Radiation portal monitors used for interdiction of illicit materials at borders include highly sensitiveneutron detection systems. The main reason for having neutron detection capability is to detect fissionneutrons from plutonium. The currently deployed radiation portal monitors (RPMs) from Ludlum andScience Applications International Corporation (SAIC) use neutron detectors based upon 3 He-filled gasproportional counters, which are the most common large neutron detector. There is a declining s ...