Efficiency calibrations of radiation detectors for source localization and characterization at long source-detector distances for gamma and neutron sources

Deployment of radiation detectors under field conditions for the purposes of security, safety or response has increased in recent years. Effective use of such instruments in the field necessitates careful consideration of the efficiency of the detector - both peak and total - at distances which may extend beyond 100 m. Difficulties in addressing the determination of both peak and total efficiencies across the energy range of interest and at long distances reduces the utility of such systems in effectively characterising radiation sources in the field. Empirical approaches to such calibrations are difficult. Approaches such as Monte Carlo simulations can become challenging with respect to time and computational requirements as source-detector distances become greater and in consideration of total efficiency. This paper presents a computationally efficient method of calculating peak efficiency at distances more than 300 m using efficiency transfer from a parallel beam geometry to point sources at extended distances. The relationship between total and peak efficiency at extended distances is explored and means of estimating the total efficiency from the peak efficiency are discussed. The ratio of the total efficiency to the peak efficiency increases as a function of the source-detector distance. The relationship is linear at distances longer than 50 m and is independent of photon energy. Usefulness of the efficiency calibration as a function of the source-detector distance was demonstrated in a field experiment. Total efficiency calibration measurements were performed for a neutron counter. An AmBe source was then successfully localized and characterised using four measurements at arbitrary locations far away from the unknown source. This kind of capability is useful for the authorities responding to nuclear accidents or security events. It has important operational implications, including the safety of the personnel involved.