Measurement of Optical Attenuation in Acrylic Light Guides for a Dark Matter Detector

Abstract

Acrylic is a common material used in dark matter and neutrino detectors for light guides, transparent vessels, and neutron shielding, creating an intermediate medium between the target volume and photodetectors. Acrylic has low absorption within the visible spectrum and has a high capture cross section for neutrons. The natural radioactivity in photodetectors is a major source of background neutrons for low background detectors making the use of acrylic attractive for shielding and background reduction. To test the optical properties of acrylic we measured the transmittance and attenuation length of fourteen samples of acrylic from four different manufacturers. Samples were evaluated at five different wavelengths between 375 nm and 632 nm. We found that all samples had excellent transmittance at wavelengths greater than 550 nm. Transmittance was found to decrease below 550 nm. As expected, UV-absorbing samples showed a sharp decrease in transmittance below 425 nm compared to UV-transmitting samples. We report attenuation lengths for the three shortest wavelengths for comparison and discuss how the acrylic was evaluated for use in the MiniCLEAN single-phase dark matter detector.Comment: Accepted by JINST, version 2 with edits from reviewer comment