The relative interfacial thermal contraction as a possible origin of the low-energy excess in cryogenic calorimeters

Abstract

Low threshold cryogenic calorimeters are a key technology for the advancement of rare-event searches. However, since a few years their sensitivity reach is challenged by the presence of a rising spectrum at low energies named low-energy excess (LEE), ascribed to an unknown background. In this work, we describe the LEE as absorber events induced by the relative thermal-contraction coefficient mismatch between the absorber and the SiO$_2$ amorphous layer underneath the transition-edge sensors (TESs), present for example in the case of CRESST detectors. The relative contraction in processes with temperature changes, such as during sensor fabrication and cooldown from room temperature to the temperature of operation, can induce surface dislocation nucleation. Other interfaced materials with thermal-expansion mismatch can also generate dislocations during temperature-variation processes. We formulate a simple elastic model to bridge this solid-state effect and the LEE observations. Double-TES modules have been designed to provide surface background rejection. We highlight that the presence of the LEE in the coincident event band of double-TES modules does not exclude the explanation given in this work. Exemplary, we discuss the role of the thermal boundary resistance between absorber and sensor as explanation for the presence of the LEE in the coincident-event band. We propose detector designs to test these hypotheses and mitigate the LEE.