Ge Detectors and 0νββ : The Search for Double Beta Decay with Germanium Detectors: Past, Present and Future
TThe Search for Double Beta Decay withGermanium Detectors: Past, Present andFuture
Frank T. Avignone III , and Steven R. Elliott , ∗ University of South Carolina, Department of Physics, Columbia , SC, USA Los Alamos National Laboratory, Physics Division, Los Alamos , NM, USA
Correspondence*:Corresponding [email protected]
ABSTRACT
High Purity Germanium Detectors have excellent energy resolution; the best among thetechnologies used in double beta decay. Since neutrino-less double beta decay hinges on thesearch for a rare peak upon a background continuum, this strength has enabled the technologyto consistently provide leading results. The Ge crystals at the heart of these experimentsare very pure; they have no measurable U or Th contamination. The added efforts to reducethe background associated with electronics, cryogenic cooling, and shielding have been verysuccessful, leading to the longevity of productivity. The first experiment published in 1967 by theMilan group of Fiorini, established the benchmark half-life limit > × yr. This bound wasimproved with the early work of the USC-PNNL, UCSB and Milan groups yielding limits above yr. The Heidelberg-Moscow and USC-PNNL collaborations pioneered the use of enrichedGe for detector fabrication. Both groups also initiated techniques of analyzing pulse waveforms toreject γ -ray background. These steps extended the limits to just over yr. In 2000, a subsetof the Heidelberg-Moscow collaboration claimed the observation of double beta decay. Morerecently, the M AJORANA and GERDA collaborations have developed new detector technologiesthat optimize the pulse waveform analysis. As a result, the GERDA collaboration refuted the claimof observation with a revolutionary approach to shielding by immersing the detectors directly inradio-pure liquid argon. In 2018, the M
AJORANA collaboration, using a classic vacuum cryostatand high-Z shielding, achieved a background level near that of GERDA by developing very purematerials for use nearby the detectors. Together, GERDA and M
AJORANA have provided limitsapproaching yr. In this article, we elaborate on the historical use of Ge detectors for doublebeta decay addressing the strengths and weaknesses. We also summarize the status and futureas many M AJORANA and GERDA collaborators have joined with scientists from other efforts togive birth to the LEGEND collaboration. LEGEND will exploit the best features of both experimentsto extend the half-life limit beyond yr with a ton-scale experiment. Keywords: double beta decay, neutrino, Ge detectors, Majorana, Dirac
The very earliest calculation of the rate for two-neutrino double-beta decay ( νββ ) is credited to MariaGoeppert-Mayer who predicted the half-life of the decay of Te in 1935 [1]. In 1937, Ettore Majoranabuilt his theory in which neutrinos are their own anti-particles [2], and in 1939, Wendell Furry proposed a r X i v : . [ nu c l - e x ] J a n vignone and Elliott Ge Detectors and νββ testing Majorana’s theory by searching for neutrinoless double-beta decay ( νββ ) [3]. While there weremany early efforts to measure double beta decay in the laboratory, the first direct observation of νββ wasin Se by Elliott, Hahn and Moe [4] using a Time-Projection-Chamber. The next direct measurementsof νββ were made using Ge detectors [5, 6, 7]. However, as interesting these experiments were, themost important efforts in building low background Ge detectors were aimed at searching for νββ via thedecay, Ge → Se + 2e − . In this article we attempt to recall the main highlights of the history of thesedevelopments.Germanium detectors have many advantages and therefore have provided the most sensitive limits onthe νββ half-life ( T ν / ), and the effective Majorana neutrino mass ( (cid:104) m ββ (cid:105) ), for much of the recenthistory of neutrino physics. Recently, Xe experiments [8, 9] have been more restrictive, although Geremains highly competitive and the technology is poised to potentially regain its previous supremacy. Thesearch for νββ is fundamentally a search for a rare peak superimposed on a background continuum.Therefore, the excellent energy resolution of Ge detectors, the best of any ββ technology, provides highlysensitive discovery potential for the process. This technology also presently has the lowest backgroundwhen normalized to a resolution-defined region at the νββ Q-value ( Q ββ ). The detectors are made frompure metallic Ge resulting in a high atomic density and therefore a relatively large number of atoms per kgof detector. Other benefits include the detectors being mostly insensitive to surface activity and the modestcryogenic requirements of liquid nitrogen temperatures. The technology is well established and has beenavailable as a commercial product for many decades.As with any ββ technology, all is not ideal with Ge. Relating T ν / to (cid:104) m ββ (cid:105) requires a nuclear matrixelement ( M ν ), and although Ge benefits from an expectedly high M ν , T ν / also depends on a phasespace factor ( G ν ) as, ( T ν / ) − = G ν M ν (cid:104) m ββ (cid:105) .The modest atomic number and Q ββ result in a relatively small G ν compared to other isotopes. It hasbeen calculated by Kotila and Iachello [10] to be . · − /yr and by Stoica and Mirea [11] to be . · − /yr. (For these units of G ν , (cid:104) m ββ (cid:105) is taken in units of the electron mass.) Since the Ge Q ββ is low compared to the other most commonly used isotopes and given that T ν / scales as Q ββ , even a smalldifference can be a significant effect. The enrichment cost has been decreasing but is still a concern. Thiscost is off-set, however, by the reduced number of detectors that must be fabricated to acquire a given Gecontent. It should also be noted that the yearly production of Ge is large compared to the requirements foreven a ton-scale experiment, so producing the required isotope will not perturb the economics of the Gemarket significantly.The long and important history Ge has played in ββ has resulted in numerous nuclear physicsstudies dedicated to the isotope. The calculation of M ν is described elsewhere in this volume andnot addressed here. However, one significant example is that of neutron occupancy numbers. These weremeasured for Ge [12, 13] followed by reconsideration of M ν in light of the additional nuclear structureinformation. The outcome was that shell model [14] values increased a bit and the quasi-random phaseapproximation [15, 16] results decreased a bit bringing them closer to agreement. Other important nuclearphysics measurements include a precise value for Q ββ = 2039.061 ± M ν to others, here we indicate the key references for Ge.The popular nuclear structure models used to calculate M ν are: the interacting boson model (IBM-2) [19], This is a provisional file, not the final typeset article vignone and Elliott Ge Detectors and νββ the quasi-particle random phase approximation (QRPA) [20], the p-n pairing, QRPA [21], energy densityfunctional methods (EDF) [22, 23], and the interacting shell model (ISM) [24, 25]. The range of values for M ν for Ge varies from 2.81 to 6.13 for these calculations.
