Research Article The M ajorana emonstrator Neutrinoless ...downloads.hindawi.com › journals ›...

Research ArticleThe Majorana Demonstrator NeutrinolessDouble-Beta Decay Experiment

N Abgrall1 E Aguayo2 F T Avignone III34 A S Barabash5 F E Bertrand4 M Boswell6

V Brudanin7 M Busch89 A S Caldwell10 Y-D Chan1 C D Christofferson10

D C Combs811 J A Detwiler12 P J Doe12 Yu Efremenko13 V Egorov7 H Ejiri14

S R Elliott6 J Esterline89 J E Fast2 P Finnerty815 F M Fraenkle815

A Galindo-Uribarri4 G K Giovanetti815 J Goett6 M P Green815 J Gruszko12

V E Guiseppe16 K Gusev7 A L Hallin17 R Hazama14 A Hegai1 R Henning815

E W Hoppe2 S Howard10 M A Howe815 K J Keeter18 M F Kidd19 A Knecht12

O Kochetov7 S I Konovalov5 R T Kouzes2 B D LaFerriere2 J Leon12

L E Leviner811 J C Loach120 P N Luke21 S MacMullin815 R D Martin1 S Mertens1

L Mizouni23 M Nomachi14 J L Orrell2 C OrsquoShaughnessy815 N R Overman2

David Phillips811 A W P Poon1 K Pushkin16 D C Radford4 K Rielage6

R G H Robertson12 M C Ronquest6 A G Schubert12 B Shanks815 T Shima14

M Shirchenko7 K J Snavely815 N Snyder16 D Steele6 J Strain815 A M Suriano10

J Thompson17 V Timkin7 W Tornow89 R L Varner4 S Vasilyev13 K Vetter1

K Vorren815 B R White4 J F Wilkerson4815 T Williams4 W Xu6 E Yakushev7

A R Young811 C-H Yu4 and V Yumatov5

1 Nuclear Science Division Lawrence Berkeley National Laboratory Berkeley CA 94720 USA2 Pacific Northwest National Laboratory Richland WA 99352 USA3Department of Physics and Astronomy University of South Carolina Columbia SC 29208 USA4Oak Ridge National Laboratory Oak Ridge TN 37831 USA5 Institute for Theoretical and Experimental Physics Moscow 117218 Russia6 Los Alamos National Laboratory Los Alamos NM 87545 USA7 Joint Institute for Nuclear Research Dubna 141980 Russia8 Triangle Universities Nuclear Laboratory Durham NC 27708 USA9Department of Physics Duke University Durham NC 27708 USA10South Dakota School of Mines and Technology Rapid City SD 57701 USA11Department of Physics North Carolina State University Raleigh NC 27695 USA12Center for Experimental Nuclear Physics and Astrophysics and Department of Physics University of WashingtonSeattle WA 98195 USA

13Department of Physics and Astronomy University of Tennessee Knoxville TN 37996 USA14Research Center for Nuclear Physics and Department of Physics Osaka UniversityIbaraki Osaka 567-0047 Japan

15Department of Physics and Astronomy University of North Carolina Chapel HillNC 27599 USA

16Department of Physics University of South Dakota Vermillion SD 57069 USA17Centre for Particle Physics University of Alberta Edmonton AB Canada T6G 2G718Department of Physics Black Hills State University Spearfish SD 57799 USA19Tennessee Tech University Cookeville TN 38505 USA20Shanghai Jiaotong University Shanghai 200240 China21Engineering Division Lawrence Berkeley National Laboratory BerkeleyCA 94720 USA

Hindawi Publishing CorporationAdvances in High Energy PhysicsVolume 2014 Article ID 365432 18 pageshttpdxdoiorg1011552014365432

2 Advances in High Energy Physics

Correspondence should be addressed to S R Elliott elliottslanlgov

Received 28 June 2013 Accepted 7 August 2013 Published 29 January 2014

Academic Editor Vincenzo Flaminio

Copyright copy 2014 N Abgrall et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited Thepublication of this article was funded by SCOAP3

The Majorana Demonstrator will search for the neutrinoless double-beta (120573120573(0])) decay of the isotope 76Ge with a mixedarray of enriched and natural germanium detectors The observation of this rare decay would indicate that the neutrino is its ownantiparticle demonstrate that lepton number is not conserved and provide information on the absolute mass scale of the neutrinoTheDemonstrator is being assembled at the 4850-foot level of the SanfordUnderground Research Facility in Lead SouthDakotaThe array will be situated in a low-background environment and surrounded by passive and active shielding Here we describe thescience goals of the Demonstrator and the details of its design

1 Introduction

11 Neutrinoless Double-Beta Decay Despite being discov-ered well over a decade ago [1ndash3] the incorporation of neu-trino mass and mixing into the standard model (SM) of par-ticle physics remains elusive A minimalistic Higgs couplingleaves the SM fine-tuned with the neutrino masses lyingsome 6 orders-of-magnitude or more below that of theother SM leptons Avoiding such unnaturalness requires newphysics One highly attractive option afforded by the electricneutrality of the neutrino is the addition of a lepton-number-violating Majorana mass term [4] Majorana neutrinos havethe novel property that particle and antiparticle are distin-guished only by chirality A Majorana mass term provides anatural explanation for the lightness of the SM neutrino viathe seesaw mechanism [5 6] Majorana neutrinos also pro-vide plausible scenarios for leptogenesis capable of account-ing for the excess of matter over antimatter in the observableuniverse [7 8]

Neutrinoless double-beta (120573120573(0])) decay searches rep-resent the only viable experimental method for testing theMajorana nature of neutrinos [9]Theobservation of this pro-cess would immediately imply that lepton number is violatedand that neutrinos areMajorana particles [10]The decay ratemay be written as

(1198790]12)minus1

= 1198660]10038161003816100381610038161198720]10038161003816100381610038162

(⟨119898120573120573⟩

119898119890

)

2

(1)

where 1198660] is a phase space factor including the couplings1198720] is a nuclear matrix element119898

119890is the electron mass and

⟨119898120573120573⟩ is the effective Majorana neutrino mass The latter is

given by

⟨119898120573120573⟩ =

1003816100381610038161003816100381610038161003816100381610038161003816

3

sum119894=1

1198802

119890119894119898119894

1003816100381610038161003816100381610038161003816100381610038161003816

(2)

where119880119890119894specifies the admixture of neutrinomass eigenstate

119894 in the electron neutrinoUntil very recently the most sensitive limits on 120573120573(0])

decay came from Ge detectors enriched in 76Ge namely

theHeidelberg-Moscow experiment [11] and the IGEX exper-iment [12ndash14] Recent results from the EXO-200 experiment[15 16] and from the KamLAND-Zen experiment [17 18]have claimed stronger bounds on neutrinomass It is difficulthowever to determine the best limit because of the uncertain-ties in the nuclear matrix elements Previous-generation 76Geexperiments also yielded a claim of the direct observation of120573120573(0]) decay by Klapdor-Kleingrothaus [19] This claim hasnot been widely accepted by the neutrino community [20ndash22] While the EXO-200 and KamLAND-Zen results are inconflict with this claim the recent results from GERDA [23ndash25] show that the observed peak is not an indication of120573120573(0])decay in a 76Ge experiment which can compare its resultswith the Klapdor-Kleingrothaus claim without dependingon the nuclear matrix elements For recent comprehensiveexperimental and theoretical reviews see [26ndash35]

A measurement of the 120573120573(0]) decay rate would yieldinformation on the absolute neutrino mass Measurementsof atmospheric solar and reactor neutrino oscillation [36]indicate a large parameter space for discovery of120573120573(0]) decayjust beyond current experimental bounds below ⟨119898

120573120573⟩ sim

50meV Moreover evidence from the SNO experiment [2] ofa clear departure from nonmaximal mixing in solar neutrinooscillation implies a minimum effective Majorana neutrinomass of sim15meV for the inverted hierarchy scenario Thistarget is within reach of next-generation 120573120573(0]) searches Anexperiment capable of observing this minimum rate wouldtherefore definitively determine theMajorana orDirac natureof the neutrino for inverted hierarchical neutrino masses

Recent developments in germanium detector technologymake a 120573120573(0])-decay search feasible using 76Ge In thispaper we describe the Majorana Demonstrator as anexperimental effort under construction in the SanfordUnder-ground Research Facility (SURF) whose goal is to demon-strate the techniques required for a definitive next-generation120573120573(0])-decay experiment with enriched Ge detectors TheDemonstrator will also test the Klapdor-Kleingrothausclaim and will be sensitive to other non-120573120573(0]) physicssignals in Ge A complementary effort in Ge with similar sen-sitivity the GERDA experiment [37] is presently operating

Advances in High Energy Physics 3

in the Laboratori Nazionali del Gran Sasso (LNGS) TheGERDA and Majorana collaborations intend to join in aproposal for the construction of a tonne-scale experiment Anearly background-free tonne-scale 76Ge experiment wouldbe sensitive to effective Majorana neutrino masses below sim20meV potentially covering the parameter space correspondingto the inverted neutrino-mass hierarchy

12 Non-120573120573(0])Physics with theMajoranaDemonstratorThe Ge-detector design used by Majorana has an energythreshold of sim500 eV This low threshold not only is criticalfor reducing 120573120573(0]) background (Section 5) but also incombination with low backgrounds opens up new physicsprograms for theMajoranaDemonstrator Recent exper-iments [38ndash41] have shown the sensitivity of P-type Point-Contact (P-PC) Ge detectors to light WIMP (lt10GeV1198882)dark matter via direct detection A very recent excess oflow-energy events reported by the CDMS collaboration [42]lends further motivation for doing such a measurement TheDemonstratormay improve the current lightWIMP limitsby two orders of magnitude [43]

In addition to lightWIMPSMajoranawill also be sensi-tive to solar axions that interact in the Ge crystals via severalpossible axion-electron coupling mechanisms One of thesemechanisms of particular interest to Majorana relies on thePrimakoff conversion of axions into photons within the Gecrystal latticewhen aBragg condition is satisfied [44 45]Thistechnique requires knowledge of the crystal axis orientationrelative to the sun at all times to maximize sensitivity Thecollaboration will measure the detector crystal orientationfor this purpose The Majorana Demonstrator can alsosearch for solar axions generated by the bremsstrahlungmechanism in the sun [46] and detected by the axioelectriceffect [47] Since this axion spectrum peaks at about 06 keVand falls sharply by an order of magnitude by about 3 keV thelow threshold and background are keys for thismeasurement

Majorana will also be sensitive to Pauli exclusion prin-ciple violating (PEPV) decays [48] In this process an atomicelectron in a Ge atom spontaneously transitions from anupper shell to the K-shell resulting in 3-ground shell elec-trons During this deexcitation a photon of energy close tothat of a K-shell X-ray (10 keV) is emitted The slight differ-ence in energy is due to the extrascreening of the nucleus bythe two K-shell electronsThe detection of this photon wouldindicate a PEPV decay Given the large number of atomspresent in 40 kg of Ge this will be a sensitive test of PEPVeffects

P-PC detectors were originally proposed for detectingcoherent nuclear scattering of reactor neutrinos [49] andthere is interest in usingHPGe detectors to do a similarmeas-urement with higher energy neutrinos at the spallationneutron source at Oak Ridge National Laboratory or similarsources [50 51] A cryostat full of natural germanium detec-tors similar to that planned for the Demonstratordeployed at a shallow underground site near the spallationneutron source target should have sufficient sensitivity tomake an observation of this process Such an effort coulddemonstrate the feasibility of P-PC technology for reactormonitoring and nuclear treaty verification

2 The Majorana DemonstratorAn Overview

The Majorana Demonstrator is an array of enriched andnatural germanium detectors that will search for the 120573120573(0])decay of the isotope 76GeThe specific goals of theMajoranaDemonstrator are

(1) demonstrate a path forward to achieve a backgroundrate at or below 1 cnt(ROI-t-y) in the 4 keV regionof interest (ROI) around the 2039 keV 119876-value for76Ge 120573120573(0]) decay This is required for tonne-scale germanium-based searches that will probe theinverted hierarchy parameter space for 120573120573(0]) decay

