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Scintillating bolometers based on ZnMoO4 and Zn100MoO4 crystals to search for0ν2β decay of100Mo (LUMINEU project): first tests at the Modane

Underground Laboratory

D.V. Podaa,b,1, E. Armengaudc,2, Q. Arnaudd,2, C. Augierd,2, A. Benoıtd,2, A. Benoıte,2, L. Bergea,1,2, R.S. Boikob,1,T. Bergmannf,2, J. Blumerg,h,2, A. Broniatowskia,2, V. Brudanini,2, P. Camuse,2, A. Cazesd,2, B. Censierd,2,

M. Chapelliera,1,2, F. Charlieuxd,2, D.M. Chernyaka,b,1, N. Coronj,1, P. Coulterk,2, G.A. Coxg,2, F.A. Danevichb,1,T. de Boissierec,2, R. Decourtl,1, M. De Jesusd,2, L. Devoyonm,1, A.-A. Drilliena,1,2, L. Dumoulina,1,2, K. Eitelh,2,

C. Enssn,1, D. Filosofovi,2, A. Fleischmannn,1, N. Fourchesc,2, J. Gascond,2, L. Gastaldon,1, G. Gerbierc,2,A. Giuliania,o,p,1, M. Grosc,1,2, L. Hehnh,2, S. Henryk,2, S. Hervec,1,2, G. Heuermanng,2, V. Humberta,1,2,

I.M. Ivanovq,1, A. Juillardd,2, C. Kefeliand,g,2, M. Kleifgesf,2, H. Kluckg,2, V.V. Kobychevb,1, F. Koskasm,1,V. Kozlovh,2, H. Krausk,2, V.A. Kudryavtsevr,2, H. Le Sueura,2, M. Loidls,1, P. Magnierc,1, E.P. Makarovq,1,

M. Mancusoa,t,1, P. de Marcillaca,1, S. Marnierosa,1,2, C. Marrache-Kikuchia,1, A. Menshikovf,2, S.G. Nasonovq,1,X-F. Navickc,1,2, C. Nonesc,1,2, E. Olivieria,1,2, P. Pariu,2, B. Paulc,1,2, Y. Penichotc,1, G. Pessinap,v,1, M.C. Piroa,2,

O. Plantevina,1, T. Redonj,1, M. Robinsonr,2, M. Rodriguess,1, S. Rozovi,2, V. Sanglardd,2, B. Schmidtg,2,V.N. Shlegelq,1, B. Siebenbornh,2, O. Strazzerm,1, D. Tcherniakhovskif,2, M. Tenconia,1, L. Torresj,1, V.I. Tretyakb,1,

L. Vagnerond,2, Ya.V. Vasilievq,1, M. Velazquezl,1, O. Viraphongl,1, R.J. Walkerh,2, M. Weberf,2, E. Yakushevi,2,X. Zhangk,2, V.N. Zhdankovw,1, (for the LUMINEU and the EDELWEISS Collaborations)

aCSNSM, Centre de Sciences Nucleaires et de Sciences de la Matiere, CNRS/IN2P3, Universite Paris-Sud, 91405 Orsay, FrancebInstitute for Nuclear Research, MSP 03680 Kyiv, Ukraine

cCEA, Centre d’Etudes Saclay, IRFU, 91191 Gif-Sur-Yvette Cedex, FrancedIPNL, Universite de Lyon, Universite Lyon 1, CNRS/IN2P3, 69622 Villeurbanne Cedex, France

eInstitut Neel, CNRS/UJF, 38042 Grenoble Cedex 9, FrancefKarlsruhe Institute of Technology, Institut fur Prozessdatenverarbeitung und Elektronik, 76021 Karlsruhe, Germany

gKarlsruhe Institute of Technology, Institut fur Experimentelle Kernphysik, 76128 Karlsruhe, GermanyhKarlsruhe Institute of Technology, Institut fur Kernphysik, 76021 Karlsruhe, Germany

iLaboratory of Nuclear Problems, JINR, 141980 Dubna, Moscowregion, Russiaj IAS, CNRS, Universite Paris-Sud, 91405 Orsay, France

kUniversity of Oxford, Department of Physics, Oxford OX1 3RH, UKl ICMCB, CNRS, Universite de Bordeaux, 33608 Pessac Cedex, France

mCEA, Centre d’Etudes Saclay, Orphee, 91191 Gif-Sur-Yvette Cedex, FrancenInstitut fur Angewandte Physik, Universitat Heidelberg, D-69120 Heidelberg, Germany

oDipartimento di Scienza e Alta Tecnologia dellUniversit‘adellInsubria, 22100 Como, ItalypINFN, Sezione di Milano-Bicocca, 20126 Milano, Italy

qNikolaev Institute of Inorganic Chemistry, 630090 Novosibirsk, RussiarDepartment of Physics and Astronomy, University of Sheffield, Hounsfield Road, Sheffield S3 7RH, UK

sCEA, Centre d’Etudes Saclay, LNAB, 91191 Gif-Sur-Yvette Cedex, FrancetDipartimento di Scienza e Alta Tecnologia dell’Universit´a dell’Insubria, I-22100 Como, Italy

uCEA, Centre d’Etudes Saclay, IRAMIS, 91191 Gif-Sur-YvetteCedex, FrancevDipartimento di Fisica dell’Universita di Milano-Bicocca, 20126 Milano, Italy

wCML Ltd., 630090 Novosibirsk, Russia

Abstract

The technology of scintillating bolometers based on zinc molybdate (ZnMoO4) crystals is under development withinthe LUMINEU project to search for 0ν2β decay of100Mo with the goal to set the basis for large scale experimentscapable to explore the inverted hierarchy region of the neutrino mass pattern. Advanced ZnMoO4 crystal scintillatorswith mass of∼ 0.3 kg were developed and Zn100MoO4 crystal from enriched100Mo was produced for the first timeby using the low-thermal-gradient Czochralski technique.One ZnMoO4 scintillator and two samples (59 g and 63 g)cut from the enriched boule were tested aboveground at milli-Kelvin temperature as scintillating bolometers showinga high detection performance. The first results of the low background measurements with three ZnMoO4 and twoenriched detectors installed in the EDELWEISS set-up at theModane Underground Laboratory (France) are presented.

Keywords: Double beta decay, Scintillating bolometer, ZnMoO4 crystal scintillator, Low counting experiment

D.V. Poda et al./ 00 (2015) 1–7 2

1. Introduction

Scintillating bolometers — cryogenic detectors witha heat-light double read-out — can play a crucial rolein next-generation experiments to study neutrino prop-erties and weak interaction via investigating neutrino-less double beta (0ν2β) decay, as discussed in Refs.[1, 2]. This technique is extensively developing nowwithin the LUCIFER [3, 4], the AMoRE [5, 6], and theLUMINEU [7] 0ν2β projects. This paper describes therecent achievements in the framework of the LUMINEUprogramme (Luminescent Underground MolybdenumInvestigation for NEUtrino mass and nature).

LUMINEU is devoted to the development of a tech-nology based on zinc molybdate (ZnMoO4) scintillat-ing bolometer as a basis for the realization of a high-sensitivity 0ν2β experiment. The good prospects of thismaterial for the bolometric technique are clearly shownin recent investigations [1, 8–13]. An important point inthe realization of LUMINEU is concerned with the tech-nology of growing high-quality radiopure large mass(0.3–0.5 kg) ZnMoO4 single crystals with the aim toproduce scintillators enriched in100Mo (Zn100MoO4).Here we report a significant progress in the develop-ment of ZnMoO4 crystal scintillators using deeply pu-rified compounds (containing molybdenum with naturalisotopic composition and enriched in100Mo). We alsopresent results of both aboveground and undergroundlow temperature tests of new scintillating bolometersbased on natural ZnMoO4 and enriched Zn100MoO4

crystal scintillators in light of their possible applicationto next-generation 0ν2β decay experiments.

