Year
2015
Abstract
The normal use of gamma radiation portal monitors is for the monitor to detect a passing source and then do a secondary inspection. If the monitor is at a remote location, such as along a highway immediate secondary inspection is impractical. The identification of the isotope(s) causing the detection, as well as other information on the source (such as the shielding material, thickness and source strength) depends entirely on the gamma spectrum obtained by the remote detector. Due to the limited dwell time, from ~ 2 seconds for a 5 mph source to less than 0.2 seconds for a source moving at highway speed, the detection methods must be very sensitive. The impact of the highly sensitive detection methods needed in these situations on the isotope identification is that the amount of information available for processing the spectrum obtained by the detector information may be very limited. Algorithms exist which allow for reliable detections of sources which provide only 20-30 counts above background. Given the very low counts levels possible when using a high sensitivity detection system the amount of information available may be very limited, making use of high level sophisticated spectra processing models ill advised. This paper discusses the problems involved, and the impact of signal strength on the ability for the system to identify various factors in the signal causing the detection. As the number of counts in the spectrum increases, increasingly more sophisticated models can be used, increasing the reliability of the estimate of shielding, shielding material and source strength.