Complete event-by-event α/γ(β) separation in a full-size TeO2 CUORE bolometer by simultaneous heat and light detection

The CUORE project began recently a search for neutrinoless double-beta decay ($0\nu\beta\beta$) of $^{130}$Te with a $\mathcal{O}$(1 ton) TeO$_2$ bolometer array. In this experiment, the background suppression relies essentially on passive shielding, material radiopurity and anti-coincidences. The lack of particle identification in CUORE makes $\alpha$ decays at the detector surface the dominant background, at the level of $\sim$0.01 counts/(keV kg y) in the region of interest ($Q$-value of $0\nu\beta\beta$ of the order of 2.5 MeV). In the present work we demonstrate, for the first time with a CUORE-size (5$\times$5$\times$5 cm) TeO$_2$ bolometer and using the same technology as CUORE for the readout of the bolometric signals, an efficient $\alpha$ particle discrimination (99.9\%) with a high acceptance of the $0\nu\beta\beta$ signal (about 96\%). This unprecedented result was possible thanks to the superior performance (10 eV RMS baseline noise) of a Neganov-Luke-assisted germanium bolometer used to detect a tiny (70 eV) light signal dominated by $\gamma$($\beta$)-induced Cherenkov radiation in the TeO$_2$ detector. The obtained results represent a major breakthrough towards the TeO$_2$-based version of CUPID, a ton-scale cryogenic $0\nu\beta\beta$ experiment proposed as a follow-up to CUORE with particle identification

Charge-to-heat transducers exploiting the Neganov-Trofimov-Luke effect for light detection in rare-event searches

International audienceIn this work we present how to fabricate large-area (15 cm 2 ), ultra-low threshold germanium bolometric photo-detectors and how to operate them to detect few (optical) photons. These detectors work at temperatures as low as few tens of mK and exploit the Neganov-Trofimov-Luke (NTL) effect. They are operated as charge-to-heat transducers: the heat signal is linearly increased by simply changing a voltage bias applied to special metal electrodes, fabricated onto the germanium absorber, and read by a (NTD-Ge) thermal sensor. We fabricated a batch of five prototypes and ran them in different facilities with dilution refrigerators. We carefully studied how impinging spurious infrared radiation impacts the detector performances, by shining infrared photons via optical-fiber-guided LED signals, in a controlled manner, into the bolometers. We hence demonstrated how the radiation-tightness of the test environment tremendously enhances the detector performances, allowing to set electrode voltage bias up to 90 volts without any leakage current and signal-to-noise gain as large as a factor 12 (for visible photons). As consequence, for the first time we could operate large-area NTD-Ge-sensor-equipped NTL bolometric photo-detectors capable to reach sub 10-eV baseline noise (RMS). Such detectors open new frontiers for rare-event search experiments based on low light yield Ge-NTD equipped scintillating bolometers, such the CUPID neutrinoless double-beta decay experiment

First scintillating bolometer tests of a CLYMENE R&D on Li2_2MoO4_4 scintillators towards a large-scale double-beta decay experiment

International audienceA new R&D on lithium molybdate scintillators has begun within a project CLYMENE (Czochralski growth of Li2MoO4 crYstals for the scintillating boloMeters used in the rare EveNts sEarches). One of the main goals of the CLYMENE is a realization of a Li2MoO4 crystal growth line to be complementary to the one recently developed by LUMINEU in view of a mass production capacity for CUPID, a next-generation tonne-scale bolometric experiment to search for neutrinoless double-beta decay. In the present paper we report the investigation of performance and radiopurity of 158-g and 13.5-g scintillating bolometers based on a first large-mass (230 g) Li2MoO4 crystal scintillator developed within the CLYMENE project. In particular, a good energy resolution (2–7 keV FWHM in the energy range of 0.2–5 MeV), one of the highest light yield (0.97 keV/MeV) amongst Li2MoO4 scintillating bolometers, an efficient alpha particles discrimination (10σ) and potentially low internal radioactive contamination (below 0.2–0.3 mBq/kg of U/Th, but 1.4 mBq/kg of 210Po) demonstrate prospects of the CLYMENE in the development of high quality and radiopure Li2MoO4 scintillators for CUPID

Assessment of Cs₂HfCl₆ crystal applicability as low-temperature scintillating bolometers by their thermodynamic characteristics

