Development of an SNM Test Source Simulator

To optimize detection sensitivity and minimize the effects of ambient background radiation, portal monitors for the detection of special nuclear materials (SNM) are ordinarily calibrated to accept signals with amplitudes corresponding to the radiation emitted from the material to be detected and to reject those falling outside of this range1-2. In the testing of portal monitors in service; it is best to utilize actual samples of the SNM to be detected (235U or 239Pu) since no other radioactive materials duplicate the gamma-ray spectra emitted by these nuclides, and a test with a check source emitting gamma rays with other energies may not necessarily disclose a de-calibration arising from gain drifts or other equipment malfunctions. The use of SNM samples for this purpose, however, may be impeded or even prevented by material control, security, and health and safety requirements. A user need has been identified for a gamma-ray source whose spectrum resembles closely that of a sample of 235U or 239Pu, but which in fact contains no SNM.