2. Development of zinc molybdate based scintillat-ing bolometers

A precursor of the LUMINEU programme, a slightlyyellow colored 313 g ZnMoO4 sample with irreg-ular shape, was produced from the first large vol-ume ZnMoO4 crystal boule grown by the low-thermal-gradient Czochralski (LTG Cz) technique [14, 15] inthe Nikolaev Institute of Inorganic Chemistry (NIIC,Novosibirsk, Russia). The second sample (with mass329 g) produced from this boule was tested as a scintil-lating bolometer at the Gran Sasso National Laborato-ries (LNGS, Assergi, Italy) [11].

Advanced ZnMoO4 crystal boules with mass of∼ 1kg have been produced recently at the NIIC by using

Email address:Denys.Poda@csnsm.in2p3.fr (D.V. Poda)1The LUMINEU Collaboration2The EDELWEISS Collaboration

the LTG Cz growth technique and molybdenum puri-fied by sublimation in vacuum and double recrystalliza-tion from aqueous solutions [13]. The crystals wererecrystallized to improve quality of the material, andtwo colorless ZnMoO4 cylindrical samples (with size⊘50×40 mm and mass 336 and 334 g) were producedfrom them. Moreover, a zinc molybdate crystal boule(with mass 171 g) enriched in100Mo to 99.5% was de-veloped for the first time at the NIIC [16, 17], and twoscintillation elements (with mass 59 and 63 g) were cutfrom the boule. The enriched molybdenum was purifiedby sublimation and recrystallization from aqueous solu-tions. It is worth noting the high yield of the Zn100MoO4

crystal boule from the initial charge (84%) and low levelof total irrecoverable losses of enriched material (4%)achieved in the frame of this R&D [16]. Some col-oration of the crystal (in contradiction with the practi-cally colorless samples produced from natural molyb-denum) can be explained by remaining traces of iron inthe enriched molybdenum and by crystallization proce-dure performed only one time [16].

In order to construct scintillating bolometers, all theabove described samples were held inside Copper hold-ers by using PTFE clamps. Both Zn100MoO4 crys-tals were mounted in one Copper holder. The crys-tal scintillators were surrounded by a reflector foil (3MVM2000/2002) to improve light collection. Thin ultra-pure Ge wafers (⊘50×0.25 mm) were used for detect-ing scintillation light. The 313 g crystal was viewedby two light detectors fixed on the opposite sides. TheZnMoO4 / Zn100MoO4 crystals and the Ge photode-tectors were instrumented with Neutron TransmutationDoped (NTD) Ge thermistors used as temperature sen-sors. All the crystals were also assembled with an indi-vidual heating element based on a heavily-doped siliconmeander. Such devices provide a stable resistance valueand are used to inject periodically a certain amount ofthermal energy with the aim to control and stabilize thethermal bolometric response. All the detector modulesare shown in Fig. 1.

3. Aboveground low temperature tests

The 313 g ZnMoO4 precursor and both Zn100MoO4

crystals were tested in aboveground cryogenic facilitiesof the Centre de Sciences Nucleaires et de Sciences de laMatiere (CSNSM, Orsay, France) with “wet” and “dry”3He/4He dilution refrigerators, respectively.

Both cryostats are surrounded by passive shield madeof low activity lead to minimize signals pile-up causedby environmental gamma rays due to a slow time re-sponse of the bolometers (hundreds millisecond). The

D.V. Poda et al./ 00 (2015) 1–7 3

(a)

(b)

(c)

Figure 1: Photographs of scintillating bolometers based onultra-pureGe photodetectors and ZnMoO4 precursor with mass of 313 g (a),advanced quality (see text) ZnMoO4 crystals with masses of 336 and334 g (b), and enriched Zn100MoO4 crystals with masses of 59 and63 g (c).

stream data were recorded by a 16 bit ADC with a sam-pling frequency of 30 kHz and 10 kHz for natural andenriched detectors, respectively. The ZnMoO4 precur-sor was operated at 17 mK during the measurements(over 38 h), while the Zn100MoO4 array was tested at13.7 mK (18 h), 15 mK (5 h), and 19 mK (24 h) basetemperatures. Both detectors were irradiated by gammaquanta from a weak232Th source, while the photodetec-tors were calibrated with the help of55Fe sources fixed

close to the Ge slabs.The data treatment (here and below) was performed

by using the optimum filtering [18]. The spectromet-ric performances of the precursor-based bolometer weredeteriorated by the pile-ups effect due to considerablyhigh counting rate≈ 2.5 Hz (e.g. see in Table 1 the en-ergy resolution of the 2615 keVγ peak). In spite of this,the test shows normal operability of the detector and al-lows us to estimate the scintillation light yield for theregisteredγ(β) events and muons, as well as the pos-sibility of particle discrimination betweenγ(β) andαevents due to the quenching of scintillation forα parti-cles. All these data are reported in Table 1.

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214Bi, 609228Ac, 911, 969

208Tl, 2615

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Cou

nts

/ 2 k

eV

Figure 2: (a) The scatter plot of the light-to-heat signal amplitude ra-tio as a function of the heat signal amplitude accumulated at13.7 mKin aboveground test with the 59 g Zn100MoO4 scintillating bolometerduring 18 h of calibration measurements with the232Th source. Thevisible band is related toγ(β) events (below 2.6 MeV) and cosmicmuons. Three sigma intervals of the light yield for theγ(β) band areshown by solid red curves together with the median value. (b)Theenergy spectrum built from the data presented in the upper plot. Thepeaks observed in the energy spectrum belong to the232Th source andenvironmental gamma’s (daughters of226Ra). The energy of markedpeaks are given in keV.

Both enriched crystals demonstrate similar perfor-mance at all the temperatures [16]. The 2-dimensionalhistogram obtained from the heat-light double read-outof the 59 g Zn100MoO4 bolometer at 13.7 mK is shownin Fig. 2 (a). The light and the heat signals detectedsimultaneously allow to get a clear discrimination be-tweenα andγ(β) particles. The absence in Fig. 2 (a)of peculiarities related with the detection ofα events(except a small structure possibly caused by210Po, as

D.V. Poda et al./ 00 (2015) 1–7 4

Table 1: List of achieved performances with ZnMoO4 and Zn100MoO4 detectors tested in aboveground and underground measurements. We reportthe energy resolution for the heat channels (FWHM — Full Width at the Half of Maximum) estimated as filtered baseline and measured forγquanta andα particles of internal210Po. We report also the light yield forγ(β) events (LYγ(β)) and quenching factor forα particles (QFα).

Detector FWHM (keV) LYγ(β) QFαCrystal Mass (g) Baseline 133Ba 214Bi 208Tl 210Po (keV/MeV)

356 keV 609 keV 2615 keV 5407 keVZnMoO4 313 1.4(1) 6.4(1) 6(1)∗ 24(2)∗ / 9(2) 19(1) 0.77(11)∗ 0.15(2)∗ / 0.14(1)

336 1.5(2) 6(1) – – 29(4) – –334 1.06(3) 3.8(4) – – 15(1) – 0.19(2)

Zn100MoO4 59 1.4(1)∗ – 5.0(5)∗ 11(3)∗ – 1.01(11)∗ ≈ 0.15∗

63 1.8(1)∗ – 10(1)∗ – – 0.93(11)∗ ≈ 0.15∗

* — results based on the aboveground measurements

often occurs in scintillators) indicates on encouragingradiopurity of the tested enriched crystals. Good spec-trometric properties of the enriched detectors, even ataboveground conditions, are well visible from Fig. 2(b), while some further information about their perfor-mances is presented in Table 1.

