Background of the BULLKID detector array operated with moderate shield on surface

TL;DR

This paper reports on the operation of the BULLKID cryogenic detector array with moderate shielding at a surface lab, analyzing background spectra for dark matter and neutrino detection.

Contribution

It demonstrates the detector's background performance and compares measured spectra with simulations, highlighting discrepancies at very low energies.

Findings

Measured background spectrum aligns with simulations above 600 eV.

Observed background rise between 225 eV and 600 eV disagrees with simulations.

High energy spectrum matches expected particle-induced X-ray emissions.

Abstract

We present the operation with moderate radiation shield in a surface laboratory of BULLKID (BULky and Low-threshold Kinetic Inductance Detector), a cryogenic detector for searches of light Dark Matter or Coherent Elastic Neutrino-Nucleus Scattering. The detector consists of an array of 60 cubic silicon particle absorbers of 0.34 g each, sensed by cryogenic kinetic inductance detectors. The analysis presented focuses on data from 15 elements of the array, with two central units used to evaluate the background and with their surrounding elements used as veto. The low energy spectrum resulting from an exposure of 290 hours to ambient backgrounds, acquired with the use of external and internal radiation shields, is compatible with the simulations at the level of $(6.8\pm0.4\,{\rm stat.}\pm0.1\,{\rm syst.})\times10^4$ counts / keV kg days from 2 keV down to an energy of 600 eV. The region between 225 eV and 600 eV shows a rise in background in disagreement with the simulations, while not sharing some of the key traits of the low energy excess observed in other cryogenic experiments. The high energy spectrum shape is in overall agreement with the simulations and displays the typical particle-induced X-ray emission of the surrounding lead.