Germanium detectors have been the mainstay of nuclear spectroscopy and related fields for more thana half a century. They replaced NaI(Tl) scintillation detectors because the energy resolution is almost40 times better for γ rays with energies near 1 MeV. They consist of single crystals of Ge grown by theCzochralski method [26]. Germanium crystals have a diamond structure and Ge has 4 valence electrons. Ifa Ge crystal has impurities with only 3 valence electrons, then there will be holes throughout the lattice.This is called p-type germanium. In Ge detectors, one or more contact surfaces are heavily doped withlithium to create a surface region of n-type Ge with extra electrons. This configuration constitutes a p-ndiode. To operate a Ge detector, a reverse bias voltage is applied which sweeps free holes to the negativecontact and conduction-band electrons to the positive contact, essentially clearing the body of the detectorof almost all electrical carriers. The crystals are cooled to about 90 K to freeze carriers from thermalexcitation to the conduction band. When a γ ray, for example, interacts with an electron in the crystal, thatelectron cascades through the lattice creating electron-hole pairs that migrate towards the opposite signcontacts, creating a displacement current. The carriers reach the electrical contacts, and are detected with acharge-sensitive preamplifier. The number of detected charges is proportional to the energy deposited.The early Ge detectors had Li diffused throughout the crystal to create an n-type crystal. These werecalled GeLi detectors and required cooling at all times to prevent the Li from migrating out of the activevolume. GeLi detectors were limited in mass by the ability to drift Li uniformly throughout large crystals.Later, so called intrinsic or high-purity Ge detectors were developed in which the natural occurrenceof periodic-table, group 3 impurities in the lattice constituted the content of electrical impurities. Afterzone-refinement and crystallization via the Czochralski technique [26], the electrical impurity level in atypical Ge detector is (2 − · electrical impurities per cm in the finished detector. This is hyper-puremetal when one considers that there are almost Ge atoms/cm in solid germanium.The first search for ββ using a Ge detector (described below) was by E. Fiorini and his colleagues inthe 1960s [27]. The detector was a 90-gram GeLi detector. Major improvements in technology since thenhave made searches for νββ far more sensitive. The fabrication of intrinsic Ge detectors, which can havemasses of several kilograms, and the use of Ge enriched to 87% in the candidate parent isotope Ge,from the natural abundance of 7.8%, led to large sensitivity improvements. Finally, the development ofenriched point-contact Ge detectors of about 800 g has revolutionized the ability to discriminate betweenbackgrounds from γ rays and ββ by pulse shape discrimination. The progress in understanding the originsof background has been substantial. Although the Ge itself is very pure due to the crystal growing process,nearby cables, electronics and shielding may include trace amounts of U/Th. All of these components haveseen significant purity improvement.These developments have resulted in experimental lower bounds on T ν / of Ge from the earliest, · yr to present bounds nearing yr. These recent results have inspired the formation of the LEGENDProject with the goal of reaching a sensitivity of T ν / ∼ . This sensitivity would probe the entireinverted neutrino-mass hierarchy for Majorana neutrinos. In this article we discuss the subject from ahistorical perspective, while also attempting to project into the future. Frontiers 3 vignone and Elliott
Ge Detectors and νββ In this section we summarize the results of the early experiments culminating in the first use of enrichedGe. We provide the νββ results from detectors fabricated from natural-abundance Ge in Table 1 and fromenriched detectors in Table 2. Table 1.
Results of Ge νββ experiments using natural abundance Ge. It is interesting to note that in 25years, the half-life sensitivity of these early experiments increased by more than three orders of magnitude.Experiment Year T ν / limit (yr) Confidence Limit ReferenceUniversity of Milan 1967 ·
68% [27]University of Milan 1973 ·
68% [28]Battelle-Carolina 1983 . ·
90% [29]University of Milan 1984 . ·
68% [30]Guelph, Aptec, Queens 1984 . ·
95% [31]Caltech 1984 . ·
68% [32]Battelle-Carolina 1985 . ·
68% [33]Battelle-Carolina 1986 . ·
68% [34]UCSB, LBNL 1986 . ·
68% [35]University of Milan 1986 . ·
68% [36]Osaka University 1987 . ·
68% [37]Caltech, Neuchˆatel, PSI 1991 . ·
68% [38]UCSB, LBNL 1991 . ·
90% [39]Caltech, Neuchˆatel, PSI 1992 . ·
68% [40]
Table 2.