(2) show technical and engineering scalability toward atonne-scale instrument

(3) test the Klapdor-Kleingrothaus claim [19](4) perform searches for physics beyond the standard

model such as the search for dark matter and axions

Majorana utilizes the demonstrated benefits of enrichedhigh-purity germanium (HPGe) detectors These includeintrinsically low-background source material understoodenrichment chemistry excellent energy resolution andsophisticated event reconstruction The main technical chal-lenge is the reduction of environmental ionizing radiationbackgrounds by about a factor 100 below what has beenachieved in previous experiments

We have designed a modular instrument composed oftwo cryostats built fromultrapure electroformed copper witheach cryostat capable of housing over 20 kg of P-PC detectorsP-PC detectors were chosen after extensive RampD by thecollaboration and each has a mass of about 06ndash10 kg Thebaseline plan calls for 30 kg of the detectors to be built fromGematerial enriched to 86 in isotope 76 and 10 kg fabricatedfrom natural Ge (78 76Ge) The modular approach willallow us to assemble and optimize each cryostat independ-ently providing a fast deployment with minimum interfer-ence on already-operational detectors

Starting from the innermost cavity the cryostats will besurrounded by an inner layer of electroformed copper anouter layer of oxygen-free high thermal conductivity (OFHC)copper high-purity lead an active muon veto polyethyleneand borated polyethylene The cryostats copper and leadshielding will all be enclosed in a radon exclusion box Theentire experiment will be located in a clean room at the 48501015840level (1478m) of the Sanford Underground Research Facility(SURF) in Lead South Dakota

3 The P-PC-Detector Technology

At the heart of Majorana is its enriched p-type point-contact HPGe detectors [49 52] These detectors have allthe benefits of coax HPGE detectors traditionally used for120573120573(0]) but also possess superb pulse-shape analysis (PSA)discrimination between single-site interactions (such as120573120573(0])-decay events) and multisite interaction events (suchas Compton scattering of 120574-ray backgrounds) making them


highly suitable for 120573120573(0]) searches Their small capacitanceresults in superb energy resolution and a low-energy thresh-old making them suitable for event correlation techniquesusingX-rays Furthermore they are relatively robust and sim-ple to produceTheir simplicity has the advantage of reducingthe characterization studies and detector-to-detector tuningrequired for an effective PSA algorithm

Like coaxial Ge detectors P-PC detectors are cylindricalin shapeThe electron holes however are collected on a smallshallow contact rather than a long-extended electrode as ina coaxial detector In the detectors made for Majorana thisldquopoint contactrdquo varies in diameter fromabout 2 to 65mmandin depth from less than amicron (for implanted contacts) to afew mm Since they have no long inner contact P-PC detect-ors are generally limited in their length-to-diameter aspectratio If a crystal is too long it can result in having a ldquopinch-offrdquo island of undepleted material in the center especially ifthe Point-Contact end of the crystal has a lower net impurityconcentration than the opposite end This can be alleviatedby using crystals with larger impurity gradients and ensuringthat the point contact is placed at the end of the crystal wherethe material is of higher impurity (generally the seed end)

The Majorana collaboration has procured 20 kg ofnatural-germanium modified-BEGe detectors from CAN-BERRA industries [53] modified so as not to have the thinfront window that permits sensitivity to low-energy external120574-rays These detectors typically have masses in the range of600ndash700 g and use a thin implanted contact Detectors fromenriched 76Ge material are being produced by AMETEKORTEC [54] These detectors have a mass of around 1 kgeach with a greater length-to-diameter ratio than that of theBEGe detectors We anticipate that approximately 30 kg ofthese detectors will be produced for the Majorana Demon-strator from the 416 kg of 86 enriched 76Ge materialsupplied to AMETEKORTEC

Figure 1 illustrates our modeling of a sample P-PC detec-tor 5 cm in diameter and 5 cm long The color scale showshole drift speeds in mmns the black lines show charge drifttrajectories and the light grey lines show ldquoisochronesrdquo-loci ofequal hole drift time for events in the detector bulk We haveadapted such drift-time calculations to create a PSA heuristicfor Geant4 simulations (see Section 53) of the remainingbackground in theMajoranaDemonstrator following thePSA cut For this PSA heuristic multiple interactions withinan event are examined for their relative drift times and thatinformation is combined with each individual energy depositto determine whether the PSA algorithmwould be capable ofrejecting the event

Measured signals from a P-PC detector are shown inFigure 2 Both current and charge pulses are shown for botha single-site (a) and a multisite (b) 120574-ray events The differ-ence in signal shape is readily apparent with four distinctinteractions evident in (b)TheMajorana collaboration usestwo different types of PSA algorithm to discriminate betweenthese two classes of eventsThe first of these developed by theGERDA collaboration [55] compares the maximum heightof the current pulse (119860) to the total energy of the event (119864)as determined from the height of the charge pulse Multiple

20

10

0

009

008

007

006

005

004

003

0020 10 20 30 40 50

Radi

us (m

m)

lt100gt

lt110gt

lt001gt

minus10

minus20

300 600

(ns)900

Z (mm)

Figure 1 Calculated hole drift in a 5 cm diameter times 5 cm long P-PCdetector

interactions result inmultiple charge pulses separated in timeand therefore in a reduced value of 119860119864

An alternative approach [56] uses a library of uniquemeasured single-site signals to perform event-by-event 1205942fitting of experimental pulse shapes A method for buildingthis library from a large number of measured signals hasbeen developed and tested with simulation and experimentalstudies Results of this optimized PSA algorithm on P-PCdata are shown in Figure 3 where the high spectrum is for allevents from a 232Thsource and the low spectrum is for eventsthat pass the PSA cut The strong peak remaining is thedouble-escape peak from the 2615 keV 208Tl 120574-ray which isa proxy for single-site 120573120573(0])-decay events The algorithmretains at least 95 of these events while rejecting up to 99of the single-escape multisite events One should comparethis to the AE results of [55] where the double-escape peakevents are accepted at 89 and the single-escape peaks arerejected at 93

More recently we have also developed a model for slowpartial-energy events from interactions within the lithiumcontact layer on the outer edge of P-PC detectors This layeris typically 1mm in thickness but is not entirely inactivematerial Hence events within that layer can produce signalswith long rise-times and partial-energy collection We nowunderstand these signals as the result of competition betweenthe diffusion of holes out of the Li layer with the recombina-tion of electrons and holes at Li precipitates [57 58] Sincethese slow events can potentially generate much of the back-ground at very low energies a detailed and comprehensiveunderstanding of this process is crucial for darkmatter axionand other low energy-dominated physics sensitivity

4 The Majorana Demonstrator

Construction and Facility

41 Enrichment Ge Reduction and Refinement and DetectorProduction TheDemonstratorbaseline plan calls for 30 kgof enriched Ge detectors The Collaboration acquired 425 kg


0

100

200

300

Time (ns)

Char

ge si

gnal

200 600 1000 1400

0

20

40

60

Time (ns)200 600 1000 1400

Curr

ent s

igna

l

70

(a)

0

100

200

300

400

0

20

40

60

80

Time (ns)200 600 1000 1400

Time (ns)200 600 1000 1400

Char

ge si

gnal

Curr

ent s

igna

l

(b)

Figure 2 Current and charge pulse responses of a P-PC detector to single- andmultisite 120574-ray eventsThe pulse shapes in (a) show the charge(top) and current (bottom) signals resulting from a typical single-site interaction while (b) shows how the pulse-shape response to amultisiteinteraction is clearly different

of 76Ge in the form of 605 kg of 76GeO2 which was produced

by the Joint Stock Company Production Association Electro-chemical Plant (ECP) in Russia The order was delivered toOak Ridge TN United States in two shipments The first20 kg was delivered in September 2011 while the rest wasdelivered in October 2012 A special steel container was con-structed to minimize the exposure of the enriched 76Ge tocosmic rays during transport The calculated cosmic ray pro-duction of 68Ge and 60Co is reduced by a factor 10 and 15respectively for samples transported within this containerShielded storage for the enriched material being processed

in Oak Ridge is provided by a cave located about 8 kmfrom the processing and detector manufacturing facilitiesThe cave has an overburden of 40m of rock which is morethan adequate for shielding the enriched material from thehadronic component of cosmic rays

Electrochemical Systems Inc (ESI) in Oak Ridge TNprovided the first stage of material preparation Before pro-cessing any enriched material pilot tests with natural GeO

2

were performed to qualify the procedures The delivered76GeO

2from ECP underwent a high-temperature reduc-

tion in a hydrogen atmosphere When the resistivity of


20

60

100

140

180

Cou

nts

Double-escape peak1592keV

1450 1550 1650 1750 1850

Gamma energy (keV)

Figure 3 Pulse-shape analysis results for P-PC data The red spec-trum is for all events within the energy range while the blue spec-trum is for events that pass the PSA cut

the reduced material was greater than 3Ω cm the materialwas then purified by zone refinement to reach a resistivity ofgt47Ω cm The processing by ESI provided a yield of 98 ofenriched material suitable for further refinement and detec-tor manufacturing In addition to the process qualification ofthe refinement at ESI AMETEKORTEC produced two P-PCdetectors fabricated from natural Ge that had been reducedand purified by ESI

The refined enriched material from ESI was further puri-fied by zone refining at AMETEKORTEC before being usedas charge in a Czochralski crystal puller Detector blanks werecut from the pulled crystals (sim70mm diameter) followedby the standard detector manufacturing steps of lithiationimplantation of the p+ contact and passivation At eachstep the mass of the enriched materials being handled wasrecorded for inventory control In the production of theenriched P-PC detectors slurries from detector cutting andshaping processes as well as small samples cut from thepulled crystal for evaluation were saved and reprocessed byESI for reuse in detector production Acids that were usedin the detector manufacturing process were not saved Atall stages of the detector fabrication process any enrichedmaterials that were not being worked on were transportedback to the cave for storage

Once an enriched detector ismanufactured it is mountedin a PopTop capsule and tested for performanceThe PopTopdetector capsule which can be detached from the cold fingeroffers superior portability and ease of disassemblymdashqualitiesthat are essential for transferring the detector from thecapsule to the low-background string mounts used in theDemonstrator In order to mitigate contamination of themounted detector in a PopTop capsule the parts that are indirect contact with the detector aremade fromonly radiopurematerials For example indium contacts are replaced by goldor clean tin contacts and synthetic charcoal with low radonemanation rate is used tomaintain the vacuum in the capsule

The production of enriched detectors began inNovember2012 As of April 2013 ten enriched detectors with a totalmass of approximately 95 kg have been fabricated with eightdelivered to SURFThe detectors were transported by groundto SURF and a portable muon counter [59] was used to logthe cosmic-ray exposure during the trip

42 Detector Array Configuration The detector array forDemonstrator is designed with many goals in mind Thefunctional requirements are as follows

(i) Only the most radiopure materials are used to con-struct the detector holder All the detector and stringcomponents are made out of two possible materialsunderground electroformed Cu (UGEFCu) from thetemporary clean room (TCR) (Section 432) or NXT-85 (a Teflon that is specially manufactured in a cleanroom environment Section 5) The UGEFCu has amaximum thickness of 127 cm and the NXT-85 partsare fabricated from 15875 cm long rods that are3175 cm in diameter The NXT-85 mass is minimizedand used only where electrical insulation is required

(ii) UGEFCu and NXT-85 parts are processed in thecleanroom machine shop (Section 47) so designsmust be compatible with themachine tools purchasedfor this shop Use of wire electric dischargemachining(EDM) is preferred as a clean material removal tech-nique

(iii) A 5mm vacuum gap or 1mm of NXT-85 is requiredto isolate high voltage from neutral components

(iv) The detectors have variable dimensions in order tomaximize the yield of enriched germanium Uniqueparts are minimized in order to allow a wide range ofdetector sizes to be packaged while still providing ahigh packing factor of germanium in each cryostat

(v) Threaded connections are difficult to produce andkeep clean We minimized the number of threadedconnections and where that was not practical specialmethods are employed to ensure quality cleanlinessand repeatability of threaded connections

