Report on the development of a low background proportional counter for monitoring plutonium in the lung

Techniques developed for use in X-ray astronomy detectors to reduce the internal background in proportional counters for the energy range 1–20 keV have been applied in the construction of a prototype low background proportional counter for monitoring plutonium in the lung. The most commonly used device at present is the phoswich which is mainly limited in its precision by the background count rate induced by the normal radioactivity of the subject being measured. A properly constructed proportional counter using the latest background rejection techniques should offer better rejection of these background events and improve the precision of the measurement. The design of such a counter is described together with preliminary results from the evaluation of a prototype. The limiting performance is shown to be due to radioactive contamination (tritium) of the counter and reduction of this to a low level gives a background lower than that of the phoswich, though not under operational conditions. Future improvements are described.