  Nowadays, the current threat of international terrorism is set to a severe
level, demanding worldwide enhanced security. Radioactive materials that could
be fashioned into a radiation dispersal device typically emit gamma rays, while
fissile materials such as uranium and plutonium emit both neutrons and gamma
rays via spontaneous or induced fission. Therefore, the detection and
identification of hazardous materials has become increasingly important. We
present the results of GEANT4 Monte Carlo simulation of an active neutron
interrogation system based on highly segmented neutron/gamma-ray detector and
pulsed neutron generator. This system is capable of detecting and imaging
radioactive and special nuclear materials, explosives and drugs. The segmented
scintillation detector works as a scatter camera, allowing selection of a gamma
ray events that undergo multiple interaction in detector blocks for radioactive
source localization. The detector consist of blocks made of plastic
scintillator which provide scattering and blocks of CsI, used as an absorber,
used as an absorber, which has to be efficient to detect the characteristic
gamma radiation for the identification. Because of this imaging capability
background events can be significantly rejected, decreasing the number of
events required for high confidence detection and thereby greatly improving its
sensitivity. A scatter imager for the detection of shielded radioactive
materials has been conceptualized, simulated, and refined to maximize
sensitivity while minimizing cost.
