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International audience; The double-beta decay of Cd116 has been investigated with the help of radiopure enriched Cd116WO4 crystal scintillators (mass of 1.162 kg) at the Gran Sasso underground laboratory. The half-life of Cd116 relative to the 2ν2β decay to the ground state of Sn116 was measured with the highest up-to-date accuracy as T1/2=(2.63-0.12+0.11)×1019 yr. A new improved limit on the 0ν2β decay of Cd116 to the ground state of Sn116 was set as T1/2≥2.2×1023 yr at 90% C.L., which is the most stringent known restriction for this isotope. It corresponds to the effective Majorana neutrino mass limit in the range ⟨mν⟩≤(1.0–1.7) eV, depending on the nuclear matrix elements used in the estimations. New improved half-life limits for the 0ν2β decay with majoron(s) emission, Lorentz-violating 2ν2β decay, and 2β transitions to excited states of Sn116 were set at the level of T1/2≥1020–1022 yr. New limits for the hypothetical lepton-number violating parameters (right-handed currents admixtures in weak interaction, the effective majoron-neutrino coupling constants, R-parity violating parameter, Lorentz-violating parameter, heavy neutrino mass) were set.

We report recent achievements in the development of scintillating bolometers to search for neutrinoless double-beta decay of $^{100}$Mo. The presented results have been obtained in the framework of the LUMINEU, LUCIFER and EDELWEISS collaborations, and are now part of the R\&D activities towards CUPID (CUORE Update with Particle IDentification), a proposed next-generation double-beta decay experiment based on the CUORE experience. We have developed a technology for the production of large mass ($\sim$1 kg), high optical quality, radiopure zinc and lithium molybdate crystal scintillators (ZnMoO$_4$ and Li$_2$MoO$_4$, respectively) from deeply purified natural and $^{100}$Mo-enriched molybdenum. The procedure is applied for a routine production of enriched crystals. Furthermore, the technology of a single detector module consisting of a large-volume ($\sim 100$~cm$^3$) Zn$^{100}$MoO$_4$ and Li$_2$$^{100}$MoO$_4$ scintillating bolometer has been established, demonstrating performance and radiopurity that are close to satisfy the demands of CUPID. In particular, the FWHM energy resolution of the detectors at 2615 keV --- near the $Q$-value of the double-beta transition of $^{100}$Mo (3034~keV) --- is $\approx$ 4--10~keV. The achieved rejection of $\alpha$-induced dominant background above 2.6~MeV is at the level of more than 99.9\%. The bulk activity of $^{232}$Th ($^{228}$Th) and $^{226}$Ra in the crystals is below 10 $\mu$Bq/kg. Both crystallization and detector technologies favor Li$_2$MoO$_4$, which was selected as a main element for the realization of a CUPID demonstrator (CUPID-0/Mo) with $\sim$7 kg of $^{100}$Mo.

International audience; Random coincidences of nuclear events can be one of the main background sources in low-temperature calorimetric experiments looking for neutrinoless double-beta decay, especially in those searches based on scintillating bolometers embedding the promising double-beta candidate$^{100}$ Mo, because of the relatively short half-life of the two-neutrino double-beta decay of this nucleus. We show in this work that randomly coinciding events of the two-neutrino double-beta decay of$^{100}$ Mo in enriched Li$_2^{100}\,\mathrm{MoO}_4$ detectors can be effectively discriminated by pulse-shape analysis in the light channel if the scintillating bolometer is provided with a Neganov–Luke light detector, which can improve the signal-to-noise ratio by a large factor, assumed here at the level of ${\sim }750$ on the basis of preliminary experimental results obtained with these devices. The achieved pile-up rejection efficiency results in a very low contribution, of the order of ${\sim }6\times 10^{-5}$ counts/(keV $\cdot $ kg $\cdot $ y), to the background counting rate in the region of interest for a large volume ( ${\sim }90$ cm$^3$ ) Li$_2^{100}\,\mathrm{MoO}_4$ detector. This background level is very encouraging in view of a possible use of the Li$_2^{100}\,\mathrm{MoO}_4$ solution for a bolometric tonne-scale next-generation experiment as that proposed in the CUPID project.

The technology of scintillating bolometers based on zinc molybdate (ZnMoO4) crystals is under development within the LUMINEU project to search for decay of 100Mo with the goal to set the basis for large scale experiments capable to explore the inverted hierarchy region of the neutrino mass pattern. Advanced ZnMoO4 crystal scintillators with mass of ∼0.3 kg were developed and Zn100MoO4 crystal from enriched 100Mo was produced for the first time by 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 showing a high detection performance. The first results of the low background measurements with three ZnMoO4 and two enriched detectors installed in the EDELWEISS set-up at the Modane Underground Laboratory (France) are presented.

