Scintillating bolometers based on ZnMoO$_4$ and Zn$^{100}$MoO$_4$ crystals to search for 0$\nu$2$\beta$ decay of $^{100}$Mo (LUMINEU project): first tests at the Modane Underground Laboratory

The technology of scintillating bolometers based on zinc molybdate (ZnMoO$_4$) crystals is under development within the LUMINEU project to search for 0$\nu$2$\beta$ decay of $^{100}$Mo with the goal to set the basis for large scale experiments capable to explore the inverted hierarchy region of the neutrino mass pattern. Advanced ZnMoO$_4$ crystal scintillators with mass of $\sim$~0.3 kg were developed and Zn$^{100}$MoO$_4$ crystal from enriched $^{100}$Mo was produced for the first time by using the low-thermal-gradient Czochralski technique. One ZnMoO$_4$ scintillator and two samples (59 g and 63 g) cut from the enriched boule were tested aboveground at milli-Kelvin temperature as scintillating bolometers showing a high detection performance. The first results of the low background measurements with three ZnMoO$_4$ and two enriched detectors installed in the EDELWEISS set-up at the Modane Underground Laboratory (France) are presented.


Introduction
Scintillating bolometers -cryogenic detectors with a heat-light double read-out -can play a crucial role in next-generation experiments to study neutrino properties and weak interaction via investigating neutrinoless double beta (0ν2β) decay, as discussed in Refs. [1,2]. This technique is extensively developing now within the LUCIFER [3,4], the AMoRE [5,6], and the LUMINEU [7] 0ν2β projects. This paper describes the recent achievements in the framework of the LUMINEU programme (Luminescent Underground Molybdenum Investigation for NEUtrino mass and nature).
LUMINEU is devoted to the development of a technology based on zinc molybdate (ZnMoO 4 ) scintillating bolometer as a basis for the realization of a highsensitivity 0ν2β experiment. The good prospects of this material for the bolometric technique are clearly shown in recent investigations [1,[8][9][10][11][12][13]]. An important point in the realization of LUMINEU is concerned with the technology of growing high-quality radiopure large mass (0.3-0.5 kg) ZnMoO 4 single crystals with the aim to produce scintillators enriched in 100 Mo (Zn 100 MoO 4 ). Here we report a significant progress in the development of ZnMoO 4 crystal scintillators using deeply purified compounds (containing molybdenum with natural isotopic composition and enriched in 100 Mo). We also present results of both aboveground and underground low temperature tests of new scintillating bolometers based on natural ZnMoO 4 and enriched Zn 100 MoO 4 crystal scintillators in light of their possible application to next-generation 0ν2β decay experiments.

Development of zinc molybdate based scintillating bolometers
A precursor of the LUMINEU programme, a slightly yellow colored 313 g ZnMoO 4 sample with irregular shape, was produced from the first large volume ZnMoO 4 crystal boule grown by the low-thermalgradient Czochralski (LTG Cz) technique [14,15] in the Nikolaev Institute of Inorganic Chemistry (NIIC, Novosibirsk, Russia). The second sample (with mass 329 g) produced from this boule was tested as a scintillating bolometer at the Gran Sasso National Laboratories (LNGS, Assergi, Italy) [11].
Advanced ZnMoO 4 crystal boules with mass of ∼ 1 kg have been produced recently at the NIIC by using Email address: Denys.Poda@csnsm.in2p3.fr (D.V. Poda) 1 The LUMINEU Collaboration 2 The EDELWEISS Collaboration the LTG Cz growth technique and molybdenum purified by sublimation in vacuum and double recrystallization from aqueous solutions [13]. The crystals were recrystallized to improve quality of the material, and two colorless ZnMoO 4 cylindrical samples (with size ⊘50×40 mm and mass 336 and 334 g) were produced from them. Moreover, a zinc molybdate crystal boule (with mass 171 g) enriched in 100 Mo to 99.5% was developed for the first time at the NIIC [16,17], and two scintillation elements (with mass 59 and 63 g) were cut from the boule. The enriched molybdenum was purified by sublimation and recrystallization from aqueous solutions. It is worth noting the high yield of the Zn 100 MoO 4 crystal boule from the initial charge (84%) and low level of total irrecoverable losses of enriched material (4%) achieved in the frame of this R&D [16]. Some coloration of the crystal (in contradiction with the practically colorless samples produced from natural molybdenum) can be explained by remaining traces of iron in the enriched molybdenum and by crystallization procedure performed only one time [16].
In order to construct scintillating bolometers, all the above described samples were held inside Copper holders by using PTFE clamps. Both Zn 100 MoO 4 crystals were mounted in one Copper holder. The crystal scintillators were surrounded by a reflector foil (3M VM2000/2002) to improve light collection. Thin ultrapure Ge wafers (⊘50×0.25 mm) were used for detecting scintillation light. The 313 g crystal was viewed by two light detectors fixed on the opposite sides. The ZnMoO 4 / Zn 100 MoO 4 crystals and the Ge photodetectors were instrumented with Neutron Transmutation Doped (NTD) Ge thermistors used as temperature sensors. All the crystals were also assembled with an individual heating element based on a heavily-doped silicon meander. Such devices provide a stable resistance value and are used to inject periodically a certain amount of thermal energy with the aim to control and stabilize the thermal bolometric response. All the detector modules are shown in Fig. 1.

