Lead-induced calibration of BULLKID detector

BULLKID-DM is an experiment designed for the direct searches of particle dark matter candidates with mass around 1 GeV, or below. It consists of a stack of diced silicon wafers, acting as arrays of particle absorbers, sensed by multiplexed Kinetic Inductance Detectors. In spring 2025, the BULLKID-DM experiment carried out a background measurement of a single detector in a mildly lead-shielded environment. The goal was to reduce the background rate below 10⁵ counts/(keV kg d). Additionally, the shield produced several calibration peaks in the detector, originating from fluorescence X-rays and gamma emissions induced by ²¹⁰Pb contamination. In this work, we present a detailed analysis of these peaks, focusing on the calibration of the detector during the background data acquisition.