Assessment of thermal neutron analysis applied to surface and near-surface unexploded ordnance detection

We present an analysis of the use of thermal neutron analysis (TNA) to confirm the presence of energetic materials in unexploded ordnance detection. Our analysis is based on the performance of a prototype built by ANCORE and tested at Socorro, NM, and at Yuma, AZ, as part of the Environmental Security Technology Certification Progam (ESTCP). From that data, we were able to develop a semi-empirical model for the expected signal strength as a function of the target's nitrogen content and depth. We found that the dependence on depth differs greatly between the two sites. We expect this simple model to be useful in future assessments of the feasibility of this approach. We also determined the P-d/P-FA performance of the system at the two sites and found it to correspond to a signal-to-noise ratio of order unity. We estimate that an increase in signal-to-noise of roughly three will be necessary to extend the applicability of this technology in unexploded ordnance detection. Such improvements may be possible if the NaI detectors currently employed are replaced with high purity germanium (HPGe) detectors.