Calibration of the SOlar Neutron TRACking (SONTRAC) Instrument

Solar neutrons are a secondary product of the interaction of trapped flare-accelerated charged particles with the dense chromospheric plasma. As they are electromagnetically neutral, solar neutrons and γ-rays may escape the corona and serve as probes to study flare-accelerated particles and processes that are otherwise inaccessible. However, neutrons are challenging to detect both due to high ambient backgrounds of Earth-orbiting spacecraft, and the short lifetime of free neutrons. Neutrons are traditionally measured using a time-of-flight technique requiring measurements in two detectors separated by a relatively large distance. Using this technique, measurements of fast (>0.5 MeV) neutrons require large instruments that are challenging to fly on space-constrained spacecraft. The SOlar Neutron TRACking (SONTRAC) instrument is a compact, imaging neutron spectrometer composed of orthogonally-stacked planes of scintillating optical fibers read out by miniature silicon photomultipliers (SiPM). Ionization tracks of recoil protons produced by neutron interactions within the fibers provide information to allow reconstruction of the energy and direction of incident neutrons. In this work, we discuss recent calibration efforts of SONTRAC, highlighting measurements of mono-energetic protons and neutrons at the Crocker Nuclear Laboratory at UC Davis as well as complimentary Monte Carlo modeling of the instrument. These measurements and simulations were utilized to compute conversion factors from instrumental units to physically meaningful values using the well-understood behavior of proton ionization behavior through material, and improve our understanding of the instrument response. Calibration efforts in preparation for the accelerator run are also presented.