The majorana 76Ge double-beta decay project

PROCEES)INGS SUPPLEMENTS ELSEVIER

Nuclear Physics B (Proc. Suppl.) 124 (2003) 247-252

THE MAJORANA

76Ge DOUBLE-BETA

www.clsevicr.com/locatc/npc

DECAY PROJECT

C.E. Aalseth”, E. Adlesb,c, D. Andersona, F.T. Avignone IlId, A. Barabashe, T.W. Bowyer”, R.L. Brodzinski”, V. Brudaninf, A. Champangneb,s, J.I. Collarh, P.J. Doei, S. Egorovf, S.R. Elliott’, H.A. Farachd, R. Gaitskellj, D. Jordana, R.K. Jain b , ’ , K. Kazkaz’, G. King IlId, 0. Kochetovf, S. Konovalov’+, R. Kouze?, H.S. Mileya, J.M. Palmsd, W.K. Pitt?, J.H. Reeve?, R.G.H. Robertson’, R. Rohmb,s, S. Sandukovskyf, L.E. Smitha, V. Stekhanov”, R.C. Thompson”, W. Tornowb,s, V. Umatove, R. Warner”, J. Webbk, J.F. Wilkerson’, and A. Youngb,C, (The Majorana Collaboration) “Pacific

Nortwest

bNorth

Carolina

“Triangle

eInstitute

Laboratory,

State University,

Universities

dUniversity

fJoint

National

Raleigh,

Nuclear Laboratory,

of South Carolina, for Theoretical

Institute

Richland,

Columbia,

and Experimental

NC, USA Durham,

NC, USA

SC 29208, USA Physics, Moscow 117259, Russia

for Nuclear Research, Dubna, Russia

gDuke University,

Durham,

NC, USA

hUniversity

of Chicago, Chicago, IL, USA

‘University

of Washington,

jBrown

WA 99352, USA

University,

Seattle, WA, USA

Providence,

kNew Mexico State University,

RI, USA NM, USA

The interest and relevance of next-generation 0, @-decay experiments is increasing. Even with nonzero neutrino mass strongly suggested by solar and atmospheric neutrino experiments sensitive to 6m2, 0, P&decay experiments are still the only way to establish the Dirac or Majorana nature of neutrinos by measuring the effective electron neutrino mass, (m,). In addition, the atmospheric neutrino oscillation experiments imply that at least one neutrino has a mass greater than about 50 meV. The Majorana Experiment expects to probe an effective neutrino mass near this critical value. Majorana is a next-generation 76Ge double-beta decay search. It will employ 500 kg of Ge, isotopically enriched to 86% in “Ge, in the form of N 200 detectors in a close-packed array. Each crystal will be electronically segmented and each segment fitted with pulse-shape analysis electronics. This combination of segmentation and pulse-shape analysis significantly improves our ability to discriminate neutrinoless double beta-decay from internal cosmogenic s8Ge and 6oCo. The half-life sensitivity is estimated to be 4.2 x 10z7 y corresponding to a (m,) range of 5 20 - 70 meV, depending on the nuclear matrix elements used to interpret the data.

1. Neutrinoless

Double-Beta

Decay

Neutrinoless double-beta decay, is a process by which two neutrons in a nucleus beta decay by ex-

changing a virtual Majorana neutrino, each emitting an electron. This violates lepton number conservation (Al = 2). There are many reviews on the subject[l - 41.

0920-5632/03/$ - see front matter 0 2003 Published by Elsevier Science B.V. doi:10.1016/S0920-5632(03)02116-O

C.E. Aolseth et al. /Nuclear Physics B (Proc. Suppl.) 124 (2003) 247-252

248

The decay rate for the process involving the exchange of a Majorana neutrino can be written as follows:

with data of similar quality. This strongly implies that these experiments with the order of 100 moles of 76Ge each are have reached their point of diminishing returns.

