Prototype for SONTRAC: a scintillating plastic fiber detector for solar neutron spectroscopy

We report the scientific motivation for and performance measurements of a prototype detector system for SONTRAC, a solar neutron tracking experiment designed to study high- energy solar flare processes. The full SONTRAC instrument will measure the energy and direction of 20 to 200 MeV neutrons by imaging the ionization tracks of the recoil protons in a densely packed bundle of scintillating plastic fibers. The prototype detector consists of a 12.7 mm square bundle of 250 micrometer scintillating plastic fibers, 10 cm long. A photomultiplier detects scintillation light from one end of the fiber bundle and provides a detection trigger to an image intensifier/CCD camera system at the opposite end. The image of the scintillation light is recorded. By tracking the recoil protons from individual neutrons the kinematics of the scattering are determined, providing a high signal to noise measurement. The predicted energy resolution is 10% at 20 MeV, improving with energy. This energy resolution translates into an uncertainty in the production time of the neutron at the Sun of 30 s for a 20 MeV neutron, also improving with energy. A SONTRAC instrument will also be capable of detecting and measuring high-energy gamma rays greater than 20 MeV as a 'solid-state spark chamber.' The self-triggering and track imaging features of the prototype are demonstrated with cosmic ray muons and 14 MeV neutrons. Design considerations for a space flight instrument are presented.