Results of Ge νββ experiments using Ge enriched in Ge. All results are quoted as 90% CL.In 2001 a subset of the Heidelberg-Moscow collaboration re-analyzed the data claiming evidence for νββ with T ν / =(0 . − . · yr (95% CL) [41] and has been subsequently refuted by succeeding limits.Experiment Year T ν / limit (yr) ReferenceITEP/Yerevan 1990 . · [5]IGEX-I 1994 . · [42]IGEX-II 1997 . · [43]Heidelberg-Moscow 2001 . · [44]IGEX-II 2002 . · [45]GERDA-I 2013 . · [46]M AJORANA . · [47]GERDA-II 2018 . · [48] The first search for νββ of Ge was performed by Fiorini and his University of Milan colleagues [27].While many of the later results were from the analyses of data from low-background Ge counting facilities,the Milan experiment was built for the express purpose of testing lepton conservation. The heart of
This is a provisional file, not the final typeset article vignone and Elliott Ge Detectors and νββ apparatus consisted of a 17 cm ( ∼
90 g) GeLi detector, with an energy resolution of ∼ Q ββ was . · − counts/(keV h), which in today’sterminology is . · counts/(keV kg yr). The data implied a bound of T ν / ≥ . · yr (68% CL).In addition to being the first high-resolution search for νββ , this was the first experiment in which thesource and detector were one and the same, yielding an excellent detection efficiency.In 1973, the Milan group published their results from a greatly upgraded experiment located in thesame location in the Mount Blanc Tunnel [28]. The detector in this case was a GeLi detector with anactive volume of 68.5 cm ( ∼
365 g). In this shield, the plastic scintillator was eliminated because itbrought background. Immediately surrounding the detector cap was a Nylon-Marinelli beaker filled withdoubly-distilled Hg. This was surrounded with 4 cm of electrolytic copper, encased in 10 cm of lowbackground lead, followed by 10 cm of ordinary lead. The outer lead shield was surrounded by a 2-mmthick cadmium sheet and the entire shield was then enclosed by 20 cm of paraffin to moderate backgroundneutrons.There were two data collection periods totaling 4400 h of live time. The background was . · − counts/(keV h). This is equivalent to ∼ . · counts/(keV kg yr), a factor of ten improvement inbackground over the 1967 result. The final result was T ν / ≥ . · yr (68% CL) [28]. The Milan Groupbuilt a new experiment with two intrinsic Ge detectors of fiducial volumes of 117 cm and 138 cm , in acommon shield [36]. There were several improvements in low background construction materials in thecryostat and shielding. There were two counting periods in which the shielding configuration underwentminor changes. The total counting time was 1.76 yr, and the resulting limit was T ν / ≥ . · yr (68%CL). The field of experimental νββ was dormant for a while after the 1973 Milan result. Renewed interestin νββ was driven by several events. First, Lubimov claimed that the electron neutrino had a mass ofabove 14 eV, from the data of the ITEP tritium-end-point experiment [49]. Second, interest in the theory ofGrand Unification was intense and third, the shell-model calculations of the nuclear matrix elements byHaxton, Stevenson and Strottman [50], indicated significant strength for the decay of Ge. With thesenew motivations, Avignone and Greenwood proposed in 1979 an experiment, based on a Monte Carlostudy with a high-purity Ge detector enclosed in a NaI(Tl) Compton suppression shield [51]. The assumedbackgrounds in this proposal were taken to be similar to the Milan experiments discussed above. A trialof the experiment suggested in Ref. [51] was proposed to the team of Brodzinsky and Wogman at theBattelle Pacific Northwest Laboratory (now PNNL). For several years the Battelle-Carolina Collaborationworked on improving the backgrounds due to the construction materials in copper cryostats. The intrinsicGe detector had an active volume of 125 cm . It was operated inside a two-inch thick NaI(Tl) Comptonsuppression shield, inside a lead shield, covered by a boron-loaded polyethylene neutron shield, with aplastic cosmic-ray shield above the entire apparatus. The experiment was operated above ground for a livetime of 4054 h, resulting in the bound T ν / ≥ . · yr [29]. The background rate was similar to previousexperiments at . · counts/(keV kg yr). The detector was then moved to a location at 4850 ft below Frontiers 5 vignone and Elliott
Ge Detectors and νββ the surface in the Homestake Gold Mine in Lead, South Dakota, in part of the Solar Neutrino Laboratoryof Raymond Davis. That location has an overburden of ∼ Q ββ and the background rate was47 counts/(keV kg yr). The detector was operated for 8089 h at the same site in the Homestake mine withthe result, T ν / ≥ . · yr (90% CL) [34]. The construction details are given in Ref. [53]. The lessonlearned was that much of the background, although reduced, was coming from the cryostat itself. This factled to a significant R&D effort by the Batelle-Carolina Collaboration to create ultra-low background copperby electroforming from CuSO solutions onto stainless steel mandrels. The results were the production ofall six of the cryostats for the International Germanium Experiment, IGEX, with electroformed copper.The IGEX experiments are discussed below. At a time shortly after the 1983 Battelle-Carolina experiment, the team of J.J. Simpson was operating acommercially built, low background Ge detector underground. The intrinsic 208 cm ( ∼ Bi, a daughter of the 22-yr
Pb inthe shield. Although the lead was between 150 and 200 yr old, this radiation still remained, demonstratingthat the lead of the shield contained a high level of
