Abstract

The fission track technique is prosperously used for determination of low-levels of 239Pu in, for example, biological and environmental samples. The lower detection limit is a few micro becquerel, comparable to the detection limit of mass spectrometers, and more sensitive than alpha spectrometry with semiconductor detectors. The fission track analysis includes a thorough chemical removal of uranium from the sample, using ion chromatography or liquid-liquid extraction before neutron irradiation since uranium, present in the sample or in chemicals used, will contribute to an increased background level of fission fragments. This is due to the high cross section of fission from thermal neutrons of 235U. The last steps in the analytical procedure are critical, in view of the background level, and have to be performed with ultra-pure acids in a dust-free environment. The chemical separations result in an isolation of plutonium in a 0.5 ml volume which is deposited on a quartz disc and irradiated with thermal neutrons in a nuclear reactor. The impinging fission fragments will create tracks in the quartz which are enlarged after etching in hydrofluoric acid and visualized in an optical microscope. The net number of tracks is a linear function of the 239Pu content.

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