Active neutron interrogation: Experimental results from the PUNITA device

The PUNITA device is a state-of-the-art active neutron interrogation facility, aimed to identify and quantify Special Nuclear Materials (SNM), by measuring induced fission neutrons. The source neutrons are generated by a powerful (D−T) tube, creating fast neutron pulses which are thermalized in a large graphite reflector. Due to the relative high detection efficiency, together with the strong thermal flux created, the PUNITA device holds some very favorable properties, and is capable of detecting SNM signature in fairly short measurement times. Yet, these exact same properties form interesting challenges and questions, due to self shielding effects and other spatial effects. In the present study, we introduce experimental results that differ from past experiments in four aspects: first, the mass range – up to 100 [gr] of ²³⁵U – is significantly larger than previous experiment. Second, as opposed to past experiments, not all samples have the same geometry and total U mass. Third, samples were repeatedly measured, to check the effect of source fatigue and source variance on the sampled values. Finally, a new mathematical formalism was used to quantify and account for the (D−T) source variance, and how it effects the sampled doubles rate.