In this work we investigate the reflectivity of highly reflective multilayerpolymer foils used in the CRESST experiment. The CRESST experiment searchesdirectly for dark matter via operating scintillating CaWO$_4$ crystals astargets for elastic dark matter-nucleon scattering. In order to suppressbackground events, the experiment employs the so-called phonon-light techniquewhich is based on the simultaneous measurement of the heat signal in the mainCaWO$_4$ target crystal and of the emitted scintillation light with a separatecryogenic light detector. Both detectors are surrounded by a highly reflectiveand scintillating multilayer polymer foil to increase the light collectionefficiency and to veto surface backgrounds. While this study is motivated bythe CRESST experiment, the results are also relevant for other rare eventsearches using scintillating cryogenic bolometers in the field of the search ofdark matter and neutrinoless double beta decay ($0\nu\beta\beta$). In this worka dedicated experiment has been set up to determine the relative reflectivityat 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 thefoil is measured with a CCD camera. The ratio of the intensities at 300 K and20 K corresponds to the relative reflectivity change. The measurementsperformed in this work show no significant change in the reflectivity withtemperature for all foils studied.
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