Aboveground low temperature tests
The 313 g ZnMoO 4 precursor and both Zn 100 MoO 4 crystals were tested in aboveground cryogenic facilities of the Centre de Sciences Nucléaires et de Sciences de la Matière (CSNSM, Orsay, France) with "wet" and "dry" 3 He/ 4 He dilution refrigerators, respectively.
Both cryostats are surrounded by passive shield made of low activity lead to minimize signals pile-up caused by environmental gamma rays due to a slow time response of the bolometers (hundreds millisecond). The stream data were recorded by a 16 bit ADC with a sampling frequency of 30 kHz and 10 kHz for natural and enriched detectors, respectively. The ZnMoO 4 precursor was operated at 17 mK during the measurements (over 38 h), while the Zn 100 MoO 4 array was tested at 13.7 mK (18 h), 15 mK (5 h), and 19 mK (24 h) base temperatures. Both detectors were irradiated by gamma quanta from a weak 232 Th source, while the photodetectors were calibrated with the help of 55 Fe sources fixed close to the Ge slabs.
The data treatment (here and below) was performed by using the optimum filtering [18]. The spectrometric performances of the precursor-based bolometer were deteriorated by the pile-ups effect due to considerably high counting rate ≈ 2.5 Hz (e.g. see in Table 1 the energy resolution of the 2615 keV γ peak). In spite of this, the test shows normal operability of the detector and allows us to estimate the scintillation light yield for the registered γ(β) events and muons, as well as the possibility of particle discrimination between γ(β) and α events due to the quenching of scintillation for α particles. All these data are reported in Table 1. Both enriched crystals demonstrate similar performance at all the temperatures [16]. The 2-dimensional histogram obtained from the heat-light double read-out of the 59 g Zn 100 MoO 4 bolometer at 13.7 mK is shown in Fig. 2 (a). The light and the heat signals detected simultaneously allow to get a clear discrimination between α and γ(β) particles. The absence in Fig. 2 (a) of peculiarities related with the detection of α events (except a small structure possibly caused by 210 Po, as Table 1: List of achieved performances with ZnMoO 4 and Zn 100 MoO 4 detectors tested in aboveground and underground measurements. We report the energy resolution for the heat channels (FWHM -Full Width at the Half of Maximum) estimated as filtered baseline and measured for γ quanta and α particles of internal 210 Po. We report also the light yield for γ(β) events (LY γ(β) ) and quenching factor for α particles (QF α ).  Table 1.

Underground cryogenic measurements
The 313 g detector was moved deep underground (≈4800 m w.e.) to the Modane Underground Laboratory (Laboratoire Souterrain de Modane, LSM, France) and tested during the EDELWEISS-III commissioning runs. The ZnMoO 4 bolometer together with fifteen ultra-pure Ge detectors (0.8 kg each) fully covered with interleaved electrodes (FID) were installed inside the 3 He/ 4 He inverted dilution refrigerator with a large experimental volume (50 l) [19]. The EDELWEISS setup, located inside a clean room (ISO Class 4) and supplied by deradonized (≈30 mBq/m 3 ) air flow, is surrounded by a massive shield made of low background lead (20 cm thick) and polyethylene (50 cm). The setup is surrounded by a 5 cm thick plastic scintillator muon veto (95% coverage), and equipped by neutron and radon counters.
The triggered signals were recorded by a 14 bit ADC in 2 s window with 2 kHz sampling rate (the half of the window contains the baseline data). The base temperature was stabilized around 19 mK. One light detector was very sensitive to microphonic noise and could not be used for measurements. The energy scale of the ZnMoO 4 detector has been measured in calibration runs with 133 Ba and 232 Th γ sources, performed over 546 h and 70 h, respectively. The background data were accumulated over 305 h.
The powerful discrimination capability achieved with the 313 g ZnMoO 4 scintillating bolometer is well illustrated in Fig. 3 (a), which shows a full separation of γ(β)-induced events from populations of α particles caused by trace impurity by radionuclides from U/Th chains (mainly, 210 Po, see below). The energy spectrum accumulated with the 232 Th gamma source (see Fig. 3 (b)) demonstrates high spectrometric properties of the detector. An overview of the detector's performances during underground measurements is given in Table 1. After completing the EDELWEISS-III commissioning runs, other two ZnMoO 4 -based scintillating bolometers (⊘50 × 40 mm) and the Zn 100 MoO 4 array together with 36 FID Ge detectors were assembled. The EDELWEISS set-up was also upgraded: a) a polyethylene shield at the 1 K plate was added; b) new ultra radiopure NOSV Copper [20] screens were installed; c) all detectors were provided with individual low background Copper-Kapton cables. In addition, a pulser system to assist to the calibration of the thermal response of the ZnMoO 4 / Zn 100 MoO 4 detectors will be implemented soon. After the upgrade of the set-up the data are recorded by a 16 bit ADC with 1 kHz sampling rate (the length of pulse profile is 2 s with the half of the window for the baseline data). The working temperature is stabilized at 18 mK. The energy scale of the detectors was measured with the 133 Ba gamma source (the measurements with the 232 Th source are foreseen).
The set-up is still under optimization, especially as far as the control of the vibration-induced noise is concerned. Therefore, we discuss here, as an illustrative example, only the results achieved with the 334 g natural ZnMoO 4 scintillating bolometer. This detector exhibits full α/γ(β) separation, as shown in Fig. 4 (a), as well as excellent spectrometric properties, as demonstrated in Fig. 4 (b). Other relevant information about performances of ⊘50 × 40 mm ZnMoO 4 detectors are reported in Table 1.

