Multi-layer fast neutron detectors based on composite heavy-oxide scintillators for detection of illegal nuclear materials

We developed and characterized a new type (designated ZEBRA) of multi-layer composite heavy-oxide scintillatordetectors for fast neutron detection for homeland security and nuclear safeguards applications. In this heterogeneous detector medium, composite layers comprised of micro-granules of heavy-oxide scintillators (ZWO, CWO, PWO, BGO, GSO(Ce), GOS(Ce) and others) dispersed in transparent plastic are alternated with layers of clear plastic that serve as scintillation light guides and as a neutron moderator material. The physical peculiarities of the neutron interactions and the principal mechanisms of fast neutron registration in these detectors are discussed in detail. The fast neutron intrinsic detection efficiencies and sensitivities of ZEBRA-detectors based on BGO, ZWO and GSO(Ce) composite scintillators in response to neutrons from 239Pu-Be and  252Cf sources were measured. These detectors had cross-sectional areas ranging from 16 to 100 cm2. The sensitivities of such detectors of size 100×100×41 mm3 were found to be 40-51 cps/(nps × cm−2), a level that is comparable to the sensitivity of a typical  3He counter of 1600 cm2 area. The intrinsic efficiencies and sensitivities of the ZEBRA-detectors also compare favorably with those of fast neutron detectors based on large-size heavy-oxide single crystals, but the multi-layer composite ZEBRA structures are much less expensive and can be easily manufactured in much larger dimensions. This work represents a significant advance from earlier single-crystal detector types as part of our effort to explore alternatives and improvements to conventional  3He counters.