4. Underground cryogenic measurements

The 313 g detector was moved deep underground(≈4800 m w.e.) to the Modane Underground Labora-tory (Laboratoire Souterrain de Modane, LSM, France)and tested during the EDELWEISS-III commissioningruns. The ZnMoO4 bolometer together with fifteenultra-pure Ge detectors (0.8 kg each) fully covered withinterleaved electrodes (FID) were installed inside the3He/4He inverted dilution refrigerator with a large ex-perimental volume (50l) [19]. The EDELWEISS set-up, located inside a clean room (ISO Class 4) and sup-plied by deradonized (≈30 mBq/m3) air flow, is sur-rounded by a massive shield made of low backgroundlead (20 cm thick) and polyethylene (50 cm). The set-up is surrounded by a 5 cm thick plastic scintillatormuon veto (95% coverage), and equipped by neutronand radon counters.

The triggered signals were recorded by a 14 bit ADCin 2 s window with 2 kHz sampling rate (the half ofthe window contains the baseline data). The base tem-perature was stabilized around 19 mK. One light detec-tor was very sensitive to microphonic noise and couldnot be used for measurements. The energy scale of theZnMoO4 detector has been measured in calibration runswith 133Ba and232Th γ sources, performed over 546 hand 70 h, respectively. The background data were accu-mulated over 305 h.

The powerful discrimination capability achieved withthe 313 g ZnMoO4 scintillating bolometer is well il-lustrated in Fig. 3 (a), which shows a full separationof γ(β)-induced events from populations ofα particles

caused by trace impurity by radionuclides from U/Thchains (mainly,210Po, see below). The energy spectrumaccumulated with the232Th gamma source (see Fig. 3(b)) demonstrates high spectrometric properties of thedetector. An overview of the detector’s performancesduring underground measurements is given in Table 1.

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208Tl, 2615

(b)

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Cou

nts

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eV

Figure 3: (a) Plot reporting the light-to-heat signal amplitude ratio asa function of the heat signal amplitude for the 313 g ZnMoO4 detectorinstalled in the EDELWEISS set-up. The detector was cooled downto 19 mK and irradiated over 51 h byγ quanta from the232Th source.Two visible bands correspond toγ(β) events andα particles. Thepositions of theα events are shifted from the nominal values due tothermal energy overestimation forα particles in case of using calibra-tion data forγ’s. Three sigma intervals of the light yield for theγ(β)band and its median value are drawn. (b) The energy spectrum of the232Th source measured by the 313 g ZnMoO4 scintillating bolometerduring 51 h of underground cryogenic run.

After completing the EDELWEISS-III commis-sioning runs, other two ZnMoO4-based scintillatingbolometers (⊘50× 40 mm) and the Zn100MoO4 arraytogether with 36 FID Ge detectors were assembled. The

D.V. Poda et al./ 00 (2015) 1–7 5

EDELWEISS set-up was also upgraded: a) a polyethy-lene shield at the 1 K plate was added; b) new ultra ra-diopure NOSV Copper [20] screens were installed; c)all detectors were provided with individual low back-ground Copper-Kapton cables. In addition, a pulser sys-tem to assist to the calibration of the thermal responseof the ZnMoO4 / Zn100MoO4 detectors will be imple-mented soon.

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Figure 4: (a) Scatter plot of the light versus the heat signals mea-sured by the 334 g ZnMoO4 scintillating bolometer in a 15 h cali-bration run with the133Ba gamma source in the EDELWEISS set-up.A cluster of events located far from theγ(β) population correspondsto α particles of210Po. The data for the light channel are presentedin ADU (Analogue-to-Digital Unit). (Insert) Part of the scatter plotcorresponding to the energy range of the used source. (b) Theen-ergy spectrum of the133Ba source measured over 15 h by the 334 gZnMoO4 scintillating bolometer.