Radiopurity of ZnMoO 4 and Zn 100 MoO 4 crystals
The radiopurity level of the ZnMoO 4 crystals was estimated by analysis of the α events selected from the underground runs, while the data of the aboveground measurements were used in case of the Zn 100 MoO 4 samples. The position of the 5.4 MeV α peak of the internal 210 Po, clearly visible in the data for the natural crystals, was used to stabilize the thermal response of the detectors. For instance, the spectra of the α events registered by the detectors based on 313 g (a) and 334 g (b) ZnMoO 4 crystals over 851 h and 527 h, respectively, are shown in Fig. 5. The crystals are slightly polluted by 210 Po detected through 5.4 MeV α peak confirming a broken equilibrium in the radioactive chain. 226 Ra (and its daughters 222 Rn, 218 Po, and 214 Bi-214 Po events), and 228 Th (with daughter 224 Ra 3 ) were detected in the 313 g crystal, while the ZnMoO 4 scintillators produced by recrystallization have shown a much better level of radiopurity, particularly in 226 Ra. It is also evident a higher surface contamination by 210 Po of the 313 g crystal or/and of the bolometer components close to it (a peak at 5.3 MeV corresponds to E α of 210 Po). In addition, excess counts around 5.8 MeV also indicate a possible surface contamination but its origin has not been identified.
The activity of internal 210 Po was derived from the fit of the 5.4 MeV peak, while 3σ intervals (according to the energy resolution of the internal 210 Po -see Table 1) centered at the Q α value were used for the calculation of the area of the peaks of other radionuclides from U/Th chains. The background contribution was evaluated in two energy regions (3.3-4 and 4.35-4.7 MeV) with a flat α continuum in which no peaks are expected. The number of counts excluded with 90% C.L. were calculated by using the Feldman-Cousins procedure [21]. Data (or limits) on radioactive contamination of the ZnMoO 4 and Zn 100 MoO 4 scintillators are summarized in Table 2, where the results for another ZnMoO 4 sample, produced from the same boule as the 313 g crystal was, are presented for comparison. As it is seen from Table 2, the improved purification and crystallization procedure adopted for the LUMINEU crystals of 334 and 336 g has lead to a significant reduction of the internal contamination, especially for 226 Ra which is not detectable now while it was clearly present in both precursor crystals (313 and 329 g). In particular, the radiopurity levels ( ≤0.01 mBq/kg) achieved for 228 Th and 226 Ra are fully compatible with next-generation 0ν2β experiments capable to explore the inverted hierarchy region of the neutrino mass pattern [1,2].

Conclusions
A significant progress is achieved in development of ZnMoO 4 crystal scintillators for the LUMINEU project. Large volume crystal boules (∼ 1 kg each) were grown by the low-thermal-gradient Czochralski technique from deeply purified molybdenum. A Zn 100 MoO 4 crystal boule with a mass of 0.17 kg was produced from enriched 100 Mo (to 99.5%) for the first time. Three natural (∼ 0.3 kg) and two enriched (∼ 0.06 kg) scintillation elements were produced for low temperature studies. Production of large volume Zn 100 MoO 4 crystal scintillators from enriched 100 Mo is in progress.
The cryogenic scintillating bolometric tests of the natural and enriched crystals showed a high performance of the detectors. The deep purification of molybdenum and recrystallization significantly improve the radioactive contamination of ZnMoO 4 crystals by 228 Th and 226 Ra to the level of ≤0.01 mBq/kg requested by the LUMINEU project.
The results of this study clarify the excellent prospects of ZnMoO 4 scintillating bolometers for the next generation 0ν2β experiments aiming to approach the inverted hierarchy region of the neutrino mass pattern.