Effects of Gain Changes on RPM Performance
The mission of the U.S. Department of Energy/National Nuclear Security Administration's (DOE/NNSA's) Office of the Second Line of Defense (SLD) is to strengthen the capability of foreign governments to deter, detect, and interdict the illicit trafficking of special nuclear and other radioactive materials across international borders and through the global maritime shipping system. The goal of this mission is to reduce the probability of these materials being fashioned into a weapon of mass destruction or radiological dispersal device that could be used against the United States or its international partners. This goal is achieved primarily through the installation and operation of radiation detection equipment at border crossings, airports, seaports, and other strategic locations around the world. In order to effectively detect the movement of radioactive material, the response of these radiation detectors to various materials in various configurations must be well characterized. Oak Ridge National Laboratory (ORNL) investigated two aspects of Radiation Portal Monitor (RPM) settings, based on a preliminary investigation done by the Los Alamos National Laboratory (LANL): source-to-detector distance effect on amplifier gain and optimized discriminator settings. This report discusses this investigation. A number of conclusions can be drawn from the ORNL testing. First, for increased distance between the source and the detector, thus illuminating the entire detector rather than just the center of the detector (as is done during detector alignments), an increase in gain may provide a 5-15% increase in sensitivity (Fig. 4). However, increasing the gain without adjusting the discriminator settings is not recommended as this makes the monitor more sensitive to electronic noise and temperature-induced fluctuations. Furthermore, if the discriminators are adjusted in relation to the increase in gain, thus appropriately discriminating against electronic noise, the sensitivity gains are less than 5% (Fig. 6). ORNL does not consider this slight increase in sensitivity to be a worthwhile pursuit. Second, increasing the ULD will increase sensitivity a few percent (Fig. 7); however, it is not clear that the slight increase in sensitivity is worth the effort required to make the change (e.g., reliability, cost, etc.). Additionally, while the monitor would be more sensitive to HEU, it would also be more sensitive to NORM. Third, the sensitivity of the system remains approximately the same whether it is calibrated to a small source on contact or a large source far away (Fig. 6). This affirms that no changes to the existing calibration procedure are necessary.