Characterization of the Li$_2$WO$_4$ crystal as a cryogenic scintillating calorimeter

TL;DR

The study assesses a large-mass molybdenum-free Li2WO4 crystal operated as a scintillating cryogenic calorimeter to enable dual-readout heat and light for rare-event searches. It reports a baseline energy resolution of 0.50 keV RMS and a low-energy threshold of 1.5 keV, with clear separation of β/γ, α, and nuclear-recoil events above 300 keV, achieving discrimination better than 6σ. Radiopurity analyses reveal Th/U limits below 0.5 mBq/kg and identify Mo and alkali impurities likely originating from crucible material, informing production practices. The results establish Li2WO4 as a competitive material for next-generation dark matter and neutrino experiments, and outline future steps including TES integration and deeper low-background measurements.

Abstract

A wide range of scintillating bolometers is under investigation for applications in the search for rare events and processes beyond the Standard Model. In this work, we report the first measurement of a natural, non-molybdenum-doped, lithium tungstate (LWO) crystal operated underground as a scintillating cryogenic calorimeter. The detector achieved a baseline energy resolution of 0.5 keV RMS with a low-energy threshold of about 1.5 keV. The simultaneous readout of heat and light enabled particle identification, revealing a clear separation between $β/γ$, $α$, and nuclear recoil populations above 300 keV, with a light-yield-based particle discrimination better than $6σ$. These results, fully comparable with those achieved with other compounds in the field, demonstrate that LWO is a promising candidate for rare-event searches. In particular, the combination of excellent radio-purity (with U/Th levels below 0.5 mBq/kg) and sensitivity to neutron interactions via the $^6$Li(n,$α$)$^3$H reaction makes this material an attractive option for next-generation experiments on dark matter, coherent elastic neutrino-nucleus scattering, and spin-dependent interactions.

Figures (5)

• Figure 1: Top. Coherent elastic neutrino-nucleus scattering (CE$\nu$NS) cross-sections as a function of incoming neutrino energy for various target nuclei. Due to their larger neutron number, heavier elements such as tungsten (W) and lead (Pb) exhibit significantly higher cross sections than lighter nuclei. Bottom. CE$\nu$NS differential cross sections as a function of the nuclear recoil energy. Following the work in Ref. Bellenghi:2019vtc, a monochromatic neutrino of 747 keV emitted in the $^{51}$Cr decay is assumed, while considering a reactor neutrino with an energy of 3-4 MeV, the spectra shift toward higher energies by a factor of $\sim$20 (see Eq. \ref{['eq:eravg']}). Lighter elements, such as lithium (Li) and oxygen (O), have a higher energy recoil, thereby relaxing the energy threshold requirements. Figures adapted from Refs. RES-NOVA:2021gqpAlloni:2025bda.
• Figure 2: A 24 g Li$_2$WO$_4$ crystal with a size of $\varnothing$24.3$\times$25.3 mm before its assembly.
• Figure 3: Energy spectra acquired over 98 hours with the Li$_2$WO$_4$ crystal (top) and the Ge-LD (bottom) using an external 232Th calibration source. In the Li$_2$WO$_4$ spectrum, all peaks exploited for the energy calibration are highlighted and labeled following the list in Tab. \ref{['tab:energy_resolution']}. Due to its small thickness (170 $\mu$m), the light detector cannot fully absorb the 232Th $\gamma$-rays, resulting in a featureless continuum spectrum. Superimposed on this latter are the scintillation light signals from the Li$_2$WO$_4$ crystal (0--2 keV), as well as X-rays from the $^{55}$Fe calibration source (5.9 and 6.4 keV), and characteristic calcium X-rays (3.7 and 4.0 keV).
• Figure 4: Light yield as a function of the energy for the data acquired with 232Th source (top panel) and AmBe neutron source (bottom panel). In the latter case, both the data acquired with (blue) and without (orange) the water tank used as neutron moderator are reported. Four populations of events are clearly distinguishable in the two datasets: the $\beta/\gamma$s forming a continuous band at LY $\sim 0.6$ keV/MeV, the $\alpha$ particles due to crystal contamination (top) and neutron capture on $^6$Li (bottom) at LY $\sim 0.1$ keV/MeV, as well as the neutron-induced nuclear recoils (NR) at LY $\sim 0.08$ keV/MeV (bottom). The insets report the LY of different particles, exploited to evaluate the discrimination power. See the main text for more details.
• Figure 5: Energy spectrum of the $\alpha$ region acquired over 98 hours with the Li$_2$WO$_4$ crystal. The highlighted Regions of Interest (ROIs), defined as $\pm$ 20 keV around the Q-values, are used to estimate the contamination from isotopes belonging to $^{232}$Th and $^{238}$U decay chains.