After the upgrade of the set-up the data are recordedby a 16 bit ADC with 1 kHz sampling rate (the lengthof pulse profile is 2 s with the half of the window for thebaseline data). The working temperature is stabilized at18 mK. The energy scale of the detectors was measuredwith the 133Ba gamma source (the measurements withthe232Th source are foreseen).

The set-up is still under optimization, especially asfar as the control of the vibration-induced noise is con-cerned. Therefore, we discuss here, as an illustrative ex-ample, only the results achieved with the 334 g naturalZnMoO4 scintillating bolometer. This detector exhibitsfull α/γ(β) separation, as shown in Fig. 4 (a), as wellas excellent spectrometric properties, as demonstratedin Fig. 4 (b). Other relevant information about perfor-mances of⊘50×40 mm ZnMoO4 detectors are reported

in Table 1.

5. Radiopurity of ZnMoO4 and Zn100MoO4 crystals

The radiopurity level of the ZnMoO4 crystals was es-timated by analysis of theα events selected from the un-derground runs, while the data of the aboveground mea-surements were used in case of the Zn100MoO4 sam-ples. The position of the 5.4 MeVα peak of the internal210Po, clearly visible in the data for the natural crys-tals, was used to stabilize the thermal response of thedetectors. For instance, the spectra of theα events reg-istered by the detectors based on 313 g (a) and 334 g (b)ZnMoO4 crystals over 851 h and 527 h, respectively, areshown in Fig. 5.

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Unidentified

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Figure 5: Theα spectra collected in the low background measure-ments in the EDELWEISS set-up with the ZnMoO4 scintillatingbolometers based on the 313 g precursor (a) and the 334 g advancedsample (b) operated over 851 h and 527 h, respectively. The origin oftheα events providing the highest rate are indicated.

The crystals are slightly polluted by210Po detectedthrough 5.4 MeVα peak confirming a broken equilib-rium in the radioactive chain.226Ra (and its daughters222Rn, 218Po, and214Bi-214Po events), and228Th (withdaughter224Ra3) were detected in the 313 g crystal,while the ZnMoO4 scintillators produced by recrystal-lization have shown a much better level of radiopurity,particularly in226Ra. It is also evident a higher surfacecontamination by210Po of the 313 g crystal or/and ofthe bolometer components close to it (a peak at 5.3 MeV

3Taking into account a short half-life of216Po (≈145 ms), whichis comparable with the time response of the 313 g detector (hundredsms), subsequentα decays of220Rn-216Po give pile-ups and thereforewere discarded from the data by the pulse-shape analysis.

D.V. Poda et al./ 00 (2015) 1–7 6

corresponds toEα of 210Po). In addition, excess countsaround 5.8 MeV also indicate a possible surface con-tamination but its origin has not been identified.

The activity of internal210Po was derived from thefit of the 5.4 MeV peak, while 3σ intervals (accordingto the energy resolution of the internal210Po — see Ta-ble 1) centered at theQα value were used for the calcula-tion of the area of the peaks of other radionuclides fromU/Th chains. The background contribution was evalu-ated in two energy regions (3.3–4 and 4.35–4.7 MeV)with a flatα continuum in which no peaks are expected.The number of counts excluded with 90% C.L. were cal-culated by using the Feldman-Cousins procedure [21].

Table 2: Radioactive contamination of the ZnMoO4 and Zn100MoO4crystals tested as scintillating bolometers in aboveground and under-ground conditions. The mass of the crystals and the total time of theaccumulated data are also presented. The results for the large massZnMoO4 crystal which was operated as the scintillating bolometer atthe LNGS (Italy) [11] are given for comparison. The uncertainties aregiven with 68% C.L., while all the limits are at 90% C.L.