In equation (1) Go” is the two-body phase space factor including coupling constants, Mj and MgT are the Fermi and Gamow-Teller nuclear matrix elements respectively, and gA and gv are the axial-vector and vector relative weak coupling constants, respectively. The quantity (m,) is the effective Majorana neutrino msss given by:

2. The Majorana iment

(2) k=l

where AC’ is the CP eigenvalue associated with the Icth neutrino mass eigenstate (fl for CP conservation), Uli is the (I, k) matrix element of the transformation between flavor eigenstates ]vt) and mass eigenstates ]vk( for left handed neutrinos;

and mk is the mass of the lath neutrino mass eigenstate. The effective Majorana neutrino mass, (m,), is directly derivable from the measured half-life of the decay as follows: (m,)

= m,(FNT$2)-1i2eV,

(4

where FN z GovlMfoy - (gA/gV)2M&12, and m, is the electron mass. This quantity derives from nuclear structure calculations and is model dependent. The most sensitive experiments thus far utilize germanium detectors isotopically enriched in 76Ge from 7.78% abundance to N 86%. Germanium detector experiments were started with natural abundance detectors by Fiorini, et al., in Milan[4], evolving over the years to the first experiments with small isotopically enriched detectors, and finally to the two present multi-kilogram isotopically-enriched 76Ge experiments: Heidelberg Moscow[5] and IGEX[G]. Reference [5] has about four times the exposure as reference [6]

76Ge Ov @P-Decay

Exper-

The Majorana experiment is proposed for a US deep underground laboratory, and requires very little R&D. It stands on the technical shoulders of the IGEX experiment and other previous sucessful double-beta decay and low-background experiments. Furthermore, new segmented Ge detector technology has recently become commercially available, while the PNNL/USC researchers have perfected new pulse-shape discrimination techniques. The IGEX experiment terminated with 117 mole-years of data with an average background of 0.06 c/keV/kg/y in the later data sets. The resulting half life lower bound is 1.6 x 1O25 y, corresponding to an upper bound on (m,) of (0.33 - 1.35) eV depending on the nuclear matrix elements used to interpret the data with eq.(4). The Majorana experiment represents a great increase in Ge mass over IGEX with new segmented Ge detectors and the newest electronic systems for pulse-shape discrimination. It is conceived to be 500 kg of Ge detectors, fabricated from Ge isotopically enriched to 86% in 76Ge. The typical detector size will be approximately 2.0 kg, requiring about 250 detectors. The array assumed has a fiducial mass of 500 kg which contains N = 3.43 x 1O27 atoms of 76Ge, a counting time of 10 years, and an energy resolution of 3.0 keV FWHM. The starting background assumed is that achieved by IGEX, prior to pulse discrimination, which was bo = 0.2 counts/keV/kg/y at the Ov ,B@decay energy of 2039 keV. It was achieved in the Homestake mine and was dominated by the decay of cosmogenie isotopes in the germanium. The decay of 270.&day 6sGe over the lo-year counting period reduces this background by a factor of 0.107. The reduction in background from the decay of 6oC~ (T112 = 5.7 y) is by a factor of 0.579. The product is taken for the reduction due to decay of

249

C.E. Aalseth et al. /Nuclear Physics B (Proc. Suppl.) 124 (2003) 247-252 cosmogenic radioisotopes in the crystal (0.062). The product of this with the original background (0.2 c/keV/kg/y) yields a background rate, before data cuts, of 0.0124 c/keV/kg/y s b. This rate was completely accounted for as cosmogenic radioactivity. The optimum region of interest in energy was calculated to be 2.80 of the gaussian peak, or 3.57 keV. The total background in this window at 2039 keV, over a 10 year period would then be: b.AE.M.t=

kaV

(0.0124 c/keV/kg/y)(3.57

keV)(5000

kg x y) =

= 221

(5)

background events prior to data cuts. Extensive experiments by the PNNL/USC collaborators demonstrated that pulse-shape discrimination can reduce the background by a multiplicative factor of 0.265. Extensive Monte Carlo Calculations have demonstrated that the segmentation of the crystal will allow an additional reduction factor of 0.138. An example of how segmentation will reduce background in the 2 MeV region of interest is shown in Figure 1. Applying both of these cuts will reduce the background to 8.1 remaining events. If the background in the region is featureless in energy, a simple Poisson analysis yields 1.645&i = 4.7 counts to 90% CL. Accordingly, the sensitivity should be TI/,

t(ln 2)Nt N ~

years.

