SONTRAC—A low background, large area solar neutron spectrometer

SONTRAC is a scintillating fiber neutron detector designed to measure solar flare neutrons from a balloon or spacecraft platform. The instrument is comprised of alternating orthogonal planes of scintillator fibers viewed by photomultiplier tubes and image intensifier/CCD camera optics. It operates by tracking the paths of recoil protons from the double scatter of 20 to 200 MeV neutrons off hydrogen in the plastic scintillator, thereby providing the necessary information to determine the incident neutron direction and energy. SONTRAC is also capable of detecting and measuring high-energy gamma rays >20 MeV as a “solid-state spark chamber.” The self-triggering and track imaging features of a prototype for tracking in two dimensions have been demonstrated in calibrations with cosmic-ray muons, 14 to ∼65 MeV neutrons and ∼20 MeV protons.