Each detector is housed in a frame referred to as a detectorunit (Figure 4) The crystal mounting plate (CMP) is thefoundation while 3 hollow hex rods and high-voltage (HV)nuts provide connection to the HV ring which clamps thedetector in place The crystal insulators provide electricalisolation between theHV surface of the detector and the neu-tral CMP The crystal insulators are also sized to compensatefor the different coefficients of thermal expansion of copperand germanium providing equivalent clamping force whenoperating warm and cold

The crystal insulators snap into place on the CMP as doesthe contact pin bushing The contact pin and low mass frontend (LMFE) board are held in place by the spring clip whichprovides contact pressure between the pin and detector Thespring clip is held in place and tensioned by parylene-coatednumber 4ndash40Cu nuts on thread-milled studs in the CMPParylene coating of these nuts provides a higher strengthconnection than that of NXT-85 nuts while providing threadlubrication to prevent copper galling

Up to five detector units are stacked into a string(Figure 5) The string is clamped together with tie rods Thestring adapter plate connects the string to the coldplate Thetie rod bottom nuts and adapter plate nuts also parylenecoated provide a strong clamping force for thermal contact


HV ring

HV nut

Hollow hex rod

Crystal insulator

CMP

Contact pinbushing

Contact pin

Hex nut

LMFE

Spring clip

Figure 4 A rendering of the detector unit design (see text for details)

Adaptor plate bolt

Adaptor plate nut

String adaptor plate

Tapered nut

Bottom nut

Tie rod

Figure 5 A rendering of the string design (see text for details)

The LMFE board is built on a 0025 cm thick fused silicasubstrate with all of the cold electronics mounted on theboardTheLMFEassembly is then inserted into spring fingerson the spring clipThis assembly is thenmounted to the CMPAfter the contact pin and detector are mounted the springclip tensioning nut is set to apply the appropriate contact pres-sure

The detector units are designed to accommodate a widerange of detector sizes frommultiple vendors Detectors havethe shape of a right cylinder and can have a diameter of 50ndash77mm and a height up to 65mm Variations in diameter andcorner sharpness can be accounted for by producing customcrystal insulators Variations in the details of the pin contactwith respect to the overall shape are accounted for by having2 different pin lengths Variations of less than 05mm in thisgeometry can be accounted for by adjusting the spring cliptension starting point

The special methods for threaded connections includethe use of dedicated tools as with all parts machined

underground to avoid cross contamination The number 4ndash40 studs on the CMP are threadmilled from bulkmaterial soinstead of having independent screws there are in placestuds that protrude from the interior surface Interior threadsare made using roll-form taps which produce a consistentthread quality and no burr All interior threads are of thesmallest depth necessary for strength and there are no blindtapped holes This makes cleaning and drying parts easierAll threaded parts are verified by hand before release for finalcleaning

43 The Electroformed Copper Cryostats the Thermosyphonand the Vacuum System

431 The Majorana Demonstrator Module The Major-ana Demonstrator is a modular instrument as the detec-tor strings are deployed in two copper cryostats each isoutfitted with its own vacuum and cryogenic systems forindependent operation This modular scheme allows forphased deployment of detectors as they become availableand suggests a scheme for development of a tonne-scale76Ge experiment a larger experiment can be constructed bydeploying several similar cryogenic modules The cryostatsdesigned for the Demonstrator are each capable of housingseven of the previously described detector strings for atotal capacity of sim20 kg of HPGe detectors apiece The firstcryostat Cryostat 1 will contain detectors produced fromboth natural and enriched germanium The second cryo-stat Cryostat 2 will only contain detectors produced fromenriched germanium The cryostats are constructed fromcopper a design decision motived by the ability to produceultrapure copper through chemical electroforming Cryostats1 and 2 are fabricated from this ultrapure copper while aninitial prototype cryostat is fabricated from commerciallysourced copper The prototype cryostat will only contain


two strings of detectors produced from natural germaniumIt serves as a testbed for mechanical designs fabricationmethods and assembly procedures that will be used for theconstruction of the electroformed copper Cryostats 1 and 2

432 Electroforming The primary requirement for the cop-per used in the Majorana Demonstrator is that it is suf-ficiently purified This includes removal of naturally occur-ring radioactivity from U and Th as well as the eliminationand prevention of reformation of cosmogenic radioisotopesspecies Due to its large total mass the radiopurity goal forthe copper that is used in the inner shield and detector com-ponents is very stringent To attain the background goal of3 cnts(ROI-t-y) the required purity levels arelt03 120583Bq 238Ukg Cu (or 24times 10minus14 g 238Ug Cu) and lt03 120583Bq 232Thkg Cu(or 75times10minus14 g 232ThgCu) Electroforming copper in a care-fully controlled manner within a clean environment allowsone to produce copper with the required radiopurity [60]

A secondary requirement for the electroformed copperrelates to its physical properties The mechanical propertiesof electroformed copper can vary drastically depending onthe conditions under which it was formed Conditions thatfavor high purity can form large crystalline structures withpoor mechanical strength Small polycrystalline formationscan exhibit adequate tensile strength but lower puritiesTheseconditions which are seemingly at odds with one anotherrequire that a careful balance of operational parametersshould be obtained in the electroforming production process

Design considerations for load bearing components werecarried out using conservative estimates for material prop-erties such as yield strength The design yield stress valueused for electroformed copper was estimated to be 48MPaMechanical testing and evaluation is necessary to prove theplated materialrsquos ability to withstand the loading conditionswithout failure Mechanical evaluation has shown the yieldstrength to be 832MPa [61] on average with a significantdegree of strain hardening observed The UGEFCu hastherefore shown compliance with the design criteria

The electroformed material for the Demonstrator hasbeen fabricated mostly from cylinders that are up to 356 cmin diameter (the inside diameter of the cryostats) The ther-mosyphonwas formed on amandrel 190 cm in diameterThecopper produced can range from a few tens ofmicrons to verythick plates near 14 cmTime constraints are the primary lim-itation when producing very thick electroforms The currentplating rate for the demonstratorrsquos copper is typically from38 to 64 120583m per day depending on a variety of parametersWhile this rate can be increased it is at the expense of purityand mechanical properties of the electrodeposited materialFrom the plating rate indicated a 14 cm thick electroformtakes approximately 8ndash12 months to complete

433 CryostatDesign Thecryostat is a copper vacuumenclo-sure that includes an electron-beam-welded vessel assemblyalong with removable top and bottom lids (see Figure 6)The vacuum seals between these components are a customMajorana design that uses thin (51 120583m thick) parylene gas-kets sandwiched between tapered surfaces machined into

the copper components Copper rail sectors and clamp boltsare used to maintain parallelism during assembly and pumpdown Vacuum forces are sufficient to maintain the seal sobolt strength is not a factor in effective sealing

Detector strings aremounted to a copper coldplate whichrests onVespel pins that provide support and alignmentwhilemaintaining a thermal break from the room temperaturevacuum vessel An infrared (IR) shield is mounted to theunderside of the coldplate to reduce detector leakage currentgenerated by IR radiation The cryostat is supported at itscrossarm by a copper frame (not shown in Figure 6) insidethe lead stack A transition is made to stainless steel conflatvacuum hardware at the far end of the crossarm tube via acopperstainless explosion-bonded transition flange All ofthe stainless steel vacuum hardware is located outside of theDemonstratorrsquos passive shielding

434 Vacuum System Each cryostat is mounted to its ownvacuum system and constructed from all-metal ultrahighvacuum components A 200 lpm oil-free diaphragm pumpprovides rough vacuum a 300 lps turbo-molecular pump isused for initial pump down to UHV pressures and a 1500lps cryopump is used for steady state operation A nonevap-orable getter (NEG) pump is used to remove built-up non-condensable gasses A residual gas analyzer provides massspectrometry analysis of the vacuumAll of the active compo-nents including valves are remotely operable and an appli-cation has been developed tomonitor and control the systemallowing full-remote operation (see Section 45) Pressuresare continuously uploaded to a slow-control database forhistory viewing

435 Cryogenics The detector strings are cooled via thecoldplate by means of a thermosyphon [62] The thermosy-phon is a closed tube within the crossarm that joins the cold-plate to a condenser volume residing outside the shield andinside a dewar containing liquid nitrogenThe thermosyphoncontains nitrogen that is liquefied at the condenser andtransported by gravity down the length of the crossarm to thecoldplate where as it evaporates it cools the coldplate Theevaporated nitrogen travels back to the condenser where it isreliquefied In this cycle heat is transported from the cold-plate to liquid nitrogen in the dewar The liquid nitrogen inthe dewar then evaporates and is replenished froman externalsupply The dual-phase nitrogen within the thermosyphonhas a large effective thermal conductivity providing therequired cooling power for the Demonstrator The oper-ating temperature can be tuned by adjusting the amount ofthermosyphon nitrogen By producing only a thin layer ofcondensed nitrogen at the coldplate microphonics fromevaporation is minimized

The thermosyphon system consists of a thermosyphontube custom liquid nitrogen dewar gaseous N

2plumbing

for loading nitrogen into the tube and an external ballasttank The thermosyphon tube is constructed from the same


Detector strings

LN dewar

Ballast tank

Pressure monitor and relief

Vacuum system

Vacuum vessel

Condenser

Cold plate Thermosyphon

Figure 6 The Majorana Demonstrator module Detector Strings are housed within ultralow background cryostats each of which aresupplied with its own vacuum and cryogenic systems (see text for details of vacuum and cryogenic system function)

grade of UGEFCu as that from which the cryostat is fab-ricated The liquid nitrogen dewar is a custom-fabricateddevice which includes the condenser volume with an in-vac-uum connection to the thermosyphon tube and an in-airconnection to the N

2supply The external ballast tank is pri-

marily a safety feature allowing for evaporation of condensedthermosyphon-tube nitrogen in the case of a loss of liquidnitrogen supply without creating a hazardous overpressurecondition Additionally since the ballast tank is isolated fromthe thermosyphon tube nitrogen can be stored for sev-eral 222Rnhalf-lives (38 days) before it is loaded into the ther-mosyphon tube In this way we can ensure that the nitrogencirculated in the thermosyphon tube is radon-free

44 Detector Acceptance Characterization and CalibrationThere are a number of experimental characteristics that needto be monitored during the course of the experiment includ-ing

(i) energy scale and linearity(ii) absolute efficiency for double-beta decay within each

detector(iii) energy resolution and peak shape

(iv) background and signal tagging efficiencies

(v) pulse waveforms response

We have developed a three-phase plan to ensure that therequired data are acquired The phases include acceptancecharacterization and calibration The acceptance testing isthe initial phase of evaluating the detector performanceThistype of testing is done upon receipt of the detectors from theirmanufacturers and is performed in the transport cryostatswith the original manufacturer-supplied preamplifiers Thetests conducted during this phase are relatively cursory andmeant solely to establish that the received detectors meetsome minimum qualifications The basic tests performed areenergy scale and resolution relative efficiency leakage cur-rent or capacitance at depletion initial estimate of the deadlayer and detector mass and dimensions

Characterization measurements are conducted to fullydetermine the operating behavior of a detector This includesenergy scale resolution capacitance measurements single-site and multiple-site event separation performance dead-layermeasurements and crystal-axismeasurements Charac-terization measurements are done with detectors in the finalstring configuration both within a test cryostat and within


the Demonstrator cryostat prior to operation that is priorto commissioning

The calibration measurements are designed to monitorthe stability of the system during run-time operation Ini-tially hour-long calibrations with a 228Thsource will be con-ducted on aweekly basisThese runswillmeasure the stabilityof the energy scale and resolution efficiency and pulse-shapeanalysis (PSA) efficiency Once the stability of the detect-ors has been established the time period between the sourcecalibrations can be extended to twice monthly or evenmonthly