Unfortrmately, the data cutting process comes with the loss of efficiency. Eliminating events that leave energy in more than one of the 12 segments leaves an efficiency of 0.907. The pulse shape dis crimination system can misidentify almost 20% of the pulses, yielding an efficiency of 0.802. The total counting efficiency is then 0.727. Accordingly the sensitivity is: Tp/“2 (sew)

This the will The

=

(0.727)(.693)(3.43 x 1027) 10 4.7 ‘v 3.7 x 1O27 y.

(6)

analysis does not account for the fact that close-packed granularity of -250 crystals further improve the background significantly. background computed above is therefore con-

Figure 1. Monte-Carlo simulation of internal 6OCo background. Left shows a spectrum before and after a onesegment-only cut is applied. Right shows histogram of number-of-segments-hit for events falling in 2.0 - 2.1 MeV ROI.

servative. The upper bounds on (m,) then range from 0.023 to 0.086 as shown in Table 1. TABLE 1: Nuclear structure factor FN and Majorana neutrino mass parameters (m,) for Tp,v, = 3.7 x 102’ y. -I -

F~,yr-’ 1.56 9.67 1.21 1.12 1.41

x x x x x

] Model [reference] ] (m,) (eV) 0.023 10-l’ ] Shell Model 171 ] lo-l5 QRPA 181. 1 0.086 Qm-4 191 0.024 10-1s & W A PO1 0.026 10-13 10-14 Shell Model 1111 0.072

If Majorana fulfills its design parameters, still makes no discovery, it can be simply panded in volume and continued. 3. Recent nology

Progress

in Ge Detector

but ex-

Tech-

Majorana can not simply be a volume expansion of IGEX. It must have superior background rejection and better electronic stability. The summing of 200 to 250 individual energy spectra could result in serious loss of energy resolution for the overall experiment. In IGEX, instabilities

250

C.E. Aalseth et al. /Nuclear

Physics B (Proc. Suppl.) 124 (2003) 247-252

lead to a 25% degradation in the energy resolution of the 117 moleyears of data. The PNNL/USC group has overcome these problems; the technology is now available. First, detectors electronically segmented into 12 individual volumes in a single n-type intrinsic Ge detector are available from two commercial companies: PerkinElmer (ORTEC) and Canberra Industries. Second, completely digital electronics from XIA (Xray Instrumentation Associates) has been used by our group to demonstrate unprecedented stability, very low energy thresholds (< 1 keV) for a 2 kg Ge detector, and a vast improvement in pulse shape discrimination[l2]. In the few years since the production of the 2-kg IGEX intrinsic Ge detectors, a new technology has evolved in the two industrial companies known to us. Large, semi-coaxial n-type detectors have been fitted with a series of azimuthal electrical contacts along their length, and one or more axial contacts in the central hole. This effectively separates the detector into pie-shaped segments from the external contacts, and into individual cylinders from the internal contact. After Monte Carlo studies and discussions with detector manufacturers, this is the general configuration that the the Majorana collaboration believes to be optimal for balancing background reduction, cost and production efficiency. The saga of pulse-shape discrimination (PSD) in the IGEX project has been slow and painful, finally culminating in success. Current techniques depend entirely on experimental calibration and do not utilize pulse-shape libraries. The ability of these techniques to be easily calibrated for individual detectors makes them practical for large detector arrays. A major contributor to this success has been the availability of commercial digital spectroscopy hardware. Digitizing a detector preamplifier signal, all subsequent operations on the signal are performed digitally. Programmable digital filters are capable of producing improved energy resolution, long-term stability, and dynamic range. The particular unit our group has been using is the Cchannel Digital Gamma Finder (DFG4C) unit developed and manufactured by XL4 Inc.

The DGF-4C has 4 independent, 40 MHz, 12bit analog-to-digital converters (ADCS). The ADCs are followed by First-in, First-out (FIFO) buffers capable of storing 1024 ADC values for a single event. In parallel with each FIFO is a programmable digital filter and trigger logic. The digital filter and trigger logic for each channel is combined into a single Field Programmable Gate Array (FPGA). Analog input data are continuously digitized and processed at 40 MHz. The DGF-4C is then a smart filter of incoming pulses. If for example a signal has a pulsewidth incompatible with the usual collection time of 200-300 ns, or is oscillatory (like microphonic noise), the filters can be programmed to reject them. This feature can be used to allow the very low energy thresholds required in Dark Matter searches as well as eliminating the broad spectrum of artificial pulses from high voltage leaks and electromagnetic interference that can add noise pulses in the region of OvPfl-decay. The 12-bit ADCs produce pulse forms that allow the discrimination between single-site interactions in the detector crystal, characteristic of Ov@decay, and the multiple-site interactions characteristic of most gamma-ray background events near 2 MeV. Experimental example pulses are shown in Figure 2 . An example single-site event from the 1592 keV double-escape peak of the ‘OsTl 2615 keV line is shown as the top signal. The bottom signal is an example multisite pulse from the full-energy peak of the 212Bi line at 1620 keV. The cheaper and more reliable digital electronics, coupled with the new, but well established; segmented Ge detector technology, were the two steps needed to perfect the Ge detector approach sufficiently to justify a large array of detectors isotopically enriched in 76Ge. This technology is ready to be deployed in a large, next generA phased ation 76Ge Ou @-decay exp eriment. approach is recommended so that all aspects of such an expensive endeavor are clearly demonstrated. The Majorana detector array would represent a large increase in msss and a significant reduction in background compared to previous ionization detector searches for cold dark matter. Figure

C.E. Aalseth et al. /Nuclear Physics B (Proc. Suppl.) 124 (2003) 247-252

I,,-

1000

1500 tlS

0

1000

1500

2000

Figure 2. Example multiple site pulse from the 1621 keV (top) full energy peak from 214Bi. Example single site pulse (bottom) from the pair production 1592.5 keV double escape peak of the decay of 232Th.

3 shows the predicted sensitivity of a Majorana CDM search. Recently, a small fraction of the Heidelberg Moscow collaboration has reanalyzed part of their data and claim to have evidence for neutrinoless double beta decay of 76 Ge [13]. Subsequently a group of authors from 14 institutions responded with specific criticisms [14]. Among their criticisms were: 1) There was no null hypothesis test demonstrating that the data require a peak, and no simulations to demonstrate that the analysis would find true peaks or not lind no peaks were there none. 2) There are 3 unidentified perks in the region that have greater statistical significance than the peak at the Ou /3p - decay energy. 3) These data give a best half life of 1.5 x 1O25 years, where their earlier publication, bared on much more data, gave an lower bound of 1.9 x 1O25 y at a 95% CL. The authors of Ref. [14] also noted that there

ns

Figure 3. Projected 95% C.L. WIMP limits with the projected low-energy background of O.O05counts/keV/kg/day,an electronic threshold of 1 keV,with 5000 kg - y of exposure.

was no discussion of how choosing the width of the analysis window in energy effected the result. Independent analysis done by them indicates a significant effect. Finally, there was no analysis of the peak strengths of all of the 214Bi peaks in the region of interest. These peaks in fact did not show the correct relative intensities to be 214Bi peaks. While the authors of Ref.(l$) noted these criticisms, they clearly recognize the high quality of the Heidelberg-Moscow experiment and the high quality of the data. The criticism is strictly of the methodology of data analysis, and with the fact that even if there were a 2 or 3 sigma effect, it is unconventional to clain evidence of the observation of a phenomen on with only this confidence level. The authors of Ref.(l4) can not clain such an effect does not exist, only that evidence for it can not be legitimately claimed based on the arguments presented in Ref. (13).

