

April 22, 2006

Themes and challenges of Modern Science•Complexity out of simplicity

How the world, with all its apparent complexity and diversity, can be constructed out of a few elementary building blocks and theirinteractions

•Simplicity out of complexityHow the world of complex systems can display such remarkable regularity and simplicity

•Understanding the nature of the physical universe

•Manipulating nature for the benefit of mankindNuclei: Two-fluid, many-body, strongly-interacting, quantal systems

provide laboratories for frontier research in all four areas

0 10000

50000

100000

150000

200000

coun

ts

energy (keV)

Astonishing simplicity in a complex many-body objectSimplicity out of complexity.

J + 8

J + 6

JJ + 2J + 4

Protons, neutrons — fermionsj = half-integer (orbital + intrinsic)

Pauli Principle: At most 2j + 1particles in a given orbit

Phonons — bosons

Two views of nuclear structureSingle-particle motion Bulk collective motion

Single-particle excitations Macroscopic shapewith residual interactions of nuclear matter

The New Frontiersof Physics with Exotic Nuclei

We can customize our system – fabricate “any” nucleus (designer nuclei) controlling the number of constituent protons and neutrons to isolate and amplify specific physics or interactions

Four Frontiers

1. Proton Rich Nuclei

2. Neutron Rich Nuclei

3. Heaviest Nuclei

4. Evolution of structure within these boundaries

Terra incognita — huge gene pool of nuclei

Scope of RIA ScienceScope of RIA ScienceThe scientific questions that RIA can address are crucial for our understanding of the universe, and are a link to our abilityto explain natural phenomena that range over distance scales spanning 42 orders of magnitude—from the proton (10–15 m) to the whole of the universe (1027 m).

Just as nuclei themselves play essential roles in the cosmos, the conceptual techniques of nuclear science have close links with those of quantum many-body physics on the nanoscale and, hence, are important in understanding the quantum world.

Moreover, nuclei are the interface between QCD and the fundamental forces and particles in nature on the one hand and the atomic and macroscopic world on the other.

Nuclear Structure and Nuclear Astrophysics

• What binds protons and neutrons into stable nuclei and rare isotopes?

• How does structure evolve with proton and neutron number

• What is the origin of simple patterns in complex nuclei?

• Where and how did the elements from iron to uranium originate?

• What causes stars to explode?

r = |ri - rj|

⇒Vij

r

Ui

Microscopy, Concept of "mean field"

Clusters of levels shell structure

Pauli Principle (≤ 2j+1 nucleons in orbit with angular momentum j) magic numbers, inert cores

Concept of valence nucleons – key to structure. Many-body few-body: each body counts. Addition of 2 neutrons in a nucleus with 150 can drastically alter structure

Ψ = Ψnl , E = EnlH.O. E = ħω (2n+l)

E (n,l) = E (n-1, l+2)E (2s) = E (1d)

1 12 4 2( ; )B E + +→

1000

400

1 12 2 0( ; )B E

4+

2+

0 0+

E (keV) Jπ

Simple Observables

+ +→

212 2

2 1( ; )i f i f

i

B E J J EJ

→ Ψ Ψ+

≡

R4/2

Classifying Structure Classifying Structure ---- The Symmetry TriangleThe Symmetry Triangle

Sph.

Deformed

Dynamical Symmetries, Phase/shape Transitions

Benchmarks

Sph.

Deformed

86 88 90 92 94 96 98 100

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

Nd Sm Gd Dy

R4/

2

N

Phase Transitions in Atomic Nuclei?

ord

er p

aram

eter

control parametercritical point

2

21 0;ξξ ξ⎡ ⎤′

′′ + + − =⎢ ⎥⎢ ⎥⎣ ⎦

%% %v

z z

X(5)Bessel equation

( ) 0.ξ β =%w

( ) 1/21 93 4+⎛ ⎞

= +⎜ ⎟⎝ ⎠

L Lv

Critical Point SymmetriesFirst Order Phase Transition – Phase Coexistence

E E

β

1 2

3

4

ββ

Energy surface changes with valence nucleon number

Iachello

First X(5) Region

Loci of P~5

−=

+p n

p n

N N p nPN N pairing

Competition between spherical-driving pairing interaction and deformation-driving p-n

interaction

Pcrit ~ 5~

Challenge to microscopy:

Why these symmetries?

In which nuclei?

Why in specific nuclei?

New symmetries in exotic nuclei?

