32Si measurement using low-level scintillation spectrometry

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Morgenstern, U.; Ditchburn, R.G. 1997 32Si measurement using low-level scintillation spectrometry. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 97/35

Abstract: Cosmogenic 32Si (half-life ca. 140 years) can be applied to the study of environmental circulation processes in the time range of 50-1000 years. Its non-gaseous nature and fairly constant production rate are favourable to quantify its role in environmental processes, and make it potentially useful in studies of groundwater flow and mixing processes, glacier dynamics, soil erosion, and sedimentation in lakes and oceans. Detection of 32Si is difficult due to its very small natural concentration. Radiometric measurement of natural 32Si was done in a few laboratories by detecting the beta radiation. The difficulties of setting up such extremely low background systems have deterred wider use of 32Si. A radiometric measurement technique has been developed to measure natural 32 Si. Commercial Scintillation Spectrometers (1220 Quantulus) were successfully introduced for detection of the extremely low-level activities. Background, detection efficiency and stability are better than with other techniques. Improvements were also made in sampling, silica extraction and purification. Sampling 32Si from water involves now much less labour; two samples can be collected on one day by one person. Separation of the precipitate from the water, extraction of the silica from the iron precipitate, and purification of the silica are significantly simplified, resulting in a decrease of the involved labour by at least a factor of two. With this detection technique and the straightforward chemistry no detectable blank was observed. For 1 cu. m effective water sample, a detection limit of 0.1 mBq per cubic metre is reached. The 32Si concentration of the first measured sample, a young river-recharged groundwater from the Hutt Valley, is with 3.29 +/- 0.25 mBq per cubic metre within the range of Northern Hemisphere rainwaters at similar latitude, and indicates a similar global distribution pattern in the Southern Hemisphere. Furthermore, this result indicates that 32Si can be found in young river-recharged groundwater, and that 32 Si may be a tool for tracing groundwater flow processes in such aquifers systems. (auths)