Effect of SiO2 coating in bolometric Ge light detectors for rare event searches

TL;DR

This study demonstrates that applying a SiO₂ anti-reflective coating on germanium light detectors enhances light collection efficiency by about 20% at millikelvin temperatures, aiding rare event searches.

Contribution

It provides a detailed measurement of SiO₂ coating effects on germanium bolometric detectors, with a methodology applicable to other coatings and luminescent materials.

Findings

SiO₂ coating increases light collection by ~20%.

Measurement performed at 25-35 mK temperature range.

Method minimizes systematic effects for accurate results.

Abstract

In germanium-based light detectors for scintillating bolometers, a SiO$_2$ anti-reflective coating is often applied on the side of the germanium wafer exposed to light with the aim to improve its light collection efficiency. In this paper, we report about a measurement, performed in the temperature range 25-35 mK, of the light-collection increase obtained thanks to this method, which resulted to be of the order of 20%. The procedure followed has been carefully selected in order to minimize systematic effects. The employed light sources have the same spectral features (peaking at $\sim 630$ nm wavelength) that will characterise future neutrinoless double beta decay experiments on the isotope $^{82}$Se and based on ZnSe crystals, such as LUCIFER. The coupling between source and light detector reproduces the configuration used in scintillating bolometers. The present measurement clarifies the role of SiO$_2$ coating and describes a method and a set-up that can be extended to the study of other types of coatings and luminescent materials.