# Low-Temperature Relative Reflectivity Measurements of Reflective and Scintillating Foils used in Rare Event Searches

## TL;DR

This study measures the low-temperature reflectivity of multilayer foils used in rare event detection experiments, finding no significant change in reflectivity between room temperature and cryogenic conditions, which is crucial for optimizing detector performance.

## Contribution

It provides the first detailed low-temperature reflectivity measurements of specific multilayer foils used in dark matter and neutrinoless double beta decay experiments, informing their use in cryogenic detectors.

## Key findings

- No significant change in reflectivity from 300 K to 20 K for all foils studied.
- Reflectivity ratios indicate stable optical properties at cryogenic temperatures.
- Results support the reliability of these foils in low-temperature rare event searches.

## Abstract

In this work we investigate the reflectivity of highly reflective multilayer polymer foils used in the CRESST experiment. The CRESST experiment searches directly for dark matter via operating scintillating CaWO$_4$ crystals as targets for elastic dark matter-nucleon scattering. In order to suppress background events, the experiment employs the so-called phonon-light technique which is based on the simultaneous measurement of the heat signal in the main CaWO$_4$ target crystal and of the emitted scintillation light with a separate cryogenic light detector. Both detectors are surrounded by a highly reflective and scintillating multilayer polymer foil to increase the light collection efficiency and to veto surface backgrounds. While this study is motivated by the CRESST experiment, the results are also relevant for other rare event searches using scintillating cryogenic bolometers in the field of the search of dark matter and neutrinoless double beta decay ($0\nu\beta\beta$). In this work a dedicated experiment has been set up to determine the relative reflectivity at 300 K and 20 K of three multilayer foils ("VM2000", "VM2002", "Vikuiti") produced by the company 3M. The intensity of a light beam reflected off the foil is measured with a CCD camera. The ratio of the intensities at 300 K and 20 K corresponds to the relative reflectivity change. The measurements performed in this work show no significant change in the reflectivity with temperature for all foils studied.