Modeling the efficiency of a fiber-coupled scintillator used for particle Time-of-Flight measurements in the iSNAP experiment*

The in-Situ counting of instantaneous Nuclear Activation Products (iSNAP) experiment was conducted using the Multi-Terawatt Laser (MTW) at the Laboratory for Laser Energetics (LLE) to determine reaction cross sections. MTW uses Target Normal Sheath Acceleration (TNSA) to generate a short intense pulse of deuterons with energies up to 10 MeV. These deuterons activated a ⁷Li target whose short half-life activation products were counted in-situ using the Short-Lived Isotope Counting System, SLICS.  A particle Time-of-Flight (pToF) detector is needed to determine the energy spectrum of the deuterons at the time the ⁷Li target was activated. The pToF system consists of an EJ-212 plastic scintillator that was fashioned in a wedge shape to sample deuterons from a 1/16th portion of the surface of the ⁷Li target. The wedge-shaped scintillator detected the TNSA-produced deuterons with an unknown efficiency. Here, we model the light collection efficiency of the pToF detector accounting for the polarization of generated light, reflection within the scintillator, and coupling into the optical fiber.  Efficiency was determined for a range of optical fibers and scintillator shapes. For each detector geometry, the sensitivity of the collection efficiency to the deuteron position within the scintillator was also calculated.  These results will lead to improved design of fiber-coupled scintillator detection systems.