Alpha Detector Pixelization Effects on NMIS Imaging

The Nuclear Materials Identification System (NMIS) conducts non intrusive scans of containers and other objects using active interrogation with 14.1 MeV neutrons from a deuterium-tritium (DT) neutron source coupled with an associated alpha particle detector. In thick target materials, especially those with a high atomic mass, neutrons may be scattered in such a manner that they are indistinguishable from directly transmitted neutrons. By subdividing the integrated alpha detector into pixels, the NMIS more accurately defines the initial neutron direction than it does with the original, unpixelated alpha detector. A scan of an annular depleted uranium (DU) casting inside 24.8 cm of lead shielding shows the improvement this change makes to imaging measurements. This improvement is expected to decrease the time required to perform Nuclear Material Control and Accountability (NMC&A) measurements with the NMIS system.