U. The detector operated for 2363 h. The result wasa bound of T ν / ≥ . · yr (68% CL), or T ν / ≥ . · yr at (95% CL). The Caltech Group began their experimental series by setting up a shielded detector above ground ina sub-basement at Caltech. The overburden was only 3 mwe. The Princeton Gamma-Tech, high-puritycoaxial Ge detector had a ∼
90 cm fiducial volume. The detector was surrounded with 15 cm of electrolyticcopper, followed by 15 cm of lead. The shield was enclosed in an airtight box to protect the detectorfrom airborne radon. The final result from 3820 h of this essentially above-ground experiment was T ν / ≥ . · yr (68% CL) [32]. The next experiment involving the Caltech group was in collaborationwith the University of Neuchˆatel and Paul Scherrer Institute [38]. It involved 8 high-purity 140-cm Gedetectors, with a combined volume of 1095 cm or 5.83 kg. The array was operated in the Gotthard Tunnelwith an overburden of 3000 mwe. The array of detectors was surrounded with 15 cm of oxygen-free, high-conductivity (OFHC) copper, followed by 18 cm of lead, all contained in an aluminum radon shield. (SeeFig. 2.) The array was operated for 6.2 kg yr of live time with resulting limits of T ν / ≥ . . · yr90%(68%) CL. The final report of this collaboration [40] reported T ν / ≥ . · yr (68% CL) from10.0 kg yr. This was the strongest bound from the natural-abundance Ge detectors. The UC Santa Barbara, Lawrence Berkeley National Laboratory experiment began with two intrinsicGe detectors of 178 cm and 158 cm operating above ground in a NaI(Tl) Compton-suppression shieldfor 1618 h. A later version was a configuration of four intrinsic detectors with a total fiducial volume of658 cm . The array had new interesting construction features, for example Si cold fingers to avoid thebackground due to the copper commonly used [35]. The array was operated for 3550 h, 200 m below groundin the Power Station in the Oroville Dam in Northern California. The final result was T ν / ≥ . · yr(68% CL). The array was later used to produce very interesting data in the search for Cold Dark Matter [54]. This is a provisional file, not the final typeset article vignone and Elliott Ge Detectors and νββ The first phase of the Osaka experiment began above ground with a 171 cm intrinsic Ge detector in a π NaI(Tl) Compton suppression shield, surrounded by a mercury shield [37]. The detector was operated for1600 h in the Kamioka Underground Laboratory, with an overburden of 2700 mwe. In the second phase, thedetector was operated for 7021 h, but without the mercury shield. The final result was T ν / ≥ . · yr(68% CL). νββ of Ge This experiment was the first search for νββ of Ge with detectors fabricated with Ge enriched in Ge [5]. This experiment consisted of three GeLi detectors, two of which were fabricated with Ge enrichedto 85% in Ge. (See Fig. 3.) The total mass of Ge was 1008 g. The three crystals were on the end of avertical cold finger inside of a NaI(Tl) Compton shield surrounded by several cm of copper followed bylead. The entire apparatus was inside a boron-loaded Polyethylene box 112 cm ×
112 cm ×
240 cm high. Theexperiment was operated 245 m underground, in the Avansk Mine in Yerevan, Armenia. The background at Q ββ was 2.5 counts/(keV kg yr) for the two enriched crystals and 2.1 counts/(keV kg yr) for the naturalcrystal. A final analysis of the results yielded a limit of T ν / ≥ . · yr (90%).In addition, this experiment was the first direct observation of the νββ decay of Ge, and onlythe second such laboratory measurement following that in Se [4]. The result of the ITEP-Yerevanexperiment was T ν / =(9 ± · yr. This result was submitted to Modern Physics Letters on 23 April1990 [5]. Later that year, the Battelle-Carolina group submitted a similar result: T ν / =1 . +0 . − . · yr(95% CL), to Physical Review Letters from data taken with two 1.05-kg, ultra-low background, naturalabundance, intrinsic Ge detectors [6]. The two groups then merged and placed one of the ITEP-Yerevanenriched GeLi detectors in the Battelle-Carolina Cryostat to re-measure the half life. The result was T ν / =(9 . +0 . − . ) · yr (2 σ ) [7]. It was later demonstrated that all three of these results were contaminatedwith internal radioactivity generated by spallation reactions of hard cosmic-ray neutrons (e.g. Co, Zn, Ge). These backgrounds produced events, which were partially attributed to νββ , resulting in deducingshorter half-lives. Results by IGEX presented at ERICE in 1993 [55], corrected for these backgroundsand found T ν / =(1 . +0 . − . ) · yr (1 σ ). Later experiments demonstrated that these corrections forinternal background, while in the correct direction, were still inadequate. Historically, the value for T ν / has increased for Ge indicating that the background subtraction is very difficult. Table 3 summarizes themeasurements of T ν / . In 1988, the Battelle-Carolina Collaboration concentrated on lowering background by electroformingthe cryostat parts from CuSO solution, and acquiring Ge enriched to 86% in Ge. A collaboration wasformed between Battelle Northwest, the University of South Carolina, the Institute of Theoretical andExperimental Physics (ITEP) Moscow, the Institute of Nuclear Research (INR) Moscow, and the Universityof Zaragoza. Over several installments, a total of 18 kg of Ge, enriched to 86% in Ge were importedto the U.S. from the two Russian institutes in oxide form. The first 5 kg from INR was reduced and zonerefined by Mr. James Meyer at Eagle Picher Inc. Three 190 cm high-purity Ge detectors were fabricatedand tested in the Homestake gold mine, at the 4850-ft level. (See Fig. 4.) While the energy resolutionand general operation of the detectors was excellent, measurements determined that the fiducial volumes Frontiers 7 vignone and Elliott
Ge Detectors and νββ Table 3.
