PNNL

Abstract

To interdict illicit radioactive sources and prevent nuclear terror, governments have deployed portal monitor systems at ports of entry. Such portal monitors rely upon the detection of spontaneously-emitted radiation signatures, however, detection scenarios become more challenging for highly enriched uranium, which emits relatively few spontaneous radiation signatures. Photon active interrogation techniques can augment current cargo inspection scenarios and improve detection capabilities for illicit special nuclear material. A high-energy photon interrogation source can induce photofission in nuclear material, producing strong radiation signatures for detection. Prompt fission signals are of specific interest because prompt neutrons are greater than 99% of the fission signal. When using detectors capable of spectroscopy, e.g., pulse shape discrimination-capable organic scintillators, the intense interrogating source will cause pulse pile-up, degrading particle classification. In this work, we demonstrate photoneutron detection via neutron activation of aluminum and iron. The experiments rely upon nuclear reactions that can only be induced by neutron interactions, and are robust against pile-up, thereby providing high-confidence neutron rates. In this work, we extend the application of neutron activation to homeland security applications, e.g., interrogating DU as an SNM surrogate, producing neutrons from (?,n) and photofission reactions that can activate samples. We find that the interrogated DU produced an aluminum activation flux of 1.77 × 105 n cm-2 s-1 and an iron-56 activation flux of 1.05 × 105 n cm-2 s-1. The ability to detect radioactivity induced by neutrons created in (?,n) and photofission reactions demonstrates a new approach for photon active interrogation to identify nuclear materials.