Cosmic Muon Rates and Neutron Background in a Backpack Radiation
Detection System
Abstract
Radiation detection systems for security applications are often equipped
with neutron sensors and required to promptly respond with an alarm if
the neutron flux exceeds the background level. As the ambient neutron
flux is essentially caused by cosmic radiation, this background could be
evaluated by assessing the rate of muons crossing the gamma-ray detector
which is comprised in such systems anyway. A study performed with a
commercial Backpack Radiation Detection System (BRD) demonstrates the
feasibility of this approach, supposed the gamma spectrometer is capable
of discriminating muon signals from energetic events caused by the
gamma-ray cascades following neutron captures. This requires a dynamic
range exceeding the usual 3-4 MeV. The instrument used provided standard
spectroscopy up to 10 MeV and facilitated charge-loss compensated
spectroscopy up to 1 GeV, which allowed a separate assessment of normal
gamma events (< 3 MeV), neutron-capture gamma events
(3-7 MeV), and muon events (> 10 MeV). In this case, the
rate of neutron-capture gamma events can be used as a neutron indicator
on its own, while the muon rate signals the ambient neutron background
level.