Results of Ge νββ experiments. The half life continues to creep up due to the complexity ofsubtracting the background. As experiments improve and the background is either reduced or better fit, T ν / increases.Experiment Year T ν / measurement ( yr) Confidence ReferenceLevelITEP/Yerevan 1990 . ± .
68% [5]Batelle-Carolina 1990 . +0 . − .
95% [6]ITEP-Yerevan/Batelle-Carolina 1991 . +0 . − .
90% [7]IGEX 1994 . +0 . − .
68% [55]Heidelberg-Moscow 1997 . ± . stat ) +0 . − . ( syst )
68% [56]IGEX 1999 . ± .
68% [57]Heidelberg-Moscow 2001 . ± . stat ) +0 . − . ( syst )
68% [44]Heidelberg-Moscow 2003 . +0 . − .
68% [58]Heidelberg-Moscow 2005 . ± . stat ) +0 . − . ( syst )
68% [59]GERDA 2013 . +0 . − .
68% [60]GERDA 2015 . ± .
68% [61]were only about 135 cm . Difficulties in crystal growth required the Li deposition on the outer surfacesto be thicker than normal. These three detectors constituted IGEX-I. One was operated in the Homestakegold mine, one in the Canfranc Underground Laboratory, in Canfranc Spain (1380 mwe), and one in theBaksan Neutrino Observatory, in Russia (660 mwe). The first results from IGEX-I were presented at theInternational Conference on Topics in Astroparticle and Underground Physics (TAUP-93), at LaboratoriNazionali del Gran Sasso (LNGS), in Assergi, Italy [42]. The data from the three detectors were combinedwith the result T ν / ≥ . · yr (90% CL). The average background was 0.3 counts/(keV kg yr). It wasalso announced at that meeting that the first of three IGEX-II detectors had been fabricated and tested. Ithad a fiducial volume of ∼
400 cm , and an energy resolution of 2.16 keV FWHM at 1332 keV. The firstIGEX results using pulse-shape discrimination to identify background events from γ rays, was presentedat Neutrino-96 at Helsinki. The result from 34.4 mole yr of data was T ν / ≥ . · yr (90% CL) [43].While IGEX was first to build and operate high-purity Ge detectors enriched in Ge, by the time of thismeeting, the Heidelberg Moscow Collaboration was already operating ∼
400 cm high-purity detectors andhad excellent results (discussed below). The IGEX technique for pulse-shape discrimination was describedin detail with IGEX-I detectors in Ref. [62], and later using the larger IGEX-II detectors in Ref. [63].During the period 1996 and 1997, the IGEX collaboration had three high-purity enriched coaxial detectorsproduced with active volumes of ∼
400 cm . The IGEX detectors had a unique configuration hanging fromthe end of the cold fingers. The cold finger rose from the liquid nitrogen bottle, made a 90 ◦ turn to horizontal,extended through the shield to the cold plate from which the detector cryostats were hung vertically down.This configuration prevented the radioactive contamination of Xeolite cryopump material from having adirect line of sight to the detector. The three IGEX-II detectors were tested at Homestake, then carriedby ship to Barcelona Spain, and installed in the Canfranc Underground Laboratory of the University ofZaragoza. It is important to point out that by this time, the experiment of the Heidelberg-Moscow groupwas operating four large enriched detectors in the LNGS, and exceeded IGEX in exposure.While there were a number of IGEX updates published in conference proceedings, the first publicationof results, including the data taken with the IGEX-II detectors was in 1999, based on 78.84 mole yr of This is a provisional file, not the final typeset article vignone and Elliott Ge Detectors and νββ exposure. The total mass of detectors was 8.1 kg. The resulting bound was T ν / ≥ . · yr (90%CL). The data were subjected to the pulse-shape discrimination techniques described in Refs. [62] and[63]. The final IGEX result was published after a total of 117 mole yr of exposure: T ν / ≥ . · yr(90% CL) [45]. The publication of this final IGEX result set a controversy in motion. A subset of theHeidelberg-Moscow Collaboration claimed that serious errors were made in the analysis of the final IGEXresults [41]. The response by the IGEX collaboration [64] clearly justified the IGEX analysis and the finalresult given in Ref. [45]. The Heidelberg-Moscow Collaboration launched a very impressive experiment with five coaxial-high-purity Ge detectors enriched to 88% in Ge, with a total mass of 11.5 kg, and an active volume with10.96 kg, operating in LNGS. The laboratory has an overburden of about 3500 mwe. The detectors wereenclosed in a shield with a 10-cm inner layer of ultrapure lead, surrounded by 20 cm of pure Bolidenlead, enclosed in a metal box flushed with high-purity nitrogen. The shield was surrounded by 10 cm ofboron-loaded polyethylene [65, 66, 67]. The experiment had an effective pulse shape analysis techniquefor identifying and removing background events [68]. It operated from 1990-2003 with a total exposure of71.71 kg y. It was the most sensitive Ge experiment until the GERDA experiment commenced. Therewere many publications presenting the results over the years. In 2001 the collaboration published thebest bound on decay: T ν / ≥ . · yr (90% CL) [69]. Later that year, a subset of the collaborationpublished a claim of direct observation of νββ of Ge, with a half-life of T ν / =(0 . − . · yr(95% CL), based on 46.5 kg yr of exposure [69, 70]. The final range of claimed values for the discovery, T ν / =(0 . − . · yr (95% CL), and the entire history of these experiments from 1990 to 2003, isgiven in Ref. [41].The claim of discovery was critiqued in an article coauthored by a broad list of authors [71], andlater excluded by results from the GERDA Experiment (discussed below). This claim has also beenexcluded by the Xe experiments (KamLAND-Zen [9] and