The Effects of Background Gamma Radiation of the Sensitivity of Hand-Held Special Nuclear Materials Monitors

The detection system of a hand-held special nuclear materials monitor was tested at a number of fixed gamma-ray background levels. The system produced an alarm when the gamma-ray counts, which were accumulated during any counting interval, equaled the trip level. Tests were conducted by rotating a standard 10-g 235u test source past the detector and recording the detection probability for a series of trip level settings at each of eight background radiation levels. The results show that with appropriate trip level settings, it is possible to exceed the required 50% detection probability at background levels that range from 38 counts (produced by our local background of 20 pR/h) to 400 counts per 0.3-s counting interval.