Comparison of the response of handheld neutron detectors in differing deployment environments: Measurements, calculations and practical implications

Highlights: • Measured and modelled (MCNP) data from and of various handheld neutron instruments. • Environmental scatter leads to a divergence from an inverse-square relationship. • Summary to assist the threat assessment at the scene of a nuclear security event. Authorities responding to nuclear security events have to be prepared for a situation in which a neutron source or nuclear material is out of regulatory control. In such a situation, handheld neutron detectors are used to search for neutron sources or nuclear material, during either an overt or a covert search. These detectors are also used to support the initial threat assessment, radiation protection at the scene and the initial characterisation of the source. Long range neutron detection, i.e. detection several tens of meters from the source, referred to here as "stand-off" detection, is of importance to nuclear security. An estimate of the neutron source strength is especially important for the initial threat assessment. However, the neutron dose rate and the neutron response are strongly dependent on the distance between the detector and the source and on the surrounding environment. These influences should be considered when estimating the neutron source strength from initial measurements. In this paper the variation of the neutron dose rate and the neutron count rate with distance from a neutron source in different, representative environments was determined. Monte-Carlo simulations were carried out in order to gain more understanding of the results. In addition, the variation of the neutron dose rate with distance from a neutron source in different surroundings is summarized in order to assist the initial threat assessment at the scene of a nuclear security event.