EXO [8]), but the direct comparison between Geexperiments removes any caveats regarding the relative matrix element values. In addition to the search for νββ , the collaboration measured T ν / = (1 . ± . stat ) +0 . − . ( syst )) · yr [44] followed later by (1 . +0 . − . ) · yr [72]. The Heidelberg-Moscow experiment was based on 13 yr of very low background operation. Hence it wouldbe a very difficult experiment to repeat. Furthermore during the 1990’s and into the 2000’s, ββ experimentstook a back seat to the interest in solar neutrino experiments due to their role in ν oscillations. As a resultthere was another hiatus in νββ results. Interest built for ββ again in the late 2000’s due to the claim of anobservation and the confirmation that ν oscillations exist and, by inference, massive light neutrinos exist.In addition, the ν physics parameters indicated by the oscillation results meant that new νββ experimentswould have discovery potential for a significant range of possible (cid:104) m ββ (cid:105) values. The field of ββ saw alarge number of new proposals advance by about 2010, including those based on Ge.One key development in Ge detector technology has greatly improved their pulse shape analysis capability.That is the use of a point-contact. Originally developed for their low capacitance [73], it was after thedevelopment of modern-day transistors that the full power of this detector design began to be exploited, inparticular for dark matter experiments [74]. The advantage for ββ arises because the weighting potential is Frontiers 9 vignone and Elliott
Ge Detectors and νββ strongly peaked at the contact for this geometry. This results in an electronic signal that predominatelyforms only when drifting charge nears the contact. Therefore, an event with multiple energy deposits withina detector will have pulse shape distinct from that of a single-site energy deposit. As ββ is a single-siteenergy deposit and many backgrounds are multiple site events, this is a powerful rejection capability andpoint-contact detectors substantially surpass the performance of the semi-coax Ge detector design that hadbeen the field’s workhorse. M AJORANA and GERDA [75] further developed and used this technology togreat success.During research and development for the M
AJORANA and GERDA programs, the use of segmenteddetectors was considered. Segmented detectors provide enhanced waveform analysis and hence improvedbackground rejection. A number of studies [76, 77, 78, 79] were done considering the added advantagesof segmentation on the reduction of background versus the disadvantages of the extra complexity andbackground due to the additional electronic channels and cables. The M
AJORANA collaboration successfullydeveloped a segmented enriched detector [80] that showed some promise. After the development of point-contact detectors, however, it became clear that the advantages of segmentation were outweighed by thedisadvantages. Segmented detectors for ββ were not further pursued. M AJORANA
Experiment
The M
AJORANA D EMONSTRATOR [81, 82] experiment was established to demonstrate that backgroundscan be controlled to a level that would justify a large (ton scale) Ge effort. Previous Ge experiments withcompact, high atomic-number shielding indicated that the classic design of a vacuum cryogenic-cryostatfilled with Ge detectors surrounded by Pb could extend the reach of ββ physics. The M AJORANA project,named in honor of Ettore Majorana and based on this concept, began construction in 2010 with initialcommissioning data collected in 2015.The ongoing experiment is sited 4300 mwe underground at the 4800-ft level of the Sanford UndergroundResearch Facility (SURF) [52]. The Ge detectors, 44.1 kg total with 29.7 kg enriched to 88% in Ge,are enclosed within two electroformed-Cu [83] cryostats. The detectors are mounted in groups of 3 to 5and hung as strings from a cold plate cooled by a thermosyphon [84]. Very low radioactivity, front-endelectronic boards [85], placed very close to the detectors, maintain signal fidelity while providing the initialamplification stage. The cryostat is contained within a 5-cm thick electrofromed Cu layer, a 5-cm thickcommercial C10100 copper layer, a 45-cm thick Pb shield, two layers of plastic-scintillator cosmic-ray vetopanels, 5 cm of borated poly and finally 25 cm of high density polyethylene. The material inside the vetolayer is contained in an Al box that is purged with boil-off N to displace Rn-laden room air. (See Fig. 5.)All materials comprising the experiment were analyzed for their radiopurity [86]. The processing of Ge forM AJORANA developed recycling techniques [87] that are critical to reduce the amount of required rawmaterial to fabricate a given mass of detectors.Initial results from the D
EMONSTRATOR were based on an exposure of 10 kg yr [82]. A second datarelease [47] based on 26 kg yr of exposure yielded a half-life limit of > . · yr (90% CL). Afterremoval of non-physical events, events in coincidence with the muon veto, events with multiple detectorsin coincidence, and pulse shape analysis to remove single-crystal events with multiple energy depositsand surface α interactions, the final background is . ± . counts/(FWHM t yr) or (4 . ± . · − counts/(keV kg yr) from the 21.3 kg yr lowest background configuration. The spectra from the full 26 kg yrexposure are shown in Fig. 6. The energy resolution, 2.5 keV FWHM at Q ββ , is the best achieved of any ββ experiment. Although the analysis is not yet complete, early studies indicate the dominate source ofbackground in the D EMONSTRATOR is not from nearby components within the detector arrays [88].
