Energy calibration of bulk events in the BULLKID detector

TL;DR

This paper demonstrates the calibration of bulk events in the BULLKID detector using 59.5 keV gamma rays, improving understanding of its response for dark matter and neutrino detection.

Contribution

It presents the first calibration of bulk events in BULLKID with gamma rays, showing improved resolution and confirming detector performance for dark matter searches.

Findings

Peak resolution of 5% (σ) achieved

Signal combination from neighboring dice improves resolution by a factor of 2

Bulk event calibration aligns closely with optical calibration results

Abstract

BULLKID is a cryogenic, solid-state detector designed for direct searches of particle Dark Matter candidates, with mass $\lesssim 1$ GeV/c$^2$, and coherent neutrino-nucleus scattering. It is based on an array of dice carved in 5 mm thick silicon crystal, sensed by phonon-mediated Kinetic Inductance Detectors. In previous works, the array was calibrated with bursts of optical photons, which are absorbed in the first hundreds nanometers of the dice and give rise to surface events. In this work, we present the reconstruction of bulk events through the 59.5 keV $\gamma$-ray generated by an $^{241}$Am source, which emulates more closely the interaction of Dark Matter and neutrinos. The peak resolution is $5\%~(\sigma)$ and its position is shifted by less than $10\%$ with respect to the optical calibration. We observe that the resolution is further improved by a factor $2$ combining the signal from neighboring dice. These results confirm the performance of the detector in view of the physics goals of the BULLKID-DM experiment for dark matter search.