Comparison of TPB and bis-MSB as VUV waveshifters in prototype LBNE photon detector paddles

The Long-Baseline Neutrino Experiment (LBNE) Project is expected to provide facilities that will enable a program in neutrino physics that can measure fundamental physical parameters, explore physics beyond the Standard Model and better elucidate the nature of matter and anti-matter. The LBNE Photon Detection subsystem is primarily designed to detect the scintillation photons produced at 128 nm as ionizing particles traverse the liquid argon. The LBNE reference design for the photon detector subsystem uses adiabatic light guides consisting of cast acrylic bars whose surface is embedded with waveshifter to convert the Vacuum Ultraviolet (VUV) 128 nm photons into the optical bandpass of silicon photomultipliers (SiPMs). In this investigation, we describe comparative studies of two VUV waveshifters — TPB and bis-MSB. We find that bis-MSB is more efficient than TPB at 128 nm. We also find that the efficiency of converting VUV photons into the optical for both waveshifters rises from 170–200 nm. Studies of the long wavelength behavior of the waveshifters supports the result that the efficiency is rising.