Activation of sodium iodide detectors in an Active Interrogation environment

Applications such as the Active Interrogation of shielded nuclear material that require the detection of specific gamma and neutron signatures in a high mixed radiation field background, require detector materials and device fabrications that are not highly susceptible to neutron or photon activation. This paper presents an activation analysis from Active Interrogation experiments where a pulsed-power accelerator was used to deliver 2.5 MeV protons onto a lithium target to produce a spectrum of neutrons up to 1 MeV. The experimental objectives were to characterize the neutron energy spectrum and spatial distribution produced from the ion beam diode utilized with the pulsed accelerator with a view to its utility for the purposes of active detection. A variety of neutron and gamma detectors were used to record the resulting signatures including sodium iodide (NaI), which is the main focus of this paper, and which is also a candidate detector technology to detect fission signatures from active detection systems. A High Purity Germanium (HPGe) detector was used to measure the amount of activation created in the NaI detector. The results showed significant activation of the core detection material, with activation product decay time constants of the order of seconds and minutes. Activation of the detector construction materials was also observed. From this type of analysis, the minimum time between active interrogations can be determined based on the recovery time of the detectors following activation, or alternatively, materials can be sought that are less susceptible to activation.