An experiment to search for double β (2β) decay of ¹⁰⁶Cd with the help of a low-background cadmium tungstate crystal scintillator developed from cadmium enriched in ¹⁰⁶Cd to 66 % (106CdWO4, 215 g) is in progress at the STELLA facility of the Gran Sasso underground laboratory (INFN, Italy). The ¹⁰⁶CdWO₄ scintillator is viewed by low-background photomultiplier through a PbWO₄ crystal light-guide produced from deeply purified archaeological lead. The detector operates in coincidence with the four low-background HPGe detectors to search for 2β processes with the emission of gamma quanta. Sensitivity of the experiment after 10678 h of data taking to different channels of 2β decay of ¹⁰⁶Cd is on the level of limT₁/₂ ~ 10¹⁹-10²¹ years.

The LUMINEU project is a demonstrator experiment that will search for the neutrinoless double beta decay of the isotope 100 Mo embedded in zinc molybdate (ZnMoO 4 ) scintillating bolometers. In this context, a zinc molybdate crystal boule enriched in 100 Mo to 99.5 % with a mass of 171 g was grown for the first time by the low-thermal-gradient Czochralski technique. The production cycle provided a high yield (the crystal boule mass was 84 % of the initial charge) and an acceptable level—around 4 %—of irrecoverable losses of the costly enriched material. Two crystals of 59 and 63 g, obtained from the enriched boule, were tested above ground at millikelvin temperatures as scintillating bolometers. They showed a good detection performance, equivalent to that of previously developed natural ZnMoO 4 detectors. These results pave the way to future sensitive searches based on the LUMINEU technology, capable of approaching and exploring the inverted hierarchy region of the neutrino mass pattern.

Random coincidence of events (particularly from two neutrino double beta decay) could be one of the main sources of background in the search for neutrinoless double beta decay with cryogenic bolometers due to their poor time resolution. Pulse-shape discrimination by using front edge analysis, mean-time and χ2 methods were applied to discriminate randomly coinciding events in ZnMoO 4 cryogenic scintillating bolometers. These events can be effectively rejected at the level of 99 % by the analysis of the heat signals with rise-time of about 14 ms and signal-to-noise ratio of 900, and at the level of 92 % by the analysis of the light signals with rise-time of about 3 ms and signal-to-noise ratio of 30, under the requirement to detect 95 % of single events. These rejection efficiencies are compatible with extremely low background levels in the region of interest of neutrinoless double beta decay of 100 Mo for enriched ZnMoO 4 detectors, of the order of 10-4  counts/(y keV kg). Pulse-shape parameters have been chosen on the basis of the performance of a real massive ZnMoO 4 scintillating bolometer. Importance of the signal-to-noise ratio, correct finding of the signal start and choice of an appropriate sampling frequency are discussed.

The LUMINEU project aims at developing a pilot double beta decay experiment using scintillating bolometers based on ZnMoO4 crystals enriched in 100Mo. In the next months regular deliveries of large-mass ZnMoO 4 crystals are expected from the Nikolaev Institute of Inorganic Chemistry (Novosibirsk, Russia). It is therefore crucial for the LUMINEU program to test systematically and in real time these samples in terms of bolometric properties, light yield and internal radioactive contamination. In this paper we describe an aboveground cryogenic facility based on a dilution refrigerator coupled to a pulse-tube cooler capable performing these measurements. A 23.8 g ZnMoO4 crystal was fully characterised in this setup. We show also that macro-bolometers can be operated with high signal-to-noise ratio in liquid-free dilution refrigerators. © 2014 Springer Science+Business Media New York.

A search for double β processes in 106Cd was carried out at the Gran Sasso National Laboratories of the INFN (Italy) with the help of a 106CdWO4 crystal scintillator (215 g) enriched in 106Cd up to 66%. After 6590 h of data taking, new improved half-life limits on the double β decay processes in 106Cd were established at the level of 1019–1021 yr; in particular, T 2νεβ+ 1/2 2.1 × 1020 yr, T 2ν2β+ 1/2 4.3 × 1020 yr, and T 0ν2ε 1/2 1.0 × 1021 yr. The resonant neutrinoless double-electron captures to the 2718-, 2741-, and 2748-keV excited states of 106Pd are restricted to T 0ν2K 1/2 4.3 × 1020 yr, T 0νKL1 1/2 9.5 × 1020 yr, and T 0νKL3 1/2 4.3 × 1020 yr, respectively (all limits at 90% confidence level). A possible resonant enhancement of the 0ν2ε processes is estimated in the framework of the quasiparticle random phase approximation (QRPA) approach. Radioactive contamination of the 106CdWO4 crystal scintillator is reported. peerReviewed