Experimental Investigation of Temperature Effects on Radiation Portal Monitor Performance

Radiation portal monitor (RPM) systems are deployed around the world in order to help detect and deter the movement of illicit nuclear material. Because these systems are often deployed in remote locations or on borders, they are exposed to and must tolerate a wide range of ambient temperature. Therefore, discovering temperature dependent behavior in RPM type detectors is increasingly important as more systems are deployed and the global political climate places a premium on ensuring that illicit trafficking of special nuclear material (SNM) and other radioactive material is detected. In this paper, results from an experimental investigation of the temperature dependence of two RPM detectors’ behavior are presented. Four hypotheses are examined, and a discussion of the results is included as well. The results presented in this paper demonstrate that the root cause of these detectors’ temperature dependence is the photomultiplier tubes. Furthermore, the results provide evidence that dark current formation in the photomultiplier tubes significantly impacts the background count rate of detectors, which in turn decreases the detectors sensitivity to low energy gamma sources such as SNM.