International audienceThe first study of some thermodynamic properties of a Cs2HfCl6 (CHC) scintillating crystal has been performed in a wide temperature range of 5–300 K. The CHC crystal exhibits a high thermal expansion coefficient (5.17 × 10−5 at 171 K), along with an extremely low heat conductivity (1.710 W m−1 K−1 at 50 K). A high heat capacity (1.22 J kg−1 K−1 at 3.8 K) along with a drastically low free mean path for phonons (about 26 Å) at temperatures around 50 K makes the CHC crystal unsuitable as a target material for cryogenic detectors. The results of the first operation of the CHC crystal as a cryogenic detector at sub-Kelvin temperatures clearly show the unfavorable phonon propagation properties of the material. On the other hand, the CHC crystal demonstrates excellent scintillating characteristics with a relatively high light yield under gamma irradiation (the detected signal is about 5200 photons per MeV). The refractive index within the (1.66–1.67) interval for light in visible and near-infrared spectral ranges was measured at room temperature. A cost- and time-effective approach for new crystalline material characterization as a target for cryogenic detectors based on the evaluation of the thermodynamic properties of the material is proposed

Growth and characterization of a Li2_2Mg2_2(MoO4_4)3_3 scintillating bolometer

International audienceLithium magnesium molybdate (Li 2 Mg 2 (MoO 4 ) 3 ) crystals were grown by the low-thermal-gradient Czochralski method. Luminescence properties of the material (emission spectra, thermally stimulated luminescence, dependence of intensity on temperature, phosphorescence) have been studied under X-ray excitation in the temperature interval from 8 to 400 K, while at the same being operated as a scintillating bolometer at 20 mK for the first time. We demonstrated that Li 2 Mg 2 (MoO 4 ) 3 crystals are a potentially promising detector material to search for neutrinoless double beta decay of  100 Mo

The CROSS Experiment: Rejecting Surface Events by PSD Induced by Superconducting Films

International audienceNeutrinoless double-beta ($0\nu \beta \beta$) decay is a hypothetical rare nuclear transition ($T_{1/2}>10^{25}$–$10^{26}$ year). Its observation would provide an important insight into the nature of neutrinos (Dirac or Majorana particle) demonstrating that the lepton number is not conserved. This decay can be investigated with bolometers embedding the double-beta decay isotope ($^{76}\mathrm{Ge}$, $^{82}\mathrm{Se}$, $^{100}\mathrm{Mo}$, $^{116}\mathrm{Cd}$, $^{130}\mathrm{Te}$...), which perform as low-temperature calorimeters (few tens of mK) detecting particle interactions via a small temperature rise read out by a dedicated thermometer. Cryogenic Rare-event Observatory with Surface Sensitivity (CROSS) aims at the development of bolometric detectors (based on $\hbox {Li}_{{2}}\hbox {MoO}_{{4}}$ and $\hbox {TeO}_{{2}}$ crystals) capable of discriminating surface $\alpha$ and $\beta$ interactions by exploiting superconducting properties of Al film deposited on the detector surface. We report in this paper the results of tests on prototypes performed at CSNSM (Orsay, France) that showed the capability of a-few-\upmu \mathrm{m}-thick superconducting Al film deposited on crystal surface to discriminate surface $\alpha$ from bulk events, thus providing the detector with the required pulse shape discrimination capability. The CROSS technology would further improve the background suppression and simplify the detector construction (no auxiliary light detector is needed to reject alpha surface events) with a view to future competitive double-beta decay searches

Neutrinoless Double-Beta Decay Searches with Enriched 116CdWO4^{116}\hbox {CdWO}_{{4}} Scintillating Bolometers

International audienceCadmium-116 is one of the favorable candidates for neutrinoless double-beta decay ($0\nu \beta \beta$) searches from both theoretical and experimental points of view, in particular thanks to the high energy of the decay (2813.49 keV), the possibility of the industrial enrichment in $^{116}\mathrm{Cd}$ and its use in the well-established production of cadmium tungstate crystal scintillators. In this work, we present low-temperature tests of two $0.6\ \mathrm{kg} \ ^{116}\hbox {CdWO}_{{4}}$ crystals enriched in $^{116}\mathrm{Cd}$ to $82\%$ as scintillating bolometers. These detectors were operated underground, with one at the Laboratoire Souterrain de Modane (LSM) in France and the second at the Laboratorio Subterraneo de Canfranc (LSC) in Spain. The two crystals are coupled to bolometric Ge light detectors in order to register the scintillation light. The double readout of heat and scintillation enables reduction in the background in the region of interest by discriminating between different populations of particles. The main goal of these tests is the study of the crystals’ radiopurity and the detectors’ performance. The achieved results are extremely promising, in particular, the detectors demonstrate a high energy resolution (11–16 keV FWHM at 2615 keV) and a high-efficiency discrimination of the alpha background ($\sim 20 \sigma$). These results, achieved for the first time with large mass enriched $^{116}\hbox {CdWO}_{{4}}$ crystals, demonstrate prospects of the bolometric technology for high-sensitivity searches of $^{116}\mathrm{Cd}0\nu \beta \beta$ decay

ZnO-based scintillating bolometers: New prospects to study double beta decay of 64^{64}Zn