This is a provisional file, not the final typeset article vignone and Elliott Ge Detectors and νββ The low background, excellent energy resolution and low energy threshold permit a variety of otherphysics measurements with M
AJORANA , including tests of the Pauli Exclusion Principle, electron decay,bosonic dark matter [89, 90], and lightly ionizing particles [91]. An important low-energy background inGe detectors is caused by spallation reactions on Ge by high-energy cosmic neutrons at the earth’s surface.The important case of Ge production yields in enriched Ge was measured in Ref. [92]. The isotope Ge is removed only at the enrichment stage, but both zone refining and crystal growth remove all othercosmogenic isotopes. Hence, surface exposure after each of these steps is a concern. This exposure wasaddressed for the M
AJORANA D EMONSTRATOR detectors in several ways. First, the enriched GeO wasshipped from Russia in a steel shipping container, developed by GERDA, that reduced the cosmogenicproduction of Ge by a factor of approximately 10. In addition, a zone-refining facility was establishedadjacent to the ORTEC, Inc. detector production facility and a ten-minute drive from the Cherokee Caverns,which allowed convenient underground storage of the Ge between processing steps. Finally, each partwas tracked through its history with a detailed database [93]. These procedures resulted in significantreductions in the low energy background, especially tritium β -decay, and opened the door to searchingfor other physics. Although GERDA did not pursue a low-energy program, the collaboration followed asimilar strategy to reduce cosmogenic backgrounds impacting νββ . The GERmanium Detector Array (GERDA) for Ge experiment arose from the idea of using liquidnitrogen (LN) as a shield because of its low radioactivity. The idea, originated by Heusser [94], was toimmerse bare Ge detectors in LN, which would act as coolant and shield. This concept was developedby the GErmanium in liquid NItrogen Underground Setup (GENIUS) collaboration [95] and realized byGERDA. The GERDA collaboration [96, 46, 97, 48], however, used liquid argon (LAr) instead of LN dueto its higher γ -ray stopping power. In addition, the LAr is an excellent scintillator, and was very effectiveas veto against background radiation external to the detector array itself.The initial GERDA goal was toconfirm or refute the claim for the observation of νββ [98, 99].The Ge detectors in GERDA are deployed in 7 strings, each enclosed within a nylon shroud that preventsradioactive ions ( K in particular) from electrostatic attraction to the detector surface. The group of stringsis submerged in a 64 m volume of LAr. The cryostat containing the LAr is, itself, contained within a590 m volume of pure-water. The neck of the LAr cryostat provides access for, not only the detectors, butall the associated utilities and data acquisition readout. The experiment is running at LNGS at a depth of3400 mwe.The experiment has progressed through 2 phases. In Phase I, 17.6 kg of enriched Ge, including thedetectors used by the HM and IGEX experiments, acquired 21.6 kg yr of data and found a half-life limitof . · yr (90% CL) [46]. The background index at Q ββ was 0.01 counts/(keV kg yr). Phase IIincreased the enriched detector mass to 35.6 kg and added a light detection system to the LAr surroundingthe detectors. Figure 7 shows the detector strings and LAr veto systems. This technique permitted a vetoof events that deposited energy in both the Ge and Ar resulting in a significant background decrease to (5 . ± . · − counts/(keV kg yr) in their BEGe dectectors [100]. This is the lowest background everachieved by a νββ experiment when normalized to the resolution at Q ββ . The reported combined exposureof Phases I and II is 82.4 kg yr resulting in a half-life limit of . · yr (90% CL) [48], convincinglyruling out the previous claim of (2 . +0 . − . ) · yr [99]. Ref. [101] strongly criticizes this claimed valueand argues that one should compare to the value in Ref. [41] of (0 . − . · yr with a quoted bestvalue of . · yr. At this time, both are excluded by the GERDA data. GERDA has also measured Frontiers 11 vignone and Elliott
Ge Detectors and νββ T ν / = (1 . +0 . − . ) · yr [60], which was followed by (1 . ± . · yr [61]. Figures 2 and 3 inthat latter paper shows a measured spectrum and fits including νββ and the key background components.The dominance of νββ is clear. νββ DECAY OF GE When normalized to the resolution at Q ββ , GERDA has the lowest background of any νββ experiment,with M AJORANA a close second. The two experiments have very modest exposures compared to othertechnologies but still have competitive or leading half-life limits. This situation has motivated the pursuitof a next-generation νββ experiment based on Ge. The Large Enriched Germanium Experiment forNeutrinoless Double Beta (LEGEND) Collaboration [102] aims to develop a phased, Ge double-betadecay experimental program with discovery potential at a half-life beyond yr, starting with existingresources as appropriate to