Li2100deplMoO4 Scintillating Bolometers for Rare-Event Search Experiments

Abstract

We report on the development of scintillating bolometers based on lithium molybdate crystals that contain molybdenum that has depleted into the double-β active isotope 100Mo (Li2100deplMoO4). We used two Li2100deplMoO4 cubic samples, each of which consisted of 45-millimeter sides and had a mass of 0.28 kg; these samples were produced following the purification and crystallization protocols developed for double-β search experiments with 100Mo-enriched Li2MoO4 crystals. Bolometric Ge detectors were utilized to register the scintillation photons that were emitted by the Li2100deplMoO4 crystal scintillators. The measurements were performed in the CROSS cryogenic set-up at the Canfranc Underground Laboratory (Spain). We observed that the Li2100deplMoO4 scintillating bolometers were characterized by an excellent spectrometric performance (∼3-6 keV of FWHM at 0.24-2.6 MeV γs), moderate scintillation signal (∼0.3-0.6 keV/MeV scintillation-to-heat energy ratio, depending on the light collection conditions), and high radiopurity (228Th and 226Ra activities are below a few µBq/kg), which is comparable with the best reported results of low-temperature detectors that are based on Li2MoO4 using natural or 100Mo-enriched molybdenum content. The prospects of Li2100deplMoO4 bolometers for use in rare-event search experiments are briefly discussed.

Keywords: bolometer; cryogenic detector; crystal scintillator; lithium molybdate; molybdenum depleted in 100Mo; rare events.

Figure 1

Photographs of Li${}_2$${}^{100\mathrm{depl}}$MoO${}_4$ low-temperature detectors LMO-depl-1 (left) and LMO-depl-2 (middle). Both crystals have two epoxy-glued sensors; the left one is an NTD Ge thermistor, while the right one is a P-doped Si heater. Each scintillator was accompanied by a circular bolometric Ge light detector, as can be seen in the transparent area of the crystal in the middle panel. An additional square-shaped Ge light detector (right) was used for the LMO-depl-2 sample. All light detectors were instrumented with an NTD Ge sensor.

Figure 2

Detector configurations in the C2U cryogenic runs at the LSC, where the Li${}_2$${}^{100\mathrm{depl}}$MoO${}_4$ scintillating bolometers LMO-depl-1 (left) and LMO-depl-2 (right) were operated. Other scintillating bolometers are based on Li${}_2$MoO${}_4$ crystals with natural (CROSS [57] and CLYMENE [58,59] R&D) and ${}^{100}$Mo-enriched (joint CROSS and CUPID R&D [23,25]) molybdenum content, as well as natural and ${}^{116}$Cd-enriched CdWO${}_4$ crystals [52,60,61,62].

Figure 3

(Left) Energy spectrum of a ${}^{232}$Th source, measured using the Li${}_2$${}^{100\mathrm{depl}}$MoO${}_4$ (LMO-depl-1) bolometer and operated at 12 mK over 125 h. The most intense $\gamma$-ray peaks observed in the spectrum are labeled by their origin; D.E. and S.E. mean double and single escape peaks, respectively. A fit to the 2615 keV peak of ${}^{208}$Tl is shown in the inset; the energy resolution is 5.8(3) keV FWHM. (Right) The energy dependence of the Li${}_2$${}^{100\mathrm{depl}}$MoO${}_4$ (LMO-depl-1) bolometer energy resolution in the calibration (red, 125 h) and background (blue, 1109 h) data acquired at 12 mK. The fit is shown by the dashed line.

Figure 4

Energy spectra of X-rays from a close ${}^{55}$Fe X-ray source (left) and Cu/Mo X-rays induced by the ${}^{232}$Th $\gamma$-ray source (right), measured by the LD-1-c (1109 h of background data) and LD-2-c (266 h, calibration) bolometers, respectively. Fitting of the spectra using two Gaussians and a linear background component are shown by solid red lines.

Figure 5

Scintillation (light-to-heat parameter) versus heat energy release measured by the Li${}_2$${}^{100\mathrm{depl}}$MoO${}_4$ scintillating bolometers. The top panel (left) shows a sum of the calibration (125 h) and background (1109 h) data of LMO-depl-1. The LMO-depl-2 events were detected in the background (top, right; 1536 h) and calibration (bottom; 111 h) measurements in coincidence with the bolometric photodetectors LD-2-c and LD-2-s, respectively.

Figure 6

Energy spectra of $\alpha$ events detected by the Li${}_2$${}^{100\mathrm{depl}}$MoO${}_4$ scintillating bolometers composed of crystals LMO-depl-1 (left; 1109 h of measurements) and LMO-depl-2 (right; 1528 h), which were operated underground in the C2U facility.

Figure 7

Energy spectra of $\gamma$($\beta$) events detected by bolometers based on 0.28 kg lithium molybdate crystals produced from molybdenum that was either depleted in ${}^{100}$Mo (pink, LMO-depl-1, and black, LMO-depl-2) or enriched in ${}^{100}$Mo (blue, LMO-enr data from [23]); the bolometers were operated in the C2U set-up at the Canfranc Underground Laboratory. The detectors LMO-depl-1 and LMO-enr were run together (see Figure 2), while the LMO-depl-2 detector was measured in the next cryogenic run, where the set-up was flushed with deradonized air. It is worth noting that the LMO-enr bolometer was irradiated by a close ${}^{238}$U/${}^{234}$U $\alpha$ source, which emits $\beta$ particles, ${}^{234}$Th ($Q_{\beta }$ = 0.27 MeV), and ${}^{234m}$Pa ($Q_{\beta }$ = 2.2 MeV). Thus, the difference between the acquired spectra is mainly explained by both the ${}^{100}$Mo two-$\nu$ double-$\beta$ activity (∼3 mHz, $Q_{\beta \beta }$ = 3.0 MeV) and the $\alpha$-source-induced $\beta$ background (about 20 mHz of ${}^{234}$Th and ${}^{234m}$Pa) of the Li${}_2$${}^{100}$MoO${}_4$ detector. A Monte Carlo distribution of the ${}^{100}$Mo two-$\nu$ double-$\beta$ decay events (green) [16] is shown for comparison.