In the detector and string characterization stage mea-surements are being performed with button sources ( 133Ba60Co and 241Am) in a clean room environment Once thedetector strings are loaded into the cryostat access will belimited Each monolith will have a low-background sourcepathway of PTFE tubing that spirals around the outside of thecryostat A line source of 228Thwill be remotely fed into thepathway enabling the calibration of the entire cryostat with asingle source either during final testing or after the monolithis placed in the shield Simulations have shown that in anhour-long run we can accumulate the necessary statistics tomonitor the efficiency PSA performance energy scale andresolution while simultaneously keeping the count rate belowthe signal pile-up threshold of sim100Hz The source will beparked in an external garage separated from the shield-pen-etrating section of the pathway by an automated valve systemDuring calibration runs the valve will be open and theentire pathway will be purged with liquid nitrogen boil-offDuring production runs the shield-penetrating section of thepathway will be sealed off from the garage The source itselfwill be encased in two plastic tubes to prevent leaving residualradioactivity within the pathway

45 Electronics and Data Acquisition

451The Detector Readout Electronics Each of the two cryo-stat modules in the Demonstrator contains seven stringswith each string holding up to five detectors Figure 7 illus-trates the basic design of the low-noise low-radioactivitysignal-readout electronics It consists of the LMFE [63] acircuit containing the input FET and feedback componentswhich is located close to the detector in order to minimizestray input capacitance It also includes the preamplifierwhich lies outside the cryostat and is connected to the LMFEby a long cable The main challenges to the practical realiza-tion of this design are sourcing components for the front endthat are low in both noise and radioactivity and dealing withthe long cable in the feedback loop

The LMFE is a resistive feedback circuitThis architecturehas various benefits over the common pulse reset alternativenamely that it is simpler and avoids interference from resetpulses a problem for multiple-detector systems The LMFEis 205 times 7mm2 in size and weights approximately 80mgThe substrate is fused silica which was chosen for its highradiopurity low dielectric losses and low thermal conduc-tivity Its low thermal conductivity means that a significanttemperature gradient can be maintained across the board By

Detector

Front end Cable Preamplifier

Figure 7 A high-level illustration of the signal readout scheme foreach detector channel (see text for details on this readout design andthe typical values for the components)

choosing an appropriate geometry and thermal conductivityfor the substrate the FET can be maintained at its optimaloperating temperature for noise performance by self-heatingwhich is adjustable by controlling drain to source voltageThe chosen FET is a bare Moxtek MX-11 JFET low-noise diewith high transconductance and very low-input capacitanceIt is attached to the silica board with silver epoxy via itsgate substrate while the source and drain pads are connectedto traces with AlndashSi wire bonds The feedback resistor isformed by sputtering a layer of amorphous Ge (a-Ge) andhas a resistance of sim10ndash100GΩ at cryogenic temperaturesThe feedback capacitance of sim02 pF is formed by the straycapacitance between traces on the front end

Extraordinarily low-noise levels can be achieved usingthe front end design previously described The equivalentnoise charge achieved without a detector was 55 eV FWHMand with a detector a small P-PC it was 85 eV The noiselevels are not representative of what will be achieved in theDemonstrator because of the greater detector capacitancebut they indicate that the noise resulting from the materialsand components of the front end board will not be thelimiting factor To minimize the material budget and thermaldissipation in the cryostat a 04mm diameter sim2m longminiature 50Ω coaxial cable is used to drive signals from theLMFE to the first stage of the preamplifier based on a classic-folded cascode transistor design The preamp is capable ofrise times below 10 ns with judicious choice of front end com-ponents and a short cable connecting it to the front endWiththe long cable used in theDemonstrator configuration thisrise time increases to sim40ndash70 ns depending on the length ofthe cable

The preamplifiers are organized by string position onmotherboards On themotherboard each detector has a low-gain and a high-gain signal output for digitization resultingin a maximum of 70 channels per cryostat The digitizationelectronics for each cryostat are in separate VME crates Eachcrate has a single-board computer to read out the digitizers inthat crate and the entire system is controlled by a central DAQcomputer A conceptual schematic of the data acquisitionelectronics is given in Figure 8

The collaboration is using GRETINA digitizer boards[64] developed as part of the GRETINA experiment (wegratefully thank the GRETINA collaboration for digitizerboard loans) The GRETINA cards are a combination of a


Veto electronics

Digitizers

SBC

VME crate

ORCAAG data storage

ORCARoot

Remotemonitoring and

control

GPS

UG raid arrayORCA backup

Clocksync

Preampspulser

Low gain

High gain

Veto

Det

ecto

r str

ings

HV

HV

Figure 8 A high-level schematic of the DAQ system Each detector uses a low- and a high-gain digitizer channel for a total of 70 channelsper crate Each VME crate has one single-board computer (SBC) that reads out all electronics in its crate sending the resulting data to ORCAData is buffered locally underground and then transferred to the surface There will be one VME crate per module The veto electronics willreside only in one of the two crates

digitizer and digital signal processor which accept 10 inputsdirectly from the detector preamplifiers and digitize at anominal frequency of 100MHz with 14 bit ADC precision Afield-programmable gate array (FPGA) performs digital lead-ing edge andor constant fraction discrimination trapezoidalshaping and pole-zero correction Some special firmwareextensions were specifically developed for the Demonstra-tor These modifications allow for independent configura-tions of the 10 input channels that allow the traces to be deci-mated in such a way as to simultaneously capture a fullysampled rising edgewith presummed regions before and afterthe edge for studying slow pulsesThe capability ofmeasuringthe input trigger rate (triggerssecond) for each individualinput channel is another feature added to the card specificallyfor the Demonstrator Data records are fixed at 2020samplesevent and the card allows these samples to be thesum of various ADC output values This summing capabilityallows the card to achieve selectable time windows (20 4080 160 and 200120583s) for the acquired data while maintaining aconstant event length In the case of the Demonstrator dif-ferent crystal geometries can be plugged into the same cardand therefore it is mandatory to be able to accommodate thedifferent time constants of the different crystals

A controller card communicates between the moth-erboard and the digitization electronics The controllercard contains 16 pulse generator outputs with individuallycontrolled amplitudes 16 DAC levels outputs for setting

the drain-to-source voltage of the FETs and 16 ADC inputs tomonitor the first stage outputs of the preamplifiersThe pulseroutputs of the controller card are used for electronics calibra-tion monitoring the gain stability of independent channelsand monitoring the trigger efficiency The controller cardsare implemented in the overall DAQ system and slow-controlprocessor

452 Other Data Acquisition and Electronic Systems Thepassive lead shield is surrounded by scintillator panels thatare used for an anticoincidence (veto) shield The veto (seeSection 46) has separate electronics in one of the VME crateswith some additional electronics in a separateNIMBINDatafrom the veto is read out by the main DAQ software and inte-grated into the detector data stream All veto events are timestamped using a scaler that counts the common clock pulses

To ensure accurate time-stamping of the signals a com-mon clock from a global positioning system (GPS) moduleis distributed among the digitizer cards and the veto systemA common reset is used as a system synchronization pulseto simultaneously reset counters on all digitization and vetoboards thus providing an absolute time reference for eachevent

Separate computer-controlledHVbias supply systems areused for the detector array and the photomultipliers of theveto shield Each detector and phototube are powered by anindependent HV channel allowing for optimization of the


Outer Cushield

Radon enclosure Veto panels

Inner Cushield

Lead bricks

Poly shield

Figure 9 The shield system in cross section shown with bothcryostats installed

HV setting for each detector and enabling any detector to betaken offline without affecting the rest of the array

Data are transferred from theDAQcomputer to an under-ground 32 TBRAID system and from there to the above-ground analysis computer system The underground RAIDstorage is used as a buffer to prevent loss of data in the case ofan underground-to-surface network failure

453 The Data Acquisition Software TheDAQ software sys-tem used by the Demonstrator is the object-oriented real-time control and acquisition (ORCA) [65] applicationORCAis a general purpose highly modular object-oriented acqui-sition and control system that can be configured at run timeto represent different hardware configurations and data read-out schemes by dragging items from a catalog of objects intoa configuration window Since each object is composed ofits own fully encapsulated data structures as well as supportand diagnostic code ORCA can easily support specific exper-iments such as the Demonstrator

46 The Shielding Configuration The passive shield consistsof graded shield materials starting outside of the cryostat andextending out to the overfloor table or base plate uponwhichthe experiment is assembled This system also includes theintegral calibration source track and drive system the trans-port mechanisms for installing and retracting the cryostatfrom the bulk of the shield for access and a radon scrubbingand nitrogen delivery system for providing purge gas to theinternal portions of the shield A high-level summary ofshield components is given in Table 1 and the complete shieldassembly is shown in Figure 9

Gamma rays from the inner region of the shieldingcontribute to the background Therefore materials withextremely low radioactivity must be used in this region Cop-per can be purchased very pure and made ultrapure viaelectroforming (see Section 432) to meet the specificationsrequired by the background budget The innermost layer ofthe shield will be 5 cm thick and built of UGEFCu sheet Thepractical upper limit on the thickness for electroforming inour baths is about 140 cm so this shield is constructed of fourlayers of 125 cm thick plates

Defector roomEforming room

Machine shopUtilities

Yates shaft

Davis CavernLUX

Davis Campus

80

Figure 10 The layout of the Davis Campus at SURF showing thethree Majorana labs and the location of the LUX experiment atthe Davis Cavern The corridor that joins the two projects is a cleanspace

The outer copper shield is made of 5 cm thick OFHCcopper plates and machined and bolted together to providethe mechanical reference points for the experiment Thisshield ldquoboxrdquo is supported off the overfloor table on fourOFHC copper legs to allow for precision alignment andreference independent of the lead shield bricks The leadshield consists of a 45 cm thick layer of 51 times 102 times 203 cm3bricks to be stacked in place and machined as needed

The copper and lead shield will be contained inside asemisealed aluminum box This box will permit for the con-trolled purging of the gas within the inner cavity that containsthe detector modules The box is constructed of a weldedand bolted aluminum structural frame with 03175 cm thickbolted Al panels All seams and openings are sealed withthe exception of the seals around the monolith which aregasketed and bolted for easy removal and replacement

Two layers of veto panels surround the radon exclusionbox Each panel consists of a 254 cm thick scintillating acrylicsheetThis sheet has small longitudinal grooves machined forwavelength-shifting fibers and is wrapped in a custom-madereflecting layer to compensate for light attenuation along thefibers Light from the fibers is read out by a single 127 cmphotomultiplier tubeThe scintillator assembly is enclosed ina light-tight aluminum boxThere are a total of 32 veto panelssurrounding the radon exclusion box including a numberthat reside within openings of the overfloor Sheets of 254 cmthick high density polyethylene (HDPE) panels stacked upto 30 cm of total thickness make up the poly shield structureThe inner 2 layers consist of borated HDPE

47 Underground Facilities TheHomestake Mine is home tothe Sanford Underground Research Facility (SURF) that hasbeen developed by the state of South Dakota as a site forexperiments requiring underground laboratory space Thestate of South Dakota along with private donor T DennySanford has committed funds that have allowed refurbish-ment and access to underground space Operation of SURFis funded by the US Department of Energy and the state ofSouth Dakota

The Majorana laboratory space at SURF consists ofthree cleanrooms in the Davis Campus complex (Figure 10)


Table 1 A summary of the shield components

Shield component Material ThicknessInner copper shield 4 layers 125 cm thick UGEFCu 5 cmOuter copper shield OFHC copper commercial 5 cmLead shield 51 times 1024 times 203 cm3 bricks 45 cmRadon exclusion box Al sheets 032ndash0635 cmVeto panels Scintillating acrylic 2 layers 254 cm eachPoly shield HDPE 30 cm inner layer borated

Figure 11 A view of the detector laboratory at the Davis Campusat SURF In this view the glove box is to the left and the prototypemodule is at the center In the rear a soft-wall cleanroom is shownthat serves the purpose of an area of increased cleanliness

near the Yates shaft at the 48501015840 level (sim4260mwe [66]) ofthe mine These rooms consist of a detector room a generalpurpose lab space and a machine shop Figure 11 showsa picture of the detector room at SURF This room willhouse the Demonstrator and has dimensions of 321015840 times 401015840Next to the detector room is the machine shop Here all thecopper and NXT-85 parts for the Demonstrator are fab-ricated thus further reducing theUGEFCursquos surface exposureto cosmic rays The machine shop is approximately 1000 ft2and includes two lathes two mills an oven a wire electricdischarge machine a press a drill press and a laser engraverThe final room is a general purpose lab and is used for testingthe detectors prior to their installation into the Demonstra-tor The room is approximately 550 ft2 and was originallydesigned for electroforming activities hence its formal nameis the electroforming room

