Beta dosimetry of potassium feldspars in sediment extracts using imaging microprobe analysis and beta counting

• Autorzy: Godfrey-Smith D. I. , Scallion P. , Clarke M. L.
• Języki publikacji: EN

Abstrakty

EN

We provide a comparison of two independent methods used to determine the K concentration of potassium feldspar extracts from sand-sized lacustrine, colluvial, and aeolian sediments. The two methods being compared are gas-flow beta counting and imaging microprobe X-ray fluorescence analysis. Imaging analysis demonstrated that the proportion of potassium feldspar grains in a K-feldspar extract varied from a low of 7% to a high of 84%. All extracts included a significant proportion of quartz, and some also contained a few plagioclase feldspar grains. However, the K2O concentration in individual potassium feldspar grains of all eight extracts examined was within the range of 15.5 š 0.7 % K2O by weight. The K2O concentrations in four of five extracts, measured using gas-flow beta counting, were significantly lower than this value. This reflects the dilution effect of significant amounts of non-K bearing grains in these extracts. This difference can result in up to an 80% underestimate in the internal beta dose rate of a potassium feldspar grain. A 14% underestimate in the total dose rate to potassium feldspar grains, and therefore a 14% overestimate in sediment ages determined by luminescence dating of nominally pure potassium feldspar extracts is thus possible. We suggest that gas-flow beta counting is suitable for the determination of K concentrations of bulk sediments and therefore the external beta dose rate, but that internal beta dosimetry is best performed by imaging microprobe X-ray analysis. An alternative, inexpensive approach would be to assume a 15.5 š 0.7 % K2O concentration for all nominal K-feldspar extracts separated using a heavy liquid with specific gravity of 2.58 gźcm-3, and use this value to determine the internal beta dose rate. Internal beta dose rate errors incurred with this approach are expected to be significantly less than those incurred by the small sample gas-flow beta counting method.

Słowa kluczowe

EN

dosimetry; gas flow beta counting; microprobe; feldspar; potassium; plagioclase; image analysis; electron backscatter; luminescence dating

• Wydawca: De Gruyter
• Czasopismo: Geochronometria
• Rocznik: 2005
• Tom: Vol. 24
• Strony: 7--12
• Opis fizyczny: Bibliogr. 10 poz., rys., tab.

Twórcy

autor

Godfrey-Smith D. I.

• Dept. of Earth Sciences, Dalhousie University, Halifax, N.S., B3H 4J1, Canada
• Defence R&D Canada – Ottawa, 3701 Carling Ave., Ottawa, ON, K1A 0Z4, Canada

autor

Scallion P.

• Dept. of Earth Sciences, Dalhousie University, Halifax, N.S., B3H 4J1, Canada

autor

Clarke M. L.

• School of Geography, University of Nottingham, University Park, Nottingham, NG7 2RD, UK

Bibliografia

• 1. Balescu S. and Lamothe M., 1994: Comparison of TL and IRSL age estimates of feldspar coarse grains from waterlain sediments. Quaternary Geochronology (Quaternary Science Reviews) 13: 437-444.
• 2. Bøtter-Jensen L. and Mejdahl V., 1985: Determination of potassium in feldspars by beta counting using a GM multicounter system. Nuclear Tracks and Radiation Measurements 10: 663-666.
• 3. Bøtter-Jensen L. and Mejdahl V., 1988: Assessment of beta dose rate using a GM multicounter system. Nuclear Tracks and Radiation Measurements 14: 187-191.
• 4. Clarke M.L., 1994: Infra-red stimulated luminescence ages from aeolian and alluvial fan deposits from the Eastern Mojave Desert, California. Quaternary Geochonology (Quaternary Science Reviews) 13: 533-538.
• 5. Clarke M.L., Richardson C.A. and Rendell H.M, 1995a: Luminescence dating of sands from the Mojave Desert. Quaternary Science Reviews 14: 783 - 789.
• 6. Clarke M.L., Wintle A.G. and Lancaster N., 1995b: Infra-red stimulated luminescence dating of sands from the Cronese Basins, Mojave Desert. Geomorphology 17: 199-206.
• 7. Dütsch C. and Krbetschek M.R., 1997: New methods for a better internal 40K dose rate determination. Radiation Measurements 27: 377-381.
• 8. Edwards S.R., 1993: Luminescence dating of sand from the Kelso Dunes, California. In: Pye K., ed, The Dynamics and Environmental Context of Aeolian Sedimentary Systems, Geological Society of London Special Publication 72: 59-68.
• 9. Godfrey-Smith D.I., Scallion P. and Clarke M.L., 1996: Beta dosimetