Experimental Investigation of Temperature Effects on Radiation Portal Monitor Performance

Radiation portal monitor (RPM) systems are deployed aroundthe world in order to help detect and deter the movement of illicitnuclear material. Because these systems are often deployedin remote locations or on borders, they are exposed to and musttolerate a wide range of ambient temperature. Therefore, discoveringtemperature dependent behavior in RPM-type detectorsis increasingly important as more systems are deployed and theglobal political climate places a premium on ensuring that illicittrafficking of special nuclear material (SNM) and other radioactivematerial is detected.In this paper, results from an experimental investigation ofthe temperature dependence of two RPM detectors’ behavior arepresented. Four hypotheses are examined, and a discussion ofthe results is included as well. The results presented in this paperdemonstrate that the root cause of these detectors’ temperaturedependence is the photomultiplier tubes. Furthermore, the resultsprovide evidence that dark current formation in the photomultipliertubes significantly impacts the background count rateof detectors, which in turn decreases the detectors sensitivity tolow energy gamma sources such as SNM.