New Features in Weakly Bound NucleiNew Features in Weakly Bound Nuclei

0 10 20

New form of matter – low density, diffuse, spatially extended, nearly pure neutron matter

Density(log)

Radius (fm)

p-ncore

n-skin

Halo Nuclei

11Li

Normal nuclear density

V (r)

r

Spatially extended wave functions

V (r)

r

Diffuse

Normal potentialNew Magic Numbers, Altered Mean Field, Shell Structure

Change in Shell Structure?Change in Shell Structure?(Reduction of spin(Reduction of spin--orbit interaction)orbit interaction)

QUESTION:Are there major new shell gapsdeveloping in the neutron-rich region, that could have major implications for structure andnucleosynthesis?

METHOD:Proton-adding reactions on Snisotopes studied with a new solenoid spectrometer

EXAMPLE:138Sn(α,t)139Sb4He target ~ 50μg/cm2

104 particles/s12 MeV/u beam5 mb/sr over at least 1 sr:~300 cts/wk for each state

Extrapolation of observed trend

Pairing Correlations

• Microscopic origin• Dependence of the range of the force on

the proton and neutron densities• Dependence on the surface

Questions one hopes to answer:

Breakdown of BCS pairing?Breakdown of BCS pairing?

QUESTION:Does BCS pairing, whichconcentrates the L=0 strength in the ground state,break down in neutron-rich nuclei?

METHOD:Neutron-pair transferon Sn isotopes studied with a new solenoid spectrometer

EXAMPLE:138Sn(t,p)140SnTritium target ~ 50μg/cm2

104 particles/s20 MeV/u beam0.5 mb/sr over at least 1 sr:~30 cts/wk for each state

In 138Sn(t,p) will it be like this withcontinued BCS pair correlations as in other Sn isotopes?

- or like this with disappearing of BCS correlations?

Transfer reactions with post-accelerated radioactive ion beams

Z = 82

N = 126188Po 189Po 191Po

N = 104 (midshell)

• Two proton transfer reaction on Hg isotopes to probe the π(2p-2h) component

184Hg

186Pb3He(184Hg(T1/2=31 s),n)186Pb at 10 MeV/u• σ ∼ 50 μbarn (cfr. 204Hg(3He,n)206Pb: R.E. Anderson et al., PRC19 (1979) 2138• 107 particles per second• gas cell at 5 bar/ 1 cm: 6 1019 at/cm2

• 100 reactions/hour

• One neutron transfer reaction on e.g. Pb isotopes

190Pb(d,p)191Pb191Po(d,p)192Po185Hg(d,p)186Hg

oddHg evenHg

oblateprolate

0+

2+

4+

oblate

prolate

Spectroscopic factors

“Classic” Near-Barrier Coulomb Excitationwith reaccelerated beams

NEW PHYSICS

Modification of nuclear structure due to neutron excess

Impact of single particle states and gaps

New collective modes, Influence of weak pairing

Advantages

Very sensitive to shapes and transitions (diagonal and off diagonal matrix elements)

Flexible: Multi-step to study structural evolutionSingle-Step to study strength functions

Clean...at sub-barrier energy it is the only mechanism

Precise...an exact theory

Efficient

Coulomb Excitation(with low intensity beams)

Take existing data set from beam Coulex of 138Ce on 700 μg/cm2 12C with Gammasphere.

Rescale 1pna for 14hrs to various scenarios:

105 p.p.s for 5 days

104 p.ps for 5 days

103 p.p.s for 5 days

Even at 100 particles per second spectroscopy is possible at least for first excited state.

103

104

105

1pna

SuperSuper--Heavy Elements StudiesHeavy Elements Studies

Z > 114, n-rich: Where does the periodic table end?

What is the shell structure at the highest Z’s?(what is the right theoretical description)

What are the properties of the heaviest elements? (stability, mass, decay modes,..)

Rare isotopes may well be the only way to reach the island of super-heavy elements

σ(fusion) ~ 1nb – 1pb (who knows for n-rich beams?)

σ(fusion) sharply peaked reaccelerated beams at precise

energyPossible with intense n-rich beams

90,92Kr, 90,92Sr,.. (>1011/s) 1 atom/week

Predictions

neutrons

protons

rppr

ocess

rppr

ocess

Crust proces

ses

Crust proces

sesn-Star

KS 1731-260

s-process

ss--process

process

r processr processr process

stell

ar burnin

g

stell

ar burnin

g

Mass knownHalf-life knownnothing known

RIA intensities (nuc/s)> 1012

1010

106

102

10-2

10-6

p process

p process

p process

Supernova

E0102-72.3

How does the physics of nuclei impact the physical universe?

Time (s)

331

330

329

328

327

Freq

u en c

y ( H

z)

10 15 20

4U1728-34

Nova

T Pyxidis

Masses and drip linesNuclear reaction ratesWeak decay ratesElectron capture ratesNeutrino interactionsEquation of StateFission processes

Nuclear Input(experiment and theory)

X-ray burst

• What is the origin of elements heavier than iron?• How do stars burn and explode?• What is the nucleonic structure of neutron stars?

