On the possibility of darkening epoxy due to radiation damage in the STAR EPD

The STAR EPD (event plane detector) is a subsystem of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) whose purpose is to probe the geometry of a heavy ion collision by examining the distribution of forward-going charged particles exiting the collision. The EPD consists of two wheels of scintillators wound with wavelength shifting fibers, each composed of 12 "supersectors," which are themselves connected to a Silicon photomultiplier (SiPM) via an optical fiber connection. To hold the wavelength shifting fibers in their place within each supersector, Eljen EJ-500 optical epoxy was used. The goal of this study was to determine if this epoxy darkened appreciably throughout RHIC's operation from 2018 - 2021 due to radiation damage. Analysis was performed on measurements of output (ADC) vs. bias voltage to investigate this phenomenon. The epoxy was demonstrated not to darken appreciably on this time scale due to radiation damage.