expedite physics results. This goal has led to a phased program, LEGEND-200and LEGEND-1000. LEGEND-200 will deploy up to 200 kg of Ge detectors within the existing GERDAinfrastructure at LNGS. Only modest modifications to the lock at the top of the cryostat and the piping inthe cryostat neck are required to accommodate the increased detector mass. In M AJORANA the componentsnear the detectors, such as the front-ends and cables, were very radio-pure. In GERDA, the LAr veto was avery powerful tool for rejecting background. Using the more radio-pure parts and improving the light yieldof the LAr veto system will reduce the background to 0.6 counts/(FWHM t yr) ( · − counts/(keV kgyr)). The 3 σ discovery level for this configuration is estimated to be greater than yr. Figure 9 showsthe discovery potential of a Ge experiment as a function of exposure for several background levels. Toreach the intended goal, LEGEND-200 requires about 1 t yr of exposure. The experiment is anticipated tobegin operations in 2021.LEGEND-200 is nearly fully funded with a few requests still pending. The project is under developmentat the time of this writing. LEGEND-1000 is envisioned to deploy a ton of isotope within 5 payloads intoLAr. (See Fig. 10.) The goal is to reach a limit of > yr. Germanium detectors have excellent energy resolution and very low background. As a result, limits on T ν / from Ge are very competitive even when the exposure is much less than competing technologies.Detectors fabricated from Ge have historically provided outstanding constraints on T ν / and (cid:104) m ββ (cid:105) . Fromthe first Ge-based experimental result in 1967, limits on T ν / have improved by a factor of · overthe intervening 50 year period. The technology continues to advance and an additional improvement insensitivity of more than a factor of 100 is within reach in the near future. CONFLICT OF INTEREST STATEMENT
The authors declare that the research was conducted in the absence of any commercial or financialrelationships that could be construed as a potential conflict of interest.
AUTHOR CONTRIBUTIONS
FTA and SRE equally contributed to the preparation of this manuscript.
This is a provisional file, not the final typeset article vignone and Elliott Ge Detectors and νββ FUNDING
FTA thanks the National Science Foundation for support under grant NSF1614611. SRE thanks theDepartment of Energy Office of Nuclear Physics for support under contract number DE-AC52-06NA25396.SRE acknowledges the support of the U.S. Department of Energy through the LANL/LDRD Program.
ACKNOWLEDGMENTS
The authors would like to acknowledge the collaborations that provided input for this manuscript includingM
AJORANA , GERDA, and LEGEND. We specifically thank Bernhard Schwingenheuer, Riccardo Brugnera,and Vince Guiseppe for careful readings.
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FIGURE CAPTIONS
Frontiers 17 vignone and Elliott
Ge Detectors and νββ Figure 1.
A diagram of the Milan Mont-Blanc Tunnel Double-Beta Decay Experiment.
This is a provisional file, not the final typeset article vignone and Elliott Ge Detectors and νββ Figure 2.
A diagram of the Caltech, Nuchˆatel, PSI experiment with four natural abundance, high-purityGe detectors.
Frontiers 19 vignone and Elliott
Ge Detectors and νββ Figure 3.
A diagram of the ITEP/Yerevan experiment showing the three crystals, two enriched, and one ofnatural abundance.
This is a provisional file, not the final typeset article vignone and Elliott Ge Detectors and νββ Figure 4.
A diagram of the first IGEX-II detectors in the Homestake gold mine showing the detectorshanging vertically.
Frontiers 21 vignone and Elliott
Ge Detectors and νββ Figure 5.
Left: The shield concept for the M
AJORANA D EMONSTRATOR . Right: A photograph of onecryostat ready for insertion into the shield with the other, already installed, visible in the background.Figure and photo courtesy of the M
AJORANA
Collaboration.
Energy [keV] C oun t s / ( . k e V k g y r) - Data Cleaning, Muon, & Multiplicity CutsAll Cuts M A J O R A N A - . c Energy [keV] C oun t s / ( k e V k g y r) All Cuts90% C.L. Limit M A J O R A N A - . Figure 6.
Left: the whole spectrum from the M
AJORANA D EMONSTRATOR from 26 kg-yr of exposure.Right: The specrum near the Q ββ for Ge at 2038 keV [47]. Figures courtesy of the M AJORANA
Collaboration.
This is a provisional file, not the final typeset article vignone and Elliott Ge Detectors and νββ Figure 7.
Left: A photograph of the strings of detectors for GERDA. Right: A photograph of thescintillation light collection system that surrounds the Ge detectors. Photos courtesy of the GERDACollaboration.
Frontiers 23 vignone and Elliott
Ge Detectors and νββ Figure 8.
The spectra from the various phases of GERDA near the Q ββ for Ge at 2039 keV [48]. Figurescourtesy of the GERDA Collaboration. This is a provisional file, not the final typeset article vignone and Elliott Ge Detectors and νββ Figure 9.
The 3 σ discovery potential for a Ge experiment for several potential background levels. Figurecourtesy of Jason Detwiler.
Figure 10.