Nuclide Activity (mBq/kg)Zn100MoO4 ZnMoO4

59 g 63 g 336 g 334 g 313 g 329 g [11]42 h 42 h 291 h 527 h 851 h 524 h

228Th ≤ 0.25 ≤ 0.21 ≤ 0.024 ≤ 0.007 0.010(3) ≤ 0.006238U ≤ 0.26 ≤ 0.21 ≤ 0.008 ≤ 0.002 ≤ 0.008 ≤ 0.006226Ra ≤ 0.26 ≤ 0.31 ≤ 0.021 ≤ 0.009 0.26(5) 0.27(6)210Po 0.9(3) 1.1(3) 0.94(5) 1.02(7) 0.62(3) 0.70(3)

Data (or limits) on radioactive contamination of theZnMoO4 and Zn100MoO4 scintillators are summarizedin Table 2, where the results for another ZnMoO4 sam-ple, produced from the same boule as the 313 g crystalwas, are presented for comparison. As it is seen fromTable 2, the improved purification and crystallizationprocedure adopted for the LUMINEU crystals of 334and 336 g has lead to a significant reduction of the in-ternal contamination, especially for226Ra which is notdetectable now while it was clearly present in both pre-cursor crystals (313 and 329 g). In particular, the ra-diopurity levels (≤0.01 mBq/kg) achieved for228Th and226Ra are fully compatible with next-generation 0ν2βexperiments capable to explore the inverted hierarchyregion of the neutrino mass pattern [1, 2].

6. Conclusions

A significant progress is achieved in developmentof ZnMoO4 crystal scintillators for the LUMINEUproject. Large volume crystal boules (∼ 1 kg each) weregrown by the low-thermal-gradient Czochralski tech-nique from deeply purified molybdenum. A Zn100MoO4

crystal boule with a mass of 0.17 kg was produced fromenriched100Mo (to 99.5%) for the first time. Three natu-ral (∼ 0.3 kg) and two enriched (∼ 0.06 kg) scintillationelements were produced for low temperature studies.Production of large volume Zn100MoO4 crystal scintil-lators from enriched100Mo is in progress.

The cryogenic scintillating bolometric tests of thenatural and enriched crystals showed a high perfor-mance of the detectors. The deep purification of molyb-denum and recrystallization significantly improve theradioactive contamination of ZnMoO4 crystals by228Thand226Ra to the level of≤0.01 mBq/kg requested by theLUMINEU project.

The results of this study clarify the excellentprospects of ZnMoO4 scintillating bolometers for thenext generation 0ν2β experiments aiming to approachthe inverted hierarchy region of the neutrino mass pat-tern.

7. Acknowledgments

The development of ZnMoO4 scintillating bolome-ters is part of the LUMINEU programme, a projectreceiving funds from the Agence Nationale de laRecherche (ANR, France). The work was supportedin part by the project “Cryogenic detector to searchfor neutrinoless double beta decay of molybdenum” inthe framework of the Programme “Dnipro” based onUkraine-France Agreement on Cultural, Scientific andTechnological Cooperation. The aboveground bolomet-ric tests were assisted by ISOTTA, a project receivingfunds from the ASPERA 2nd Common Call dedicatedto R&D activities. D.V. Poda was supported by theP2IO LabEx (ANR-10-LABX-0038) in the framework“Investissements d’Avenir” (ANR-11-IDEX-0003-01)managed by the ANR (France).

References

[1] J.W. Beeman et al., A next-generation neutrinoless double betadecay experiment based on ZnMoO4 scintillating bolometers,Phys. Lett. B 710 (2012) 318.

[2] D.R. Artusa et al., Exploring the neutrinoless double beta de-cay in the inverted neutrino hierarchy with bolometric detectors,Eur. Phys. J. C 74 (2014) 3096.

[3] F. Ferroni, LUCIFER: A new technique for Double Beta Decay,Il Nuovo Cim. C 33, N 5 (2010) 27.