REFERJZNCES 1.

H. Primakoff and S.P. Rosen, Rep. Prog. Phys. 22, 121 (1959); Phys. Rev. 184, 1925

251

252

2.

3.

4. 5.

6.

7. 8.

9.

10.

11. 12.

13.

14.

C.E. Adseth et al. /Nuclear Physics B (Proc. Suppl.) 124 (2003) 247-252 (1969). W.C. Haxton and G.J. Stevenson Jr., Prog. Part. Nucl. Phys. 12, 409 (1984); F.T. Avignone III and R.L. Brodzinski, Prog. Part. Nucl. Phys. 21,99 (1988); M. Moe and P. Vogel, Ann. Rev. Nucl. Part. Sci 44, 247 (1994). A. Morales, Nucl. Phys. B (Proc. Suppl.) 77, 335 (1999); H. Ejiri, Phys. Rept. C (2000), Int. J. Mod. Phys. E6, 1 (1997); V. Tretyak and Y. Zdesenko, At. Data, Nucl. Data Tables 61,43 (1995). See also the very recent review: Steven R. Elliott and Petr Vogel, Ann. Rev. Nucl. Part. Sci., vol. 52 (in press); hepph/0202264. E. Fiorini et al., Phys. Lett. 25 B, 602 (1967); Lett. Nuovo Cimento 3, 149 (1970). L. Baudis, et al., Phys. Rev. Lett. 83, 41 (1999); see also F.T. Avignone III, C.E. Aalseth, and R.L. Brodzinski, Phys. Rev. Lett. 85,465 (2000). (The value of 1.9 x 10z5 years is the new official Heidelberg Value Presented by H.V. Klapdor-Kleingrothaus at NANP-2001, June 2001, Dubna, Russia). C.E. Aalseth, et al., (IGEX Collaboration), Yad. Phys. 63, 1299 (2000); D. Gonzales, Nucl. Phys. B (Proc, Suppl.) 8’7, 278 (2000). W.C. Haxton, G.J. Stevenson, Jr., and D. Strottman, Phys. Rev. D 25, 2360 (1981). P. Vogel and M.R. Zirnbauer, Phys. Rev. Let. 57, 3148 (1986); J. Engel, P. Vogel, and M.R. Zirnbauer, Phys. Rev. C 37, 731 (1988). 0. Civitarese, A. Faessler, and T. Tomoda, Phys. Lett. B 194,11 (1987); T. Tomoda and A. Faessler, Phys. Lett. B 199, 475 (1987). K. Muto, E. Bender, and H.V. Klapdor, Z.Phys. A 177, 334 (1989). The matrix elements used in Table 1 are from A. Standt, K. Muto, and H.V. Klapdor, Europhys. Lett. 13, 31 (1990). E. Caurier, et al., Phys. Rev. Lett. 77, 1954 (1996). B. Hubbard-Nelson, M. Momayezi and W.K. Warburton, Nucl. Instr. and Meth. in Physics Research A 422, 411 (1999). H.V. Klapdor-Kleingro Thaus, A. Dietz, H.L. Harney and I.V. Krivosheina, Mod. Phys. Lett. A 16, 2409 (2000). C.E. Aalseth et al., “COMMENT ON EV-

IDENCE FOR NEUTRINOLESS DOUBLE BETA DECAY“, Mod. Phys. Lett. (submitted); hepex/0202018/ Feb. 2002.