The Characterization and Application of a Method to Predict the Background Neutron Response in Nuclear Security Systems
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Date
2021-04-14
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Abstract
Nuclear security operators employ gamma radiation spectroscopy and neutron counting techniques to locate radiological materials, particularly those that may be used for nuclear terrorism. One such method employed is the mobile radiological search system (MRSS). A challenge with an MRSS is its difficulty to discriminate background neutron signals. However, operational data suggested a linear relationship between the neutron count rate and the gamma spectrometer’s high energy (>3 MeV) count rate. Such a relationship would enable the prediction of the neutron background, which would improve the MRSS’s ability to discriminate non-environmental background responses. The purpose of this research was to verify this relationship, determine its source, and establish a method to predict neutron background in an MRSS. Verification measurements used a commercially available MRSS in a series of measurements in parking garages on Texas A&M University’s campus. The natural variations of radiation background within the garages confirmed the correlation between the gamma and neutron radiation backgrounds. With the relationship verified, a cause for the correlation was sought. This was done through extended radiation background measurements using different configurations and types of gamma ray scintillators. Analysis of their high energy (4-85 MeV) spectra found that they were caused by muons. This result indicated that the correlation between the two detectors is significant: muons and neutrons are part of radiation background as both are produced by cosmic ray interactions with the Earth’s atmosphere. Additional MRSS measurements sought to characterize this relationship by measuring locations throughout the Central United States, sampling variables that affect the neutron and muon fluxes. These results showed that the neutron count rate followed a power law function of the muon count rate, indicating the neutron count rate indeed could be predicted. This prediction method was then applied to MRSS measurements of a parking garage while 252Cf (a spontaneous fission neutron source) was present. These measurements found that the alarm method which came from the neutron prediction function performed properly when alerting for the presence of that radioactive material. Overall, these results show that MRSS operators can use similar systems to make such predictions, and the resulting alarm method should improve an MRSS’s sensitivity to man-made radiological materials.
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nuclear security, neutron background prediction, neutron counting, muon measurement, scintillation spectroscopy