Atmospheric neutron measurements with the SONTRAC science model

The Solar Neutron Tracking (SONTRAC) telescope was originally developed to measure the energy spectrum and incident direction of neutrons produced in solar flares, in the energy range 20-250 MeV. While developed primarily for solar physics, the SONTRAC detector may be employed in virtually any application requiring both energy measurement and imaging capabilities. The SONTRAC science model (SM) is presently being operated at the University of New Hampshire (UNH) as a ground-based instrument to investigate the energy spectrum, zenith and azimuth angle dependence of the cosmic-ray induced sea-level atmospheric neutron flux. SONTRAC measurements are based on the non-relativistic double scatter of neutrons off ambient protons within a block of scintillating fibers. Using the n-p elastic double-scatter technique, it is possible to uniquely determine the neutron's energy and direction on an event-by-event basis. The 3D SM consists of a cube of orthogonal plastic scintillating fiber layers with 5 cm sides, read out by two CCD cameras. Two orthogonal imaging chains allow full 3D reconstruction of scattered proton tracks.