Measurement of Uranium Series Radionuclides in Rock and Groundwater at the Koongarra ore Deposit, Australia, by Gamma Sp
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MEASUREMENT OF URANIUM SERIES RADIONUCLIDES IN ROCK AND GROUNDWATER AT THE KOONGARRA ORE DEPOSIT, AUSTRALIA, BY GAMMA SPECTROMETRY NOBUYUKI YANASE AND KEIICHI SEKINE Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki 319-11, JAPAN ABSTRACT Gamma spectrometry without any self-absorption correction was developed to measure low energy gamma rays emitted by uranium and actinium series radionuclides in rock samples and groundwater residues collected at the Koongarra ore deposit, Australia. Thin samples were prepared to minimize the self-absorption by uranium in the samples. The present method gave standard deviations of 0.9 to 18% for the measurements of concentrations of uranium and actinium series radionuclides. The concentrations of 2 38U, 2 30Th and 23 5U measured by gamma spectrometry were compared with those by alpha spectrometry that requires a complicated chemical separation procedure. The results obtained by both methods were in fairly good agreement, and it was found that the gamma spectrometry is applicable to rock and groundwater samples having uranium contents up to 8.1% (103 Bq/g) and 3 Bq/l of 238U, respectively. The detection limits were calculated to be of the order of 10-2 Bq/g for rock samples and 10-' Bq/l for groundwater samples. The concentrations of uranium and actinium series radionuclides can be determined precisely in these samples using gamma spectrometry without any self-absorption correction. INTRODUCTION Natural analogue studies using uranium series radionuclides are one of the useful ways for predicting the long-term migration behaviour of actinides in the geosphere. In such studies, the distributions of uranium series radionuclides in rock and groundwater over the study site are essential for discussing the migration behaviour of these nuclides. For the measurement of uranium series nuclide concentrations, radiochemical analysis followed by alpha spectrometry is most commonly used 1. However, this method is time consuming and complicated. On the other hand, gamma spectrometry is a simple, non-destructive method and suitable for
accumulating an enormous amount of data for many radionuclides simultaneously 2 -6 . However, in most of the previous studies the gamma rays used have been limited to high intensity peaks such as 46.5 keV of 21OPb, 63.0 keV of 23 4Th, and peaks of 2 22Rn progeny. The application of gamma spectrometry to environmental samples is also restricted due to a worse detection limit than that alpha spectrometry. Regarding measurement of low energy gamma rays, the self-absorption of gamma rays by the sample matrix causes a fluctuation of counting efficiency even if the counting geometry is the same. Dickson and Snelling carried out a correction for self-absorption using the measured absorption coefficients of gamma rays for each sample matrix 7 . Monte Carlo calculations have also been used to correct for self-absorption, but an exact elemental composition of the sample was needed for the calculation 8 . Gamma spectrometry using these corrections is more
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