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. Most currently deployed radiation portal monitors (RPMs) use neutrondetectors based upon 3 He-filled gas proportional counters, which are the most common large area neutron detector. This type of neutron detector is used in the RPMs installed in international loc ...