The final Majorana laboratory is the temporary clean-room (TCR) (not pictured in Figure 11) which sits on thesame level as the Davis Campus and is approximately 1 kmaway It consists of a cleanroom building shown in Figure 12that contains 10 electroforming baths and a small annex forchanging into cleanroom garb The total area of the buildingis 121015840 times 401015840 with the annex room consuming 81015840 times 121015840 of thatarea The TCR was required prior to beneficial occupancyof the Davis Campus for initiating the slow process ofelectroforming copper in order for parts to be ready on timefor assembly of the Demonstrator

5 The Background Model and the Majorana

Demonstrator Sensitivity

The projected background in the Demonstrator is signifi-cantly improved over previous-generation experiments Thisreduction is a result of fielding the detectors in large arrays

Figure 12 A photograph of the TCR used for electroforming on the48501015840level at SURF

that share a cryostat and minimizing the amount of inter-stitial material Further background suppression is achievedthrough the aggressive reduction of radioactive impuritiesin construction materials and minimization of exposure tocosmic rays Majorana will also make use of event signa-tures to reject backgrounds that do appear including pulse-shape characteristics detector hit granularity cosmic-rayveto tags and single-site time correlations In this section wedescribe these aspects of the Majorana Demonstratordesign and their impact on the projected backgrounds andphysics sensitivity

51 Pure Materials The production process for enrichedgermanium detectors (enrichment zone refining and crystalgrowth) efficiently removes natural radioactive impuritiesfrom the bulk germanium The cosmogenic activation iso-topes 60Co and 68Ge are produced in the crystals while theyare aboveground but can be sufficiently reduced by minimiz-ing the time to deployment underground and by the use ofpassive shielding during transport and storage

For the main structural material in the innermost regionof the apparatus we choose copper for its lack of naturallyoccurring radioactive isotopes and its excellent physicalproperties By starting with the cleanest copper stock we haveidentified and then electroforming it underground to elim-inate primordial radioactivity and cosmogenically produced60Co we have achieved several orders-of-magnitude back-ground reduction over commercial alternatives Electro-formed copper will also be employed for the innermost pas-sive high Z-shield Commercial copper stock is clean enough


for use as the next layer of shielding For all uses of copper wehave certified the cleanliness of samples via assay Modernlead is available with sufficient purity for use as the bulkshielding material outside of the copper layers

Several clean plastics are available for electrical and ther-mal insulation For the detector supports we use a pure poly-tetrafluoroethylene (PTFE) DuPont Teflon NXT-85 Thinlayers of low-radioactivity parylene will be used as a coatingon copper threads to prevent galling and for the cryostat sealFor the few weight-bearing plastic components requiringhigher rigidity we have sourced pure stocks of PEEK (poly-ether ether ketone) produced by Victrex and Vespel pro-duced by DuPont

The front-end electronics are also designed to be low-mass and ultralow background because they must be locatedin the interior of the array adjacent to the detectors in order tomaintain signal fidelity The circuit board is fabricated bysputtering thin traces of pure gold and titanium on a silicawafer upon which a bare FET is mounted using silver epoxyA simGΩ-level feedback resistance is provided by depositingintrinsically pure amorphous Ge Detector contact is madevia an electroformed copper pin with beads of low-back-ground tin at either endAn electroformed copper spring pro-vides the contact force Our signal- and high-voltage cablesare extremely low-mass miniature coaxial cable We haveworked with the vendor to cleanly fabricate the final productusing pure stock thatwe provide for the conductor insulationand shield Cable connectors within the cryostat are madefrom electroformed copper PTFE and the same silica circuitboards used for the front-end electronics

The high material purities required for the MajoranaDemonstrator necessitated the development of improvedassay capabilities These capabilities are needed not only toestablish that the required purities can be achieved but alsoto monitor construction processes to verify that cleanliness ismaintained We rely primarily on three assay methods 120574-ray counting inductively coupled plasma mass spectrometry(ICP-MS) and neutron activation analysis (NAA)

52 Background Rejection One key advantage of HPGedetectors is their inherent excellent energy resolution Back-ground rejection in our P-PC detectors is significant due tonot only the energy resolution but also array granularity(interdetector coincidences) pulse-shape discriminationand event time correlation These techniques rely on thedifferentiation of the spatial and temporal distributions of120573120573(0])-decay events from most background events Back-ground signals from radioactive decay often include a 120573andor one or more 120574-rays Such 120574-rays frequently undergomultiple scatters over several centimeters Since 120573120573(0])-decay energy deposition typically occurs within a smallvolume (asymp1mm3) it is a single-site energy deposit The twodifferent topologies can be separated by PSA (see Section 3)Rejection of decay-product series using single-site time-correlation analysis techniques is also possible in theseultralow event rate experiments For example 68Ga120573+ decayscan only result in background to 120573120573(0]) decay if one of theannihilation 120574-rays interacts in the same crystal that contains

the 120573+ Hence it is always a multiple-site energy deposit andwe reject much of this background through PSA In additionhowever 68Ga decays are preceded by the electron capturedecay of the parent 68Ge The low threshold of the P-PCdetectors permits additional rejection by a time-correlationcut with the 68Ge 10 keV K and 1 keV L X-rays

53 Monte Carlo Simulations The Majorana and GERDAcollaborations have jointly developed a simulation softwareframework mage that is based on the Geant4 [67 68]simulation toolkit Mage is used to simulate the response ofultralow radioactive background detectors for ionizing radi-ation The development of a common simulation frameworkused by both collaborations reduces duplication of efforteases comparison between simulated data and experimentand simplifies the addition of new simulated detector geome-tries The Mage package is described in more detail in [69]

Mage has interfaces with numerous external packagesincluding software that simulates charge pulse generation inHPGe detectors The Mage framework contains the geome-try models of common detector objects experimental pro-totypes test stands and the Demonstrator itself It alsoimplements customized event generators Geant4 physicslists and output formats All of these features are availableas class libraries that are typically compiled into a singleexecutableThe user selects the particular experimental setupimplementation at run time via macros In the prototypingphase the simulation was used as a virtual test stand guidingdetector design allowing an estimate of the effectiveness ofproposed background reduction techniques and providingan estimate of the experimental sensitivity During operationMage will be used to simulate and characterize backgroundsto determine the ultimate sensitivity of the experiment Itwill also provide probability distribution functions (PDFs) forlikelihood-based signal extraction analyses

Majorana has developed a detailed geometry of theDemonstrator in Mage that consists of more than 3 800components The collaboration has performed detailed sim-ulations of 60 000 background contributions from differentisotopes in these components that required 60 kCPU-hrs ondifferent Linux clusters The simulations include estimates ofthe rejection efficiencies from the analysis cuts discussed inSection 52 The PSA cut efficiencies in particular are esti-mated using a heuristic calculation in which multiple inter-actions in a detector are examined for their relative drift timeusing isochrone maps such as that depicted in Figure 1 Thatinformation is combined with the energies of the interactionsto determine whether the PSA algorithmwould be capable ofrejecting the event The summary of the background expec-tation after all cuts is given in Figure 13 These simulationresults were used for the current background estimates andwill be used in future analyses of data

The collaboration has also developed an automated val-idation suite that thoroughly tests all the critical physicsprocesses that are being simulated by Mage and Geant4against validated experimental data This suite is run everytime there is amajor update toMage or geant4 to verify thatthe critical physics processes are not altered between versions


Natural radioactivityCosmogenic activationExternal environmental

120583-inducedNeutrinos

0 01 02 03 04 05 06 07 08 09 1

08880480

0195

02220270

0067

00300088

00550100

00540210

0170

01300030

0011

MJD 120573120573(0) background goals (cntsROI-t-y)

Electroformed CuOFHC Cu shielding

Lead shieldingCables

Front endsGe (UTh)

Plastics + other68Ge 60Co (enrGe)

60Co (Cu)External 120574 (120572 n)

Rn surface 120572

Ge(n n)Ge(n 120574)

Directs 120583 + other backgrounds

Ge Cu Pb (n n998400120574)

Figure 13 Estimated background contributions for the 120573120573(0])-decay search Backgrounds from natural radioactivity are shown in blueCosmogenic activation products are colored red Green denotes backgrounds from the environment or those introduced during detectorassembly Purple is for 120583-induced backgrounds at depth An upper limit on the negligible background from atmospheric and other neutrinosis shown in orange The contributions sum to 30 cnts(ROI-t-y) in the Majorana Demonstrator

54 Predicted Sensitivity of the Majorana DemonstratorThe sensitivity of a neutrinoless double-beta decay searchincreases with the exposure of the experiment but ultimatelydepends on the achieved background level This relationshipis illustrated for the Demonstrator in Figure 14 in whichwehave used the Feldman-Cousins [70] definition of sensitiv-ity in order to transition smoothly between the background-free and background-dominated regimes The backgroundexpectation for the Demonstrator is 3 cnts(ROI-t-y) in a4 keV energy window at the 120573120573(0])-decay endpoint energyA minimum exposure of about 30 kg-y is required to test therecent claim of an observation of 120573120573(0]) decay [19]

6 Status Prospects and Conclusions

61 Status of the Majorana Demonstrator Project TheMajorana collaboration obtained beneficial occupancy ofits Davis Campus underground laboratories in May 2012 Atthe time of this writing in June 2013 the collaboration hascompleted outfitting the labs established cleanliness (thedetector room is typically better than 500 particlesft3 ofdiameter 05 120583m or smaller) and is proceeding with theconstruction and assembly of the array The undergroundelectroforming laboratories which started operation in thesummer of 2011 have now produced more than 75 of therequired copper The 425 kg of 86 enriched 76Ge has beenreduced from GeO

2and refined to electronic grade Ge with

a yield of 98 Ten enriched P-PC detectors with a total massof 95 kg have been produced by ORTEC with eight nowunderground to SURF A prototype cryostat the same as the

ultraclean cryostats but fabricated from commercial copperhas been assembled and operated with its associated vacuumsystem Two strings of natural Ge detectors have been builtin the glove boxes and are undergoing testing before they areinstalled in the prototype cryostat Cryostat 1 has beenmachined from the UGEFCu Samples obtained from allmaterials being used in the Demonstrator are beingassayed Slow-control systems and their associated sensorsare in continuous operation in all UG laboratories Dataacquisition for detector acceptance testing string testing andthe main array are operational

The prototype cryostat will be commissioned in thesummer of 2013 Cryostat 1 which will contain seven stringsof both enriched and natural Ge detectors is scheduled tobe completed in late 2013 Cryostat 2 which is expected tocontain all enriched detectors is expected to be assembledin 2014 The full array should be in operation in 2015 TheDemonstrator will be operated for about 3 years in orderto collect sim100 kg-years of exposure

62 The Future Large-Scale Experiment TheMajorana andGERDA collaborations are working together to prepare fora single-tonne-scale 76Ge experiment that will combine thebest technical features of both experiments The results ofthe two experiments will be used to determine the best pathforward

The present generation of experiments will likely produceresults with limits on ⟨119898

120573120573⟩ below about 100meV The next

generationwill strive to cover the inverted hierarchy region ofthe effective Majorana neutrino mass To accomplish this


Background-free01 countsROIty

1 countROIty4 countsROIty

103

102

101010minus3 10minus2 10minus1 1

1025

1026

1027

1028

Exposure (ton-years)

m120573120573

sens

itivi

ty Mod phys lett A 21 (2006) p 1547(3120590) (130ndash355)times 1025 years

(90

CL

Q

RPA

NM

E) (m

eV)

Inverted hierarchy (ml rarr 0 eV)

76G

eT0

12

sens

itivi

ty (9

0

CL

) (y

ears

)

Figure 14 90CL sensitivity as a function of exposure for 120573120573(0])-decay searches in 76Geunder different background scenariosMatrixelements from [71] were used to convert half-life to neutrino massThe blue band shows the region where a signal would be detected ifthe recent claim [19] is correct

an experiment will require ⟨119898120573120573⟩ sensitivity down to about

20meV Such small neutrino masses would indicate a half-life longer than 1027 y As seen in Figure 14 to observe sucha long half-life one will need a tonne or more of isotope andbackgrounds below 1 cnt(ROI-t-y)