Applications• United States leadership in nuclear science is vital to

the nation's well-being as well. RIA will have profound benefits to society; it will play an important role in the 21st Century's advances in modern technology, medicine, the environment, and national security.

• The pursuit of the scientific opportunities that drive RIA will enhance the training of the next generation of nuclear scientists. This field provides a superb venue to educate those who will seek to exploit nuclei for the benefit of humankind and the security of our nation.

RIA Discovery Potential, “Spin-offs”

• Comprehensive nuclear theory• Reaching the limits of nuclear binding• Discovery/study of exotic nuclear topologies• Discovery of new structural symmetries• Study of phases of nuclei and nuclear matter• Crucial ingredients for astrophysics• Tests of fundamental symmetries• Unforeseen Discoveries

• Applications to medicine, national security, …• Training the next generation of scientists who know and

can exploit the atomic nucleus

Exotic Nuclei and RIA

• It is the overall consensus of the international nuclear structure and nuclear astrophysics communities that the future of the study ofatomic nuclei requires advanced facilities for access to nuclei far from the valley of stability.

• Discovery potential to produce a paradigm change that will transform nuclear structure and astrophysics like atomic physicswas changed by the laser or condensed matter physics by the transistor.

• The aim is not to study all newly available species, but to use this expanded gene pool of exotic nuclei to select those that isolate or amplify specific physics.

Rare Isotope Science

History of the concept: A selection of highlights: A selection of highlights

• 1980’s – Early experiments with exotic nuclei

• 1991 – LRP – Advanced radioactive beam facility cited as a possible

future initiative

• 1996 – LRP - ISL top priority for new construction upon completion of

RHIC

• 2002 – LRP - RIA as top priority for major new construction

• 2002-5 – Seven NSAC reports reaffirm support for RIA

• 2003 – DOE 20 Year Facilities Plan -- Strategic Plan – Tied for Third

LRP 2002: RECOMMENDATION 2LRP 2002: RECOMMENDATION 2

The Rare Isotope Accelerator (RIA) is our highest priority for major new construction. RIA will be the world-leading facility for research in nuclear structure and nuclear astrophysics.

The exciting new scientific opportunities offered by research with rare isotopes are compelling. RIA is required to exploit these opportunities and to ensure world leadership in these areas of nuclear science.

Facilities for the Future of Science Facilities for the Future of Science –– A Twenty Year Outlook A Twenty Year Outlook (2003)(2003)

RARE ISOTOPE SCIENCE ASSESSMENTStatement of Task

The committee will define a scientific agenda for a U.S. domestic rare-isotope facility, taking into account current government plans.

***** The committee will carry out a thorough independent assessment of the importance to the nation of the science agenda for the Rare Isotope Accelerator. ******In preparing its report, the committee will address the role that such a facility could play in the

future of nuclear physics, considering the field broadly, but placing emphasis on its potential scientific impact on nuclear structure, nuclear astrophysics, fundamental symmetries, stockpile stewardship and other national security areas, and future availability of scientific and technical personnel. The need for such a facility will be addressed in the context of international efforts in this area.

In particular, the committee will address the following questions:

• What science should be addressed by a rare isotope facility and what is its importance in the overall context of research in nuclear physics and physics in general?

• What are the capabilities of other facilities, existing and planned, domestic and abroad, to address the science agenda? What scientific role could be played by a domestic rare-isotope facility that is complementary to existing and planned facilities at home and elsewhere?

• What are the benefits to other fields of science and to society of establishing such a facility in the United States?

Summary

Exotic Nuclei Complexity – Simplicity

Links to nano-science, high energy physics, and the cosmos

Paradigm-Changing Discovery Potential

Comprehensive Understanding of Atomic Nuclei

Applications

Backups

Approaches to Nuclear StructureMicroscopic – Approximate solutions to real nuclei

• Ab initio, No core, Monte Carlo• Effective Interactions, Many degrees of freedom• Density Functional Theory

Numerically intensive. Revolutionary advances → enhanced ability to predict wide variety of nuclei → promise of a comprehensive theory.

Macroscopic – Exact solutions to ideal nuclei

Many-body symmetries. Few degrees of freedom. Simple patterns, quantum numbers, selection rules, phases.

• Analytic, intuitive understanding -- WHAT symmetries WHERE?

Challenge to microscopy – Why THESE symmetries: In which nuclei: Why in THESE nuclei?

π ν

82

50 82

126

Z ≤ 82 , N < 126

11

Z ≤ 82 , N < 126

1 2

Z > 82 , N < 126

2

3

3

Z > 82 , N > 126