Developing the Cryogenic Veto for BULLKID-DM Experiment

BULLKID-DM is a cryogenic experiment that will use phonon-sensing kinetic inductance detectors (KIDs) to search for ≤1 GeV/c$^2$ weakly interacting massive particle (WIMP) dark matter particles. The experiment will be conducted in the low background environment of the underground Gran Sasso National Laboratories in Italy. In the experiment's energy region of interest (200 eV–2 keV), it is crucial to minimize background signals from muons, γ-rays, and neutrons. For this reason, the collaboration is developing a cryogenic anticoincidence detector (veto) composed of high-density scintillating crystals read out by KID-based light detectors. We conducted a study to select the optimal veto material. Along with bismuth germanate B$_4$G$_3$O$_{12}$ (BGO), we have identified gadolinium oxyorthosilicate Gd$_2$SiO$_5$ and gadolinium aluminum gallium garnet Gd$_3$Al$_2$Ga$_3$O$_{12}$ (GAGG), which contain gadolinium and thus are ideal for absorption of thermal neutrons. We tested the scintillating properties of the three crystals at cryogenic temperatures down to 20 K. GAGG exhibited the highest light yield and submicrosecond scintillation times. ... mehrFurthermore, we validated the effectiveness of the veto through simulation of the backgrounds originating from photon and neutron fluxes in the full BULLKID-DM experimental setup. A benchmark detector composed of a large (3 × 3 inch cylinder) BGO crystal coupled to an Si wafer equipped with a KID was tested. We successfully operated this device by measuring the energy spectrum of a $^{137}$Cs gamma source. We then estimated the light yield, energy resolution, and expected energy threshold. We interpreted the results with a simulation that takes into account crystal scintillation, optical photon transport, and KID response. Our findings allow for the optimization of the coupling between a KID and a scintillating crystal to further enhance the detection performance of the BULLKID-DM veto.