Towards multiparticle position sensitivity with an opaque scintillation detector

Abstract

LiquidO is a detection technique that utilizes opaque media to stochastically confine scintillation and Cherenkov light near its production point, resulting in effective optical segmentation. A lattice of wavelength-shifting (WLS) fibers suspended in the detector volume collects and transports light to photosensors at the fiber ends. LiquidO offers the prospect of improved particle identification and directional reconstruction using time of flight, energy deposition, and topology of events that deposit energy at multiple locations in the detector volume. A critical first step is resolving multiple interaction points of Compton and neutron scattering, demanding a position resolved calibration of the single-scatter response. We performed a calibration of BPULSE (Berkeley Penn State Universities LiquidO Science Experiment), a 4.1L opaque scintillation detector prototype instrumented with 64WLS fibers read out by silicon photomultipliers. A γ-ray source, 137Cs or 60Co, is collimated into a beam that preferentially Compton-scatters in the detector. The scattered γ ray is tagged by a NaI(Tl) scintillator behind a slit aperture. The tag constrains the recoil energy and interaction vertex, providing a known electron recoil energy at a given BPULSE location. For tagged 373.4keV electrons, the detector collects 19.04 ± 0.07 photoelectrons with a 23.4 ± 0.4% energy resolution. Across four measurement points, the response is highly linear, with 48.3±1.5 p.e. produced per MeV between 182keV and 889keV. A center-of-mass vertex reconstruction localizes tagged recoils about the beam axis, with the spread governed by the 20mm beam spot.