Scaling of Testing Speed for Different Source-to-Detector Distances

Abstract

Several American National Standard Institute/Institute of Electrical and Electronics Engineers (ANSI/IEEE) and International Electrotechnical Commission (IEC) standards were developed for radiation detection systems used for homeland security applications and the detection of illicit trafficking of radioactive materials. These standards cover a wide range of tests, some of which require sources to pass by an instrument at a certain speed producing a fixed exposure rate at the distance of closest approach. Some also allow this distance to vary within a specified range. This work investigates the need to apply a scaling factor dependent on distance to the testing speed in order to compensate for the time that the instrument is exposed to the radiation field. It was observed that there is an increase in the instrument response, with factors ranging from 1.5 to 4, when sources are placed at farther distances while maintaining the same testing speed and the same exposure rate at the distance of closest approach between the source and the detector. When the speed is scaled, most of these differences are reduced to values of approximately ±20 % within the measurement uncertainties.