A Comparison Between Gross and Net Count Mrthods When Searching for Orphan Radioactive Sources

Abstract

Different numerical strategies in searching for orphan radioactive sources in the environment by means of a mobile detector system have been evaluated. A carborne 3- by 3-inch NaI(Tl) spectrometric system was used with an unshielded 2 GBq 137Cs source as a test source. In this paper, a previous method (MB method), based on a moving average algorithm applied on the gross count rate, was extended and compared with three moving average algorithm methods involving different natural background subtraction strategies. For each method the distance from the road that an orphan source can be detected with a probability of 50% (the critical distance, CD) when driving by is determined. The CD for a 2 GBq 137Cs source improved from 105 m to 130 m when the interference from 40K was subtracted from the 137Cs spectral window. However, when the contribution of other natural gamma-emitting nuclides also was subtracted, the ability to find the 137Cs source was reduced.

A deviation display method for visualising data in mobile gamma-ray spectrometry

“…With the source sealed in the steel capsule and at a distance of 130 m, the primary photon fluence rate at the road was approximately 10 % lower than that for the same distance in Hjerpe and Samuelsson (2003). This difference is balanced by the slightly larger detector used in this work, making the experiment carried out in this work comparable to those of Hjerpe and Samuelsson (2003), in terms of sensitivity.…”

“…This difference is balanced by the slightly larger detector used in this work, making the experiment carried out in this work comparable to those of Hjerpe and Samuelsson (2003), in terms of sensitivity. …”

“…Here, instead of deciding alarm levels, the background measurements were used to sample the relative deviations, q i,j . Hjerpe and Samuelsson (2003) showed that a representative background distribution could be estimated after about 45 minutes by driving around the area where the experiments should take place. Since the field experiment in this work was undertaken at the same site, this result applies here as well.…”

“…The Critical Distance (CD) is defined by Hjerpe and Samuelsson (2003) as "the distance from the road at which a source can be detected with 50% probability when driving by". Here, the source was placed at D = 130 m perpendicular to a 400 m long straight stretch of the road and driven by 10 times at a speed of 50 km h −1 .…”

“…Here, the source was placed at D = 130 m perpendicular to a 400 m long straight stretch of the road and driven by 10 times at a speed of 50 km h −1 . This distance was not arbitrarily chosen, it equals the CD of a 2 GBq 137 Cs source using a 7.62 x 7.62 cm NaI(Tl) system and the PSC-method, as shown by Hjerpe and Samuelsson (2003).…”