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors acknowledge support from the Office of NuclearPhysics in the DOE Office of Science under Grant nosDE-AC02-05CH11231 DE-FG02-97ER41041 DE-FG02-97ER41033 DE-FG02-97ER4104 DE-FG02-97ER41042DE-SCOO05054 DE-FG02-10ER41715 and DE-FG02-97ER41020 They also acknowledge support from theParticle and Nuclear Astrophysics Program of the NationalScience Foundation through Grant nos PHY-0919270 PHY-1003940 0855314 PHY-1202950 MRI 0923142 and 1003399They gratefully acknowledge support from the RussianFoundation for Basic Research Grant no 12-02-12112 andthe support of the US Department of Energy through theLANLLDRD Program

References

[1] Y Ashie J Hosaka and K Ishihara ldquoEvidence for an oscillatorysignature in atmospheric neutrino oscillationsrdquo Physical ReviewLetters vol 93 no 10 Article ID 101801 6 pages 2004

[2] S N Ahmed A E Anthony and E W Beier ldquoMeasurement ofthe total active 8B solar neutrino flux at the Sudbury NeutrinoObservatorywith enhancedneutral current sensitivityrdquoPhysicalReview Letters vol 92 no 18 Article ID 181301 6 pages 2004

[3] S Abe T Ebihara and S Enomoto ldquoPrecision measurementof neutrino oscillation parameters with KamLANDrdquo PhysicalReview Letters vol 100 no 22 Article ID 221803 5 pages 2008

[4] E Majorana ldquoTeoria simmetrica dellrsquoelettrone e del positronerdquoNuovo Cimento vol 5 no 4 pp 171ndash184 1937 English Transla-tion in Soryushiron Kenkyu vol 63 p 149 1981

[5] T Yanagida ldquoHorizontal gauge symmetry and masses of neu-trinosrdquo in Proceedings of the Workshop on Unified Theory andBaryon Number in the Universe O Sawada and A SugamotoEds KEK Tsukuba Japan 1979

[6] M Gell-Mann P Ramond and R Slansky Supergravity North-Holland Amsterdam The Netherlands 1979

[7] M Fukugita and T Yanagida ldquoBarygenesis without grand uni-ficationrdquo Physics Letters B vol 174 no 1 pp 45ndash47 1986

[8] P Di Bari ldquoAn introduction to leptogenesis and neutrino prop-ertiesrdquo Contemporary Physics vol 53 no 4 pp 315ndash338 2012

[9] M Zralek ldquoOn the possibilities of distinguishing dirac fromMajorana neutrinosrdquoActa Physica Polonica B vol 28 no 11 pp2225ndash2257 1997

[10] J Schechter and J W F Valle ldquoNeutrinoless double-120573 decayin SU(2)timesU(1) theoriesrdquo Physical Review D vol 25 no 11 pp2951ndash2954 1982

[11] L Baudis A Dietz G Heusser et al ldquoLimits on the Majorananeutrino mass in the 01 eV rangerdquo Physical Review Letters vol83 no 1 pp 41ndash44 1999

[12] C E Aalseth F T Avignone III R L Brodzinski et al ldquoNeu-trinoless double-120573 decay of 76Ge first results from the Inter-national Germanium Experiment (IGEX) with six isotopicallyenriched detectorsrdquo Physical Review C vol 59 no 4 pp 2108ndash2113 1999

[13] C E Aalseth F T Avignone III R L Brodzinski et al ldquoIGEX76Ge neutrinoless double-beta decay experiment prospects fornext generation experimentsrdquo Physical Review D vol 65 no 9Article ID 092007 6 pages 2002

[14] C E Aalseth F T Avignone III R L Brodzinski et al ldquoTheIGEX experiment reexamined a response to the critiqueof Klapdor-Kleingrothaus Dietz and Krivosheinardquo PhysicalReview D vol 70 no 7 Article ID 078302 5 pages 2004

[15] N Ackerman B Aharmim M Auger et al ldquoObservation oftwo-neutrino double-beta decay in 136Xe with the EXO-200detectorrdquo Physical Review Letters vol 107 no 21 Article ID212501 5 pages 2011

[16] M Auger D J Auty P S Barbeau et al ldquoSearch for neutrinolessdouble-beta decay in 136Xe with EXO-200rdquo Physical ReviewLetters vol 109 no 3 Article ID 032505 6 pages 2012

[17] A Gando Y Gando H Hanakago et al ldquoMeasurement of thedouble-120573 decay half-life of 136Xe with the KamLAND-ZenexperimentrdquoPhysical ReviewC vol 85 no 4 Article ID 0455046 pages 2012

[18] A Gando Y Gando H Hanakago et al ldquoLimit on neutrinoless120573120573 decay of 136Xe from the first phase of KamLAND-Zen andcomparison with the positive claim in 76Gerdquo Physical ReviewLetters vol 110 no 6 Article ID 062502 5 pages 2013

[19] H V Klapdor-Kleingrothaus and I V Krivosheina ldquoThe evi-dence for the observation of 0]120573120573 decay the identification of0]120573120573 events from the full spectrardquo Modern Physics Letters Avol 21 no 20 pp 1547ndash1566 2006

[20] C E Aalseth F T Avignone III A Barabash et al ldquoCommenton ldquoevidence for neutrinoless double beta decayrdquordquo ModernPhysics Letters A vol 17 no 22 p 1475 2002


[21] Y G Zdesenko F A Danevich and V I Tretyak ldquoHas neutri-noless double 120573 decay of 76Ge been really observedrdquo PhysicsLetters B vol 546 no 3-4 pp 206ndash215 2002

[22] F Feruglio A Strumia and F Vissani ldquoNeutrino oscillationsand signals in 120573 and 0]2120573 experimentsrdquo Nuclear Physics B vol637 no 1ndash3 pp 345ndash377 2002

[23] M Agostini M Allardt E Andreotti et al ldquoResults on neu-trinoless double-120573 decay of 76Ge from phase I of the GERDAexperimentrdquo Physical Review Letters vol 111 no 12 Article ID122503 6 pages 2013

[24] M Agostini M Allardt E Andreotti et al ldquoThe background inthe neutrinoless double beta decay experiment GERDArdquo httparxivorgabs13065084

[25] M Agostini M Allardt E Andreotti et al ldquoPulse shape dis-crimination for GERDA phase I datardquo The European PhysicalJournal C vol 73 article 2583 2013

[26] S R Elliott and P Vogel ldquoDouble beta decay rdquoAnnual Review ofNuclear and Particle Science vol 52 pp 115ndash151 2002

[27] S R Elliott and J Engel ldquoDouble-beta decayrdquo Journal of PhysicsG vol 30 no 9 article 183 2004

[28] A S Barabash ldquoDouble-beta-decay experiments present statusand prospects for the futurerdquo Physics of Atomic Nuclei vol 67no 3 pp 438ndash452 2004

[29] H Ejiri ldquoDouble beta decays and neutrino massesrdquo Journal ofthe Physical Society of Japan vol 74 pp 2101ndash2127 2005

[30] F T Avignone III G S King III and Y G Zdesenko ldquoNextgeneration double-beta decay experiments metrics for theirevaluationrdquo New Journal of Physics vol 7 article 6 no 1 2005

[31] F T Avignone III S R Elliott and J Engel ldquoDouble beta decayMajorana neutrinos and neutrino massrdquo Reviews of ModernPhysics vol 80 no 2 pp 481ndash516 2008

[32] A S Barabash ldquoDouble beta decay experimentsrdquo Physics ofParticles and Nuclei vol 42 no 4 pp 613ndash627 2011

[33] W Rodejohann ldquoNeutrino-less double beta decay and particlephysicsrdquo International Journal of Modern Physics E vol 20 no9 article 1833 2011

[34] S R Elliott ldquoRecent progress in double beta decayrdquo ModernPhysics Letters A vol 27 no 7 Article ID 1230009 16 pages 2012

[35] J D Vergados H Ejiri and F Simkovic ldquoTheory of neutrinolessdouble-beta decayrdquoReports on Progress in Physics vol 75 no 10Article ID 106301 2012

[36] J Beringer J-F Arguin R M Barnett et al ldquoReview of particlephysicsrdquoPhysical ReviewD vol 86 no 1 Article ID 010001 1528pages 2012

[37] K-H Ackermann M Agostini M Allardt et al ldquoThe GERDAexperiment for the search of 0]120573120573 decay in 76GerdquoThe EuropeanPhysical Journal C vol 73 article 2330 2013

[38] C E Aalseth P S Barbeau D G Cerdeno et al ldquoExperimentalconstraints on a dark matter origin for the dama annual modu-lation effectrdquo Physical Review Letters vol 101 no 25 Article ID251301 4 pages 2008

[39] C E Aalseth P S Barbeau N S Bowden et al ldquoResults from asearch for light-mass dark matter with a p-type point contactgermanium detectorrdquo Physical Review Letters vol 106 no 13Article ID 131301 4 pages 2011

[40] C E Aalseth P S Barbeau J Colaresi et al ldquoSearch for anannual modulation in a p-type point contact germanium darkmatter detectorrdquo Physical Review Letters vol 107 no 14 ArticleID 141301 5 pages 2011

[41] CE AalsethM Amman F T Avignone III et al ldquoAstroparticlephysics with a customized low-background broad energy ger-manium detectorrdquo Nuclear Instruments and Methods in PhysicsResearch A vol 652 no 1 pp 692ndash695 2011

[42] R Agnese Z Ahmed AJ Anderson et al ldquoSilicon detectordark matter results from the final exposure of CDMS IIrdquoPhysical Review Letters vol 111 no 25 Article ID 251301 6pages 2013

[43] G K Giovanetti E Aguayo F T Avignone et al ldquoDark mattersensitivities of the Majorana Demonstratorrdquo Journal of Physicsvol 375 no 1 Article ID 012014 2012

[44] F T Avignone III D Abriola R L Brodzinski et al ldquoExper-imental search for solar axions via coherent Primakoff conver-sion in a germanium spectrometerrdquo Physical Review Letters vol81 no 23 pp 5068ndash5071 1998

[45] R J Creswick F T Avignone III H A Farach et al ldquoTheoryfor the direct detection of solar axions by coherent Primakoffconversion in germanium detectorsrdquo Physics Letters B vol 427no 3-4 pp 235ndash240 1998

[46] A V Derbin A S Kayunov V V Muratova et al ldquoConstraintson the axion-electron coupling for solar axions produced by aCompton process and bremsstrahlungrdquo Physical Review D vol83 no 2 Article ID 023505 7 pages 2011

[47] F T Avignone III ldquoPotential for large germanium detectorarrays for solar-axion searches utilizing the axio-electric effectfor detectionrdquo Physical Review D vol 79 no 3 Article ID035015 5 pages 2009

[48] S R Elliott B H LaRoque V M Gehman M F Kidd and MChen ldquoAn improved limit on Pauli-exclusion-principle forbid-den atomic transitionsrdquo Foundations of Physics vol 42 no 8pp 1015ndash1030 2012

[49] P S Barbeau J I Collar and O Tench ldquoLarge-mass ultralownoise germanium detectors performance and applications inneutrino and astroparticle physicsrdquo Journal of Cosmology andAstroparticle Physics vol 2007 no 9 article 009 2007

[50] K Scholberg ldquoProspects for measuring coherent neutrino-nucleus elastic scattering at a stopped-pion neutrino sourcerdquoPhysical Review D vol 73 no 3 Article ID 033005 9 pages2006

[51] A J Anderson J M Conrad E Figueroa-Feliciano K Schol-berg and J Spitz ldquoCoherent neutrino scattering in dark matterdetectorsrdquo Physical Review D vol 84 no 1 Article ID 0130082011

[52] P N Luke F S Goulding N W Madden and R H Pehl ldquoLowcapacitance large volume shaped-field germanium detectorrdquoIEEE Transactions on Nuclear Science vol 36 no 1 pp 926ndash930 1989

