Phenomenology of prompt gamma neutron activation analysis in the detection of mines and near-surface ordnance

Abstract

ABSTRACT Prompt gamma neutron activation analysis (PGNAA) has been proposed for confirming the presence of energetic materials aspart of a mine or unexploded ordnance detection system. Ancore Corporation (previously SAIC Advanced NucleomcsDivision), funded through Night Vision Electro Sciences Directorate by Environmental Security Test Certification Program,has carried out proof-of-concept demonstrations of PGNAA in this confirmatory role at Socorro, NM, and Yuma, AZ. Inthis, the first part of a two-part paper addressing the use of PGNAA in the detection of surface and rear-surface UXO, weexplore the phenomenology of PGNAA signals from surface or near-surface ordnance in soil to gain insight into the results cfthose demonstrations.PGNAA uses the high-energy gamma ray (10.8 MeV) from capture on N'4 as a signature of the presence of nitrogen. This isone of the highest energy gamma rays resulting from neutron capture, and nitrogen is a major constituent of explosives, but asmall portion of soil. Thus, PGNAA might be effective at confirming the presence of explosives.The phenomenology of dry soil is dominated by the two most common elements, oxygen and silicon. Neutrons injectedinto the soil elastically scatter from nuclei (predominantly oxygen), losing energy and propagating in a random walk fashion.Once slowed, neutron capture on soil elements produces a broad gamma-ray spectrum. Capture on Si29 produces a 10.6 MeVgamma, which is not resolvable from the nitrogen signal of interest using scintillation detectors. Thus, PGNAA will needeither good resolution detectors, or robust background subtraction to estimate the silicon contribution.For any system unable to resolve the 5i29(1O.6 MeV) and N'4(1O.8 MeV) gammas there is an inherenfly low signal tobackground, resulting primarily from the silicon in the soil. After background subtraction, there remains a challenging signal