Characterization of a gamma spectrometer–neutron counter background correlation in mobile radiological search systems

Abstract

The mobile radiological search system (MRSS) is frequently employed to locate radiological material that may be used in nuclear terrorism. Because they are mobile, an MRSS has difficulties characterizing the radiation background during operations, particularly in their neutron counters. Operators of these systems noted a potential relationship between the MRSS’s background neutron count rate and the count rate in its scintillator’s “overflow” channel (a channel that counts all signals with energies greater than 3 MeV). Previous work indicated that this overflow channel is the result of muons. That result coupled with the fact that both muons and background neutrons are produced by cosmic ray interactions meant that the neutron-overflow channel relationship is non-trivial. To fully characterize the neutron–muon relationship, this research collected and analyzed MRSS data from throughout the continental United States. These data were collected by three different organizations (one university and two MRSS operators), each with a unique MRSS setup. The locations measured accounted for the changes in neutron and muon background as a result of three variables: elevation, latitude, and the local environment (i.e. nearby buildings). Analysis of this data indicated that the average neutron count rate varied with the average muon count rate by a power law function. This relationship sufficiently accounts for all three variables that change both count rates, and its presence indicates that an MRSS can estimate its background neutron count rate by simply measuring the muon count rate. However, the data also indicated one complication with applying this relationship more broadly: each organization’s relationship had slightly different power law equations, which appears to be dependent on individual MRSS setup.