The detection of fast neutrons has important applications in a variety of fields including geospace,
solar, and planetary physics. Though neutrons are ubiquitous products of nuclear interactions,
they are challenging to detect and the measurements typically suffer large backgrounds. Highenergy
neutrons (> 50 MeV) pose even greater challenges because the traditional double scatter
technique based on a time-of-flight (ToF) is limited by the finite flight path and active detector
sizes limited by small satellite platforms. At these high energies, the proton recoil is likely to
escape the detector volume, degrading the energy and angular resolution. Scintillator-based technologies
have a proven record for detecting and measuring fast neutrons. They have high stopping
power, good energy resolution, and fast timing properties. By dramatically increasing the
segmentation of scintillator arrays (down to hundreds of sub-mm fibers) proton-tracking can be
achieved, effectively supplanting the ToF measurement, thereby eliminating the need for widely
separated detectors, thereby greatly increasing the detection efficiency. It reduces the scale size
of the detector from that necessary for time of flight to the proton range in dense matter. Modern
readout devices such as silicon photomultipliers offer an ideal alternative to photomultiplier
tubes given their inherently compact size, fast response, and low operating voltages. The Solar
Neutron TRACking (SONTRAC) Concept, based on scintillating-fiber bundles, would provide
high-resolution imaging of fast neutrons at energies where the bulk of solar and magnetospheric
neutrons resides. Recent development and performance of the SONTRAC Concept are presented.
