Current special nuclear material safeguards requirements preclude the disposal of high-efficiency particulate air (HEPA) filters which have been used in air exhaust systems for areas in which enriched uranium is utilized until the amount of U-235 contained in the filter has been determined. To this end, a technique has been developed for evaluating U-235 contained in used HEPA filters by selectively measuring the emission rate of the 0.185 Mev (54 percent) photon from U-235.
Counting of the 0.185 Mev photon is performed with a Nal (Tl) planar gamma defector, driving a portable, battery-operated pulse rate meter with single-channel pulse height analyzer and pulse integrator capabilities* Filters are placed in a fixture designed to assure consistent source-detector geometry, and subjected to an appropriate count on each face of the filter medium with the detector positioned at the center of the face at a distance of 2" from the surface of the medium. The U-235 contained in the filter is determined on the basis of the observed count rate on a filter face and the ratio of the count rates on the two faces.