[53] ldquoBroad energy Ge detectorsrdquo Canberra Industries MeridenConn USA 2009

[54] ldquoORTECrdquo Oak Ridge Tenn USA 2009[55] D Budjas M B Heider O Chkvorets N Khanbekov and S

Schonert ldquoPulse shape discrimination studies with a broad-energy germanium detector for signal identification and back-ground suppression in the GERDA double beta decay exper-imentrdquo Journal of Instrumentation vol 4 no 10 Article IDP10007 2009

[56] R Cooper DC Radford K Lagergren et al ldquoA Pulse shapeanalysis technique for the Majorana experimentrdquo NuclearInstruments and Methods in Physics Research A vol 629 no 1pp 303ndash310 2011


[57] E Aguayo M Amman FT Avignone III et al ldquoCharacteristicsof signals originating near the lithium-diffused N+ contact ofhigh purity germanium p-type point contact detectorsrdquoNuclearInstruments andMethods in Physics ResearchA vol 701 pp 176ndash185 2013

[58] P Finnerty A direct dark matter search with the Majorana low-background broad energy germanium detector [PhD thesis]University of North Carolina Chapel Hill NC USA 2013

[59] E Aguayo J E Fast R T Kouzes and J L Orrell ldquoThe spl mu-witness detector a ruggedized portable flux meter for cos-mogenic activation monitoringrdquo IEEE Transactions on NuclearScience vol 60 no 2 pp 689ndash692 2013

[60] E W Hoppe E E Mintzer C E Aalseth et al ldquoMicroscopicevaluation of contaminants in ultra-high purity copperrdquo Journalof Radioanalytical and Nuclear Chemistry vol 282 no 1 pp315ndash320 2009

[61] N R Overman D J Edwards C T Overman EWHoppe andT A Kafentzis ldquoMajorana electroformed copper mechanicalanalysisrdquo Tech Rep PNNL-21315 Pacific Northwest NationalLaboratory 2012

[62] E AguayoM Busch R Daniels J E Fast M P Green andD JReid ldquoThe design of an ultra-low background thermosyphonfor the Majorana Demonstratorrdquo Nuclear Instruments andMethods in Physics Research A vol 709 pp 17ndash21 2013

[63] P Barton P Luke M Amman et al ldquoLow-noise low-massfront end electronics for low-background physics experimentsusing germanium detectorsrdquo in Proceedings of the IEEE NuclearScience and Medical Imaging Conference (NSSMIC rsquo11) pp1976ndash1979 Valencia Spain 2011

[64] S Zimmermann JT Anderson D Doering et al ldquoImple-mentation and performance of the electronics and computingsystem of the gamma ray energy tracking in-beam nuclear array(GRETINA)rdquo IEEE Transactions on Nuclear Science vol 59 no5 pp 2494ndash2500 2012

[65] M A Howe G A Cox P J Harvey et al ldquoSudbury neu-trino observatory neutral current detector acquisition softwareoverviewrdquo IEEE Transactions on Nuclear Science vol 51 no 3pp 878ndash883 2004

[66] J Heise ldquoPrivate communicationrdquo 2013[67] S Agostinelli J Allison K Amako et al ldquoGeant4mdasha simula-

tion toolkitrdquo Nuclear Instruments and Methods in PhysicsResearch A vol 506 no 3 pp 250ndash303 2003

[68] J Allison K Amako J Apostolakis et al ldquoGeant4 develop-ments and applicationsrdquo IEEE Transactions on Nuclear Sciencevol 53 no 1 pp 270ndash278 2006

[69] M Boswell Y-D Chan J A Detwiler et al ldquoMaGe-a Geant4-based Monte Carlo application framework for low-backgroundgermanium experimentsrdquo IEEE Transactions on Nuclear Sci-ence vol 58 no 3 pp 1212ndash1220 2011

[70] G J Feldman and R D Cousins ldquoUnified approach to the clas-sical statistical analysis of small signalsrdquo Physical Review D vol57 no 7 pp 3873ndash3889 1998

[71] F Simkovic A Faessler H Muther V Rodin and M Staufldquo0]120573120573-decay nuclear matrix elements with self-consistentshort-range correlationsrdquo Physical Review C vol 79 no 5 Arti-cle ID 055501 10 pages 2009

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

High Energy PhysicsAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


FluidsJournal of

Atomic and Molecular Physics

Journal of



Advances in Condensed Matter Physics

OpticsInternational Journal of



AstronomyAdvances in

International Journal of


Superconductivity


Statistical MechanicsInternational Journal of


GravityJournal of


AstrophysicsJournal of


Physics Research International


Solid State PhysicsJournal of

Computational Methods in Physics

Journal of



Soft MatterJournal of

Hindawi Publishing Corporationhttpwwwhindawicom

AerodynamicsJournal of

Volume 2014


PhotonicsJournal of


Journal of

Biophysics


ThermodynamicsJournal of


Correspondence should be addressed to S R Elliott elliottslanlgov

Received 28 June 2013 Accepted 7 August 2013 Published 29 January 2014

Academic Editor Vincenzo Flaminio

Copyright copy 2014 N Abgrall et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited Thepublication of this article was funded by SCOAP3

The Majorana Demonstrator will search for the neutrinoless double-beta (120573120573(0])) decay of the isotope 76Ge with a mixedarray of enriched and natural germanium detectors The observation of this rare decay would indicate that the neutrino is its ownantiparticle demonstrate that lepton number is not conserved and provide information on the absolute mass scale of the neutrinoTheDemonstrator is being assembled at the 4850-foot level of the SanfordUnderground Research Facility in Lead SouthDakotaThe array will be situated in a low-background environment and surrounded by passive and active shielding Here we describe thescience goals of the Demonstrator and the details of its design

1 Introduction

11 Neutrinoless Double-Beta Decay Despite being discov-ered well over a decade ago [1ndash3] the incorporation of neu-trino mass and mixing into the standard model (SM) of par-ticle physics remains elusive A minimalistic Higgs couplingleaves the SM fine-tuned with the neutrino masses lyingsome 6 orders-of-magnitude or more below that of theother SM leptons Avoiding such unnaturalness requires newphysics One highly attractive option afforded by the electricneutrality of the neutrino is the addition of a lepton-number-violating Majorana mass term [4] Majorana neutrinos havethe novel property that particle and antiparticle are distin-guished only by chirality A Majorana mass term provides anatural explanation for the lightness of the SM neutrino viathe seesaw mechanism [5 6] Majorana neutrinos also pro-vide plausible scenarios for leptogenesis capable of account-ing for the excess of matter over antimatter in the observableuniverse [7 8]

Neutrinoless double-beta (120573120573(0])) decay searches rep-resent the only viable experimental method for testing theMajorana nature of neutrinos [9]Theobservation of this pro-cess would immediately imply that lepton number is violatedand that neutrinos areMajorana particles [10]The decay ratemay be written as

(1198790]12)minus1

= 1198660]10038161003816100381610038161198720]10038161003816100381610038162

(⟨119898120573120573⟩

119898119890

)

2

(1)

where 1198660] is a phase space factor including the couplings1198720] is a nuclear matrix element119898

119890is the electron mass and

⟨119898120573120573⟩ is the effective Majorana neutrino mass The latter is

given by

⟨119898120573120573⟩ =

1003816100381610038161003816100381610038161003816100381610038161003816

3

sum119894=1

1198802

119890119894119898119894

1003816100381610038161003816100381610038161003816100381610038161003816

(2)

where119880119890119894specifies the admixture of neutrinomass eigenstate

119894 in the electron neutrinoUntil very recently the most sensitive limits on 120573120573(0])

decay came from Ge detectors enriched in 76Ge namely

theHeidelberg-Moscow experiment [11] and the IGEX exper-iment [12ndash14] Recent results from the EXO-200 experiment[15 16] and from the KamLAND-Zen experiment [17 18]have claimed stronger bounds on neutrinomass It is difficulthowever to determine the best limit because of the uncertain-ties in the nuclear matrix elements Previous-generation 76Geexperiments also yielded a claim of the direct observation of120573120573(0]) decay by Klapdor-Kleingrothaus [19] This claim hasnot been widely accepted by the neutrino community [20ndash22] While the EXO-200 and KamLAND-Zen results are inconflict with this claim the recent results from GERDA [23ndash25] show that the observed peak is not an indication of120573120573(0])decay in a 76Ge experiment which can compare its resultswith the Klapdor-Kleingrothaus claim without dependingon the nuclear matrix elements For recent comprehensiveexperimental and theoretical reviews see [26ndash35]

A measurement of the 120573120573(0]) decay rate would yieldinformation on the absolute neutrino mass Measurementsof atmospheric solar and reactor neutrino oscillation [36]indicate a large parameter space for discovery of120573120573(0]) decayjust beyond current experimental bounds below ⟨119898

120573120573⟩ sim

50meV Moreover evidence from the SNO experiment [2] ofa clear departure from nonmaximal mixing in solar neutrinooscillation implies a minimum effective Majorana neutrinomass of sim15meV for the inverted hierarchy scenario Thistarget is within reach of next-generation 120573120573(0]) searches Anexperiment capable of observing this minimum rate wouldtherefore definitively determine theMajorana orDirac natureof the neutrino for inverted hierarchical neutrino masses

Recent developments in germanium detector technologymake a 120573120573(0])-decay search feasible using 76Ge In thispaper we describe the Majorana Demonstrator as anexperimental effort under construction in the SanfordUnder-ground Research Facility (SURF) whose goal is to demon-strate the techniques required for a definitive next-generation120573120573(0])-decay experiment with enriched Ge detectors TheDemonstrator will also test the Klapdor-Kleingrothausclaim and will be sensitive to other non-120573120573(0]) physicssignals in Ge A complementary effort in Ge with similar sen-sitivity the GERDA experiment [37] is presently operating
























20

10

0

009

008

007

006

005

004

003

0020 10 20 30 40 50

Radi

us (m

m)

lt100gt

lt110gt

lt001gt

minus10

minus20

300 600

(ns)900

Z (mm)









0

100

200

300

Time (ns)

Char

ge si

gnal

200 600 1000 1400

0

20

40

60

Time (ns)200 600 1000 1400

Curr

ent s

igna

l

70

(a)

0

100

200

300

400

0

20

40

60

80

Time (ns)200 600 1000 1400

Time (ns)200 600 1000 1400

Char

ge si

gnal

Curr

ent s

igna

l

(b)






2





20

60

100

140

180

Cou

nts


1450 1550 1650 1750 1850

Gamma energy (keV)
















HV ring

HV nut

Hollow hex rod

Crystal insulator

CMP

Contact pinbushing

Contact pin

Hex nut

LMFE

Spring clip


Adaptor plate bolt

Adaptor plate nut


Tapered nut

Bottom nut

Tie rod




















2plumbing



Detector strings

LN dewar

Ballast tank


Vacuum system

Vacuum vessel

Condenser




















Detector








Veto electronics

Digitizers

SBC

VME crate

ORCAAG data storage

ORCARoot


control

GPS


Clocksync

Preampspulser

Low gain

High gain

Veto

Det

ecto

r str

ings

HV

HV









Outer Cushield


Inner Cushield

Lead bricks

Poly shield









Yates shaft

Davis CavernLUX

Davis Campus

80


































0 01 02 03 04 05 06 07 08 09 1

08880480

0195

02220270

0067

00300088

00550100

00540210

0170

01300030

0011




Front endsGe (UTh)


60Co (Cu)External 120574 (120572 n)

Rn surface 120572

Ge(n n)Ge(n 120574)


Ge Cu Pb (n n998400120574)
















103

102


1025

1026

1027

1028


m120573120573

sens

itivi


(90

CL

Q

RPA

NM

E) (m

eV)


76G

eT0

12

sens

itivi

ty (9

0

CL

) (y

ears

)






Acknowledgments


References















































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics


























20

10

0

009

008

007

006

005

004

003

0020 10 20 30 40 50

Radi

us (m

m)

lt100gt

lt110gt

lt001gt

minus10

minus20

300 600

(ns)900

Z (mm)









0

100

200

300

Time (ns)

Char

ge si

gnal

200 600 1000 1400

0

20

40

60

Time (ns)200 600 1000 1400

Curr

ent s

igna

l

70

(a)

