Analysis of Pu-Am spectra in a Xe proportional counter: effect of characteristic X-rays.

Abstract

In developing a method for the evaluation of in vivo plutonium and americium in the absence of a priori knowledge of the Pu-to-Am ratio, xenon-filled proportional counters were employed. Analysis of Pu-Am spectra indicated that many counts observed in the plutonium and americium L X-ray region were actually due to absorption by the detector of higher-energy photons followed by the escape of xenon characteristic X-rays. The higher–energy photons originated mainly in americium (Compton-scattered 59.6 KeV gammas), and to a smaller degree in plutonium. Quantitative assessment of in vivo Pu-Am thus requires correction of the count-rate in the L X-ray region for these contributions from higher-energy photons. This correction is particularly important in cases where the contaminant is located deep within the body, since the L X-rays are attenuated faster than the higher-energy photons. The relative importance of this correction thus depends upon the Pu-to-Am ratio, the thickness and type of absorber, and the particular detector used. In practice, an approximate correction factor for each case has been obtained from spectra in the 35–55 KeV region obtained simultaneously with the corresponding spectra in the L X-ray region. The contribution of these higher-energy photons to count rates observed in the L X-ray region severely limits the practical usefulness of xenon-filled proportional counters for quantitative evaluation of plutonium and americium body burdens.