Abstract
Radiation detection for nuclear security frequently employs neutron counting and scintillation systems simultaneously. One potential issue, particularly when searching a large area, is understanding the ambient (or background) response of these systems throughout the operation. This is easily mitigated for the scintillation system but remains a problem for neutron counting systems. Operational data and previous research have shown that a correlation appears between the neutron count rate and the count rate at high energies in the scintillation system (energies greater than 4 MeV) in background conditions. To understand the cause of the correlation, background measurements were performed using sodium iodide (NaI) and polyvinyl toluene (PVT) scintillation systems. These detectors were calibrated to high energy scales such that their spectra would show energies up to 70 MeV and 85 MeV, respectively. Results show that at least one statistical mode appeared in the spectra on these energy scales (particularly between 5 MeV and 60 MeV). The energy and maximum probability of these modes varied with orientation, and they were dependent upon the detector thickness with respect to the vertical axis and the detector area perpendicular to that axis, respectively. The modes’ energies also matched the expected energy deposition from background muons in the detectors with path lengths equal to one of the detectors’ dimensions. These data matched results from simulations of background muons interacting with these detectors calculated using MCNP, and they similarly matched muon energy spectra calculated from possible path lengths through the detectors using Python. These results indicate that scintillation measurements at energies higher than those employed in typical nuclear security operations are the result of background muons. Since these muons are produced similar processes as background neutrons, the count rate of these particles could potentially be applied to better characterize the background in neutron counting systems.
Highlights
The threat of nuclear terrorism is a concern for many nations
A nuclear terrorist attack would most likely occur at a public event and would use a radiological dispersal device (RDD – commonly known as a “dirty bomb”) or an improvised nuclear device (IND)
To efficiently locate sources of radiation, the mobile search system detectors must have well understood ambient responses. This is simple for a gamma spectroscopy system as the background has a spectrum that is unique from any illicit gamma ray-emitting material [1, 2]
Summary
The threat of nuclear terrorism is a concern for many nations. A nuclear terrorist attack would most likely occur at a public event and would use a radiological dispersal device (RDD – commonly known as a “dirty bomb”) or an improvised nuclear device (IND). The background response in a neutron counting system is much more difficult to characterize because the user is only provided a count rate This is compounded by the fact that the main source of background neutrons is secondary cosmic ray interactions in the Earth’s upper atmosphere (mainly through spallation processes) [3]. This and the physics of neutron interactions means the neutron count rate can vary with elevation changes along with the presence and composition of surrounding buildings or Jackson Nicholas Wagner and Craig Marianno: Identification of the Ambient Response Relationship in Neutron Counting and Scintillation Measurement Systems material. This makes it more difficult to determine if a change in neutron count rate is due to one of these background changes or from an illicit source
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