[4] J.W. Beeman et al., Current status and future perspectives of theLUCIFER experiment, AHEP 2013 (2013), Article ID 237973.

[5] H. Bhang et al., AMoRE experiment: a search for neutrino-less double beta decay of100Mo isotope with40Ca100MoO4cryogenic scintillation detector, J. Phys.: Conf. Ser. 375(2012)042023.

D.V. Poda et al./ 00 (2015) 1–7 7

[6] G.B. Kim et al., A CaMoO4 crystal low temperature detector forthe AMoRE neutrinoless double beta decay search, Accepted toAHEP (2014), Article ID 817530.

[7] M. Tenconi et al., LUMINEU: a pilote scintillating bolometerexperiment for neutrinoless double beta decay search, presentedat TAUP 2013 and to be published in the proceedings.

[8] L. Gironi et al., Performance of ZnMoO4 crystal as cryogenicscintillating bolometer to search for double beta decay of molyb-denum, JINST 5 (2010) P11007.

[9] J.W. Beeman et al., ZnMoO4: A promising bolometer for neu-trinoless double beta decay searches, Astropart. Phys. 35 (2012)813.

[10] J.W. Beeman et al., An improved ZnMoO4 scintillating bolome-ter for the search for neutrinoless double beta decay of100Mo,J. Low Temp. Phys. 167 (2012) 1021.

[11] J.W. Beeman et al., Performances of a large mass ZnMoO4 scin-tillating bolometer for a next generation 0νDBD experiment,Eur. Phys. J. C 72 (2012) 2142.

[12] D.M. Chernyak et al., Optical, luminescence and thermalproperties of radiopure ZnMoO4 crystals used in scintillatingbolometers for double beta decay search, Nucl. Instr. Meth.A729 (2013) 856.

[13] L. Berge et al., Purification of molybdenum, growth and char-acterization of medium volume ZnMoO4 crystals for the LU-MINEU program, JINST 9 (2014) P06004.

[14] A.A. Pavlyuk, Ya.V. Vasiliev, L.Yu. Kharchenko,F.A. Kuznetsov, Low Thermal Gradient technique andmethod for large oxide crystals growth from melt and flux, inProc. of the APSAM-92, Asia Pacific Society for AdvancedMaterials, Shanghai, 26–29 April 1992, Institute of MaterialsResearch, Tohoku University, Sendai, Japan, 1993, p. 164.

[15] E.N. Galashov, P.S. Galkin, P.E. Plusnin, V.N. Shlegel, Spe-cific features of the phase formation, synthesis, and growthofZnMoO4 crystals, Crystallogr. Rep. 59 (2014) 288.

[16] A.S. Barabash et al., Enriched Zn100MoO4 scintillating bolome-ters to search for 0ν2β decay of100Mo with the LUMINEUexperiment, arXiv:1405.6937 [physics.ins-det], accepted to Eur.Phys. J. C.

[17] D.N. Grigoriev, F.A. Danevich, V.N. Shlegel and Ya.V. Vasiliev,Development of crystal scintillators for calorimetry in high en-ergy and astroparticle physics, JINST 9 (2014) C09004.

[18] E. Gatti, P. Manfredi, Processing the signals from solid-state de-tectors in elementary-particle physics, Riv. Nuovo Cim. 9 (1986)1.

[19] E. Armengaud et al., Final results of the EDELWEISS-II WIMPsearch using a 4-kg array of cryogenic germanium detectorswith interleaved electrodes, Phys. Lett. B 702 (2011) 329.

[20] M. Laubenstein et al., Underground measurements of radioac-tivity, Appl. Radiat. Isotopes 61 (2004) 167.

[21] G.J. Feldman, R.D. Cousins, Unified approach to the classi-cal statistical analysis of small signals, Phys. Rev. D 57 (1998)3873.