0

100

200

300

400

0

20

40

60

80

Time (ns)200 600 1000 1400

Time (ns)200 600 1000 1400

Char

ge si

gnal

Curr

ent s

igna

l

(b)






2





20

60

100

140

180

Cou

nts


1450 1550 1650 1750 1850

Gamma energy (keV)
















HV ring

HV nut

Hollow hex rod

Crystal insulator

CMP

Contact pinbushing

Contact pin

Hex nut

LMFE

Spring clip


Adaptor plate bolt

Adaptor plate nut


Tapered nut

Bottom nut

Tie rod




















2plumbing



Detector strings

LN dewar

Ballast tank


Vacuum system

Vacuum vessel

Condenser




















Detector








Veto electronics

Digitizers

SBC

VME crate

ORCAAG data storage

ORCARoot


control

GPS


Clocksync

Preampspulser

Low gain

High gain

Veto

Det

ecto

r str

ings

HV

HV









Outer Cushield


Inner Cushield

Lead bricks

Poly shield









Yates shaft

Davis CavernLUX

Davis Campus

80


































0 01 02 03 04 05 06 07 08 09 1

08880480

0195

02220270

0067

00300088

00550100

00540210

0170

01300030

0011




Front endsGe (UTh)


60Co (Cu)External 120574 (120572 n)

Rn surface 120572

Ge(n n)Ge(n 120574)


Ge Cu Pb (n n998400120574)
















103

102


1025

1026

1027

1028


m120573120573

sens

itivi


(90

CL

Q

RPA

NM

E) (m

eV)


76G

eT0

12

sens

itivi

ty (9

0

CL

) (y

ears

)






Acknowledgments


References















































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




0

100

200

300

Time (ns)

Char

ge si

gnal

200 600 1000 1400

0

20

40

60

Time (ns)200 600 1000 1400

Curr

ent s

igna

l

70

(a)

0

100

200

300

400

0

20

40

60

80

Time (ns)200 600 1000 1400

Time (ns)200 600 1000 1400

Char

ge si

gnal

Curr

ent s

igna

l

(b)






2





20

60

100

140

180

Cou

nts


1450 1550 1650 1750 1850

Gamma energy (keV)
















HV ring

HV nut

Hollow hex rod

Crystal insulator

CMP

Contact pinbushing

Contact pin

Hex nut

LMFE

Spring clip


Adaptor plate bolt

Adaptor plate nut


Tapered nut

Bottom nut

Tie rod




















2plumbing



Detector strings

LN dewar

Ballast tank


Vacuum system

Vacuum vessel

Condenser




















Detector








Veto electronics

Digitizers

SBC

VME crate

ORCAAG data storage

ORCARoot


control

GPS


Clocksync

Preampspulser

Low gain

High gain

Veto

Det

ecto

r str

ings

HV

HV









Outer Cushield


Inner Cushield

Lead bricks

Poly shield









Yates shaft

Davis CavernLUX

Davis Campus

80


































0 01 02 03 04 05 06 07 08 09 1

08880480

0195

02220270

0067

00300088

00550100

00540210

0170

01300030

0011




Front endsGe (UTh)


60Co (Cu)External 120574 (120572 n)

Rn surface 120572

Ge(n n)Ge(n 120574)


Ge Cu Pb (n n998400120574)
















103

102


1025

1026

1027

1028


m120573120573

sens

itivi


(90

CL

Q

RPA

NM

E) (m

eV)


76G

eT0

12

sens

itivi

ty (9

0

CL

) (y

ears

)






Acknowledgments


References















































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




20

60

100

140

180

Cou

nts


1450 1550 1650 1750 1850

Gamma energy (keV)
















HV ring

HV nut

Hollow hex rod

Crystal insulator

CMP

Contact pinbushing

Contact pin

Hex nut

LMFE

Spring clip


Adaptor plate bolt

Adaptor plate nut


Tapered nut

Bottom nut

Tie rod




















2plumbing



Detector strings

LN dewar

Ballast tank


Vacuum system

Vacuum vessel

Condenser




















Detector








Veto electronics

Digitizers

SBC

VME crate

ORCAAG data storage

ORCARoot


control

GPS


Clocksync

Preampspulser

Low gain

High gain

Veto

Det

ecto

r str

ings

HV

HV









Outer Cushield


Inner Cushield

Lead bricks

Poly shield









Yates shaft

Davis CavernLUX

Davis Campus

80


































0 01 02 03 04 05 06 07 08 09 1

08880480

0195

02220270

0067

00300088

00550100

00540210

0170

01300030

0011




Front endsGe (UTh)


60Co (Cu)External 120574 (120572 n)

Rn surface 120572

Ge(n n)Ge(n 120574)


Ge Cu Pb (n n998400120574)
















103

102


1025

1026

1027

1028


m120573120573

sens

itivi


(90

CL

Q

RPA

NM

E) (m

eV)


76G

eT0

12

sens

itivi

ty (9

0

CL

) (y

ears

)






Acknowledgments


References















































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




HV ring

HV nut

Hollow hex rod

Crystal insulator

CMP

Contact pinbushing

Contact pin

Hex nut

LMFE

Spring clip


Adaptor plate bolt

Adaptor plate nut


Tapered nut

Bottom nut

Tie rod




















2plumbing



Detector strings

LN dewar

Ballast tank


Vacuum system

Vacuum vessel

Condenser




















Detector








Veto electronics

Digitizers

SBC

VME crate

ORCAAG data storage

ORCARoot


control

GPS


Clocksync

Preampspulser

Low gain

High gain

Veto

Det

ecto

r str

ings

HV

HV









Outer Cushield


Inner Cushield

Lead bricks

Poly shield









Yates shaft

Davis CavernLUX

Davis Campus

80


































0 01 02 03 04 05 06 07 08 09 1

08880480

0195

02220270

0067

00300088

00550100

00540210

0170

01300030

0011




Front endsGe (UTh)


60Co (Cu)External 120574 (120572 n)

Rn surface 120572

Ge(n n)Ge(n 120574)


Ge Cu Pb (n n998400120574)
















103

102


1025

1026

1027

1028


m120573120573

sens

itivi


(90

CL

Q

RPA

NM

E) (m

eV)


76G

eT0

12

sens

itivi

ty (9

0

CL

) (y

ears

)






Acknowledgments


References















































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics















2plumbing



Detector strings

LN dewar

Ballast tank


Vacuum system

Vacuum vessel

Condenser




















Detector








Veto electronics

Digitizers

SBC

VME crate

ORCAAG data storage

ORCARoot


control

GPS


Clocksync

Preampspulser

Low gain

High gain

Veto

Det

ecto

r str

ings

HV

HV









Outer Cushield


Inner Cushield

Lead bricks

Poly shield









Yates shaft

Davis CavernLUX

Davis Campus

80


































0 01 02 03 04 05 06 07 08 09 1

08880480

0195

02220270

0067

00300088

00550100

00540210

0170

01300030

0011




Front endsGe (UTh)


60Co (Cu)External 120574 (120572 n)

Rn surface 120572

Ge(n n)Ge(n 120574)


Ge Cu Pb (n n998400120574)
















103

102


1025

1026

1027

1028


m120573120573

sens

itivi


(90

CL

Q

RPA

NM

E) (m

eV)


76G

eT0

12

sens

itivi

ty (9

0

CL

) (y

ears

)






Acknowledgments


References















































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




Detector strings

LN dewar

Ballast tank


Vacuum system

Vacuum vessel

Condenser




















Detector








Veto electronics

Digitizers

SBC

VME crate

ORCAAG data storage

ORCARoot


control

GPS


Clocksync

Preampspulser

Low gain

High gain

Veto

Det

ecto

r str

ings

HV

HV









Outer Cushield


Inner Cushield

Lead bricks

Poly shield









Yates shaft

Davis CavernLUX

Davis Campus

80


































0 01 02 03 04 05 06 07 08 09 1

08880480

0195

02220270

0067

00300088

00550100

00540210

0170

01300030

0011




Front endsGe (UTh)


60Co (Cu)External 120574 (120572 n)

Rn surface 120572

Ge(n n)Ge(n 120574)


Ge Cu Pb (n n998400120574)
















103

102


1025

1026

1027

1028


m120573120573

sens

itivi


(90

CL

Q

RPA

NM

E) (m

eV)


76G

eT0

12

sens

itivi

ty (9

0

CL

) (y

ears

)






Acknowledgments


References















































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics










Detector








Veto electronics

Digitizers

SBC

VME crate

ORCAAG data storage

ORCARoot


control

GPS


Clocksync

Preampspulser

Low gain

High gain

Veto

Det

ecto

r str

ings

HV

HV









Outer Cushield


Inner Cushield

Lead bricks

Poly shield









Yates shaft

Davis CavernLUX

Davis Campus

80


































0 01 02 03 04 05 06 07 08 09 1

08880480

0195

02220270

0067

00300088

00550100

00540210

0170

01300030

0011




Front endsGe (UTh)


60Co (Cu)External 120574 (120572 n)

Rn surface 120572

Ge(n n)Ge(n 120574)


Ge Cu Pb (n n998400120574)
















103

102


1025

1026

1027

1028


m120573120573

sens

itivi


(90

CL

Q

RPA

NM

E) (m

eV)


76G

eT0

12

sens

itivi

ty (9

0

CL

) (y

ears

)






Acknowledgments


References















































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




Veto electronics

Digitizers

SBC

VME crate

ORCAAG data storage

ORCARoot


control

GPS


Clocksync

Preampspulser

Low gain

High gain

Veto

Det

ecto

r str

ings

HV

HV









Outer Cushield


Inner Cushield

Lead bricks

Poly shield









Yates shaft

Davis CavernLUX

Davis Campus

80


































0 01 02 03 04 05 06 07 08 09 1

08880480

0195

02220270

0067

00300088

00550100

00540210

0170

01300030

0011




Front endsGe (UTh)


60Co (Cu)External 120574 (120572 n)

Rn surface 120572

Ge(n n)Ge(n 120574)


Ge Cu Pb (n n998400120574)
















103

102


1025

1026

1027

1028


m120573120573

sens

itivi


(90

CL

Q

RPA

NM

E) (m

eV)


76G

eT0

12

sens

itivi

ty (9

0

CL

) (y

ears

)






Acknowledgments


References















































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




Outer Cushield


Inner Cushield

Lead bricks

Poly shield









Yates shaft

Davis CavernLUX

Davis Campus

80


































0 01 02 03 04 05 06 07 08 09 1

08880480

0195

02220270

0067

00300088

00550100

00540210

0170

01300030

0011




Front endsGe (UTh)


60Co (Cu)External 120574 (120572 n)

Rn surface 120572

Ge(n n)Ge(n 120574)


Ge Cu Pb (n n998400120574)
















103

102


1025

1026

1027

1028


m120573120573

sens

itivi


(90

CL

Q

RPA

NM

E) (m

eV)


76G

eT0

12

sens

itivi

ty (9

0

CL

) (y

ears

)






Acknowledgments


References















































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics






























0 01 02 03 04 05 06 07 08 09 1

08880480

0195

02220270

0067

00300088

00550100

00540210

0170

01300030

0011




Front endsGe (UTh)


60Co (Cu)External 120574 (120572 n)

Rn surface 120572

Ge(n n)Ge(n 120574)


Ge Cu Pb (n n998400120574)
















103

102


1025

1026

1027

1028


m120573120573

sens

itivi


(90

CL

Q

RPA

NM

E) (m

eV)


76G

eT0

12

sens

itivi

ty (9

0

CL

) (y

ears

)






Acknowledgments


References















































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics






103

102


1025

1026

1027

1028


m120573120573

sens

itivi


(90

CL

Q

RPA

NM

E) (m

eV)


76G

eT0

12

sens

itivi

ty (9

0

CL

) (y

ears

)






Acknowledgments


References















































































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics





























































FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics



Research Article The M ajorana emonstrator Neutrinoless ...downloads.hindawi.com › journals ›...

Documents

Transcript of Research Article The M ajorana emonstrator Neutrinoless ...downloads.hindawi.com › journals ›...