Measurement of radioactivity in mount st. helens volcanic ash by x/γ ray spectrometry

A Si(Li)-NaI(Tl) x/γ-ray spectrometer is used for assaying radionuclides in the ash from the Mount St. Helens eruption of May 18, 1980. Spectra of the ash are compared with those of local soil obtained before the eruption. The activity in the ash from the 238_U and 232_Th decay chains is 40% of that in soil. The relative 238_U activity is 50%, while the 232_Th is relatively depleted, its activity being only 25% of that in the soil. The Si(Li) detector is mounted on edge to permit counting two samples simultaneously and is sandwiched together with the samples between two large NaI(Tl) scintillators. The Si(Li) detector is normally operated in anticoincidence with the scintillators to suppress the γ-ray background thereby Increasing the sensitivity. However in this study the NaI(Tl) detectors are used in coincidence to identify the Th x rays having coincident γ rays characteristic to the 232_Th decay chain. This is a novel alternate way of using the scintillators. These results demonstrate the usefulness of this spectrometer for measuring activity levels below that of natural soil.