On the Development of the Wavelength Shifter Deposition System for the DEAP-3600 Dark Matter Search Experiment

Abstract

The DEAP-3600 dark matter search experiment uses 3600 kilograms of liquid argon as a target medium contained in a spherical acrylic vessel. Particle interactions in liquid argon produce vacuum ultraviolet light at a peak wavelength of 128 nanometers, which is not directly visible by the 255 inward-facing photomultiplier tubes. To make visible these interactions, the organic wavelength shifter 1,1,4,4-tetraphenyl-1,3-butadiene, with a re-emission spectrum peaked at 430 nanometers, was evaporatively deposited over the inner surface of the acrylic vessel. The several micron thick coating covers approximately 9 square meters. Application of the coating was performed under vacuum using an evaporation source deployed in the center of the vessel before filling with argon. Research and development on the deployment system for sources inside the acrylic vessel, small scale testing of the evaporation source, and data from test deposition thickness monitoring will be presented, as well as details on the development of the final source, deployment, and performance.