International audienceThe first detailed study on the performance of a ZnO-based cryogenic scintillating bolometer as a detector to search for rare processes in zinc isotopes was performed. A 7.2 g ZnO low-temperature detector, containing more than 80% of zinc in its mass, exhibits good energy resolution of baseline noise 1.0--2.7 keV FWHM at various working temperatures resulting in a low-energy threshold for the experiment, 2.0--6.0 keV. The light yield for $\beta$/$\gamma$ events was measured as 1.5(3) keV/MeV, while it varies for $\alpha$ particles in the range of 0.2--3.0 keV/MeV. The detector demonstrate an effective identification of the $\beta$/$\gamma$ events from $\alpha$ events using time-properties of only heat signals. %(namely, Rise time parameter). The radiopurity of the ZnO crystal was evaluated using the Inductively Coupled Plasma Mass Spectrometry, an ultra-low-background High Purity Ge $\gamma$-spectrometer, and bolometric measurements. Only limits were set at the level of $\mathcal{O}$(1--100) mBq/kg on activities of \Nuc{K}{40}, \Nuc{Cs}{137} and daughter nuclides from the U/Th natural decay chains. The total internal $\alpha$-activity was calculated to be 22(2) mBq/kg, with a major contribution caused by 6(1) mBq/kg of \Nuc{Th}{232} and 12(2) mBq/kg of \Nuc{U}{234}. Limits on double beta decay (DBD) processes in \Nuc{Zn}{64} and \Nuc{Zn}{70} isotopes were set on the level of $\mathcal{O}(10^{17}$--$10^{18})$ yr for various decay modes profiting from 271 h of acquired background data in the above-ground lab. This study shows a good potential for ZnO-based scintillating bolometers to search for DBD processes of Zn isotopes, especially in \Nuc{Zn}{64}, with the most prominent spectral features at $\sim$10--20 keV, like the two neutrino double electron capture. A 10 kg-scale experiment can reach the experimental sensitivity at the level of $\mathcal{O}(10^{24})$ yr

ZnO-based scintillating bolometers: New prospects to study double beta decay of 64^{64}Zn

International audienceThe first detailed study on the performance of a ZnO-based cryogenic scintillating bolometer as a detector to search for rare processes in zinc isotopes was performed. A 7.2 g ZnO low-temperature detector, containing more than 80% of zinc in its mass, exhibits good energy resolution of baseline noise 1.0--2.7 keV FWHM at various working temperatures resulting in a low-energy threshold for the experiment, 2.0--6.0 keV. The light yield for $\beta$/$\gamma$ events was measured as 1.5(3) keV/MeV, while it varies for $\alpha$ particles in the range of 0.2--3.0 keV/MeV. The detector demonstrate an effective identification of the $\beta$/$\gamma$ events from $\alpha$ events using time-properties of only heat signals. %(namely, Rise time parameter). The radiopurity of the ZnO crystal was evaluated using the Inductively Coupled Plasma Mass Spectrometry, an ultra-low-background High Purity Ge $\gamma$-spectrometer, and bolometric measurements. Only limits were set at the level of $\mathcal{O}$(1--100) mBq/kg on activities of \Nuc{K}{40}, \Nuc{Cs}{137} and daughter nuclides from the U/Th natural decay chains. The total internal $\alpha$-activity was calculated to be 22(2) mBq/kg, with a major contribution caused by 6(1) mBq/kg of \Nuc{Th}{232} and 12(2) mBq/kg of \Nuc{U}{234}. Limits on double beta decay (DBD) processes in \Nuc{Zn}{64} and \Nuc{Zn}{70} isotopes were set on the level of $\mathcal{O}(10^{17}$--$10^{18})$ yr for various decay modes profiting from 271 h of acquired background data in the above-ground lab. This study shows a good potential for ZnO-based scintillating bolometers to search for DBD processes of Zn isotopes, especially in \Nuc{Zn}{64}, with the most prominent spectral features at $\sim$10--20 keV, like the two neutrino double electron capture. A 10 kg-scale experiment can reach the experimental sensitivity at the level of $\mathcal{O}(10^{24})$ yr

First test of a CdMoO4\mathrm{CdMoO_4} scintillating bolometer for neutrinoless double beta decay experiments with 116Cd\mathrm{{}^{116}Cd} and 100Mo\mathrm{{}^{100}Mo} nuclides

International audienceA large cylindrical cadmium molybdate crystal with natural isotopic abundance has been used to fabricate a scintillating bolometer. The measurement was performed above ground at milli-Kelvin temperature, with simultaneous readout of the heat and the scintillation light. The energy resolution as FWHM has achieved from 5 keV (at 238 keV) to 13 keV (at 2615 keV). We present the results of the α versus β / γ events discrimination. The low internal trace contamination of the CdMoO 4 crystal was evaluated as well. The detector performance with preliminary positive indications proves that cadmium molybdate crystal is a promising absorber for neutrinoless double beta decay scintillating bolometric experiments with 116 Cd and 100 Mo nuclides in the next-generation technique