https://doi.org/10.1140/epjc/s10052-026-15523-4
Regular Article - Experimental Physics
Energy calibration of bulk events in the BULLKID detector
1
Dipartimento di Fisica, Sapienza Università di Roma, P. le A. Moro 2, 00185, Rome, Italy
2
INFN Sezione di Roma, Sapienza Università di Roma, P.le A. Moro 2, 00185, Rome, Italy
3
Dipartimento di Fisica “Enrico Fermi”, Università di Pisa, Largo Bruno Pontecorvo 3, 56127, Pisa, Italy
4
INFN Sezione di Pisa, Largo Bruno Pontecorvo 3, 56127, Pisa, Italy
5
Instituto de Física, Universidad Nacional Autónoma de México, A.P. 20-364, 01000, Ciudad de México, Mexico
6
INFN Sezione di Ferrara, Via Saragat 1, 44122, Ferrara, Italy
7
University Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, 38000, Grenoble, France
8
INFN Laboratori Nazionali del Gran Sasso, Via Sommarive 14, 67100, Assergi, AQ, Italy
9
Institute for Data Processing and Electronics, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany
10
INFN-TIFPA, Via Sommarive 14, 38123, Povo (Trento), Italy
11
Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, Via Saragat 1, 44100, Ferrara, Italy
12
Gran Sasso Science Institute, Viale Francesco Crispi, 7 Rectorate, Via Michele Iacobucci, 2, 67100, L’Aquila, AQ, Italy
a
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Received:
21
October
2025
Accepted:
6
March
2026
Published online:
23
March
2026
Abstract
BULLKID is a cryogenic, solid-state detector designed for direct searches of particle Dark Matter candidates, with mass 
GeV/c
, 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 
-ray generated by an 
Am source, which emulates more closely the interaction of Dark Matter and neutrinos. The peak resolution is 
and its position is shifted by less than 
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.
© The Author(s) 2026
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Funded by SCOAP3.
