THE INTERFERING SOURCE PROBLEM

In the performance testing of nondestructive assay systems there exists the concept of a benign or non-interfering matrix. This is understood to be a container filled with a waste simulant which is not expected to perturb the measurement technique. Since many assay systems take the void or empty (apart from air) container to be the reference case used to establish the absolute calibration parameters in the absence of matrix corrections the empty container is sometimes used as an example of a non-interfering matrix. The assay of a test source or a collection of test sources in the empty container is not however free of bias. This is because the spatial distribution of the test sources usually does not replicate the uniformly distributed source distribution of the reference calibration. Further more, and often more importantly, the construction and support of test sources containing special nuclear materials usually introduces significant attenuation of the characteristic gamma rays used to quantify the mass of the radioactive material present. This is a source of bias because the reference calibration conventionally assumes that any special nuclear materials present are dilute and not encapsulated so that self attenuation and encapsulation attenuation are negligible. For example, the counts per sec per gram formed from an experimental efficiency curve based on traceable gamma-ray standards is corrected for attenuation. In the present work we discuss the bias (relative to non-attenuating source conditions) introduced when assaying Performance Demonstration Program (PDP) type sources of Pu using a ?-ray box counter. These are made by mixing plutonium dioxide particles in diatomaceous earth and encapsulating it in stainless steel for safety reasons. It is clear that the PDP sources are inherently interfering to the relatively low-energy Pu lines of principle interest. In principle the differential attenuation between various lines from a given isotope, or a range of isotopes when the relative isotopic composition is known, gives information about the attenuation and this can be used to surmise or confirm constructional details. In the present work we compare measured data to expectations based on published fabrication details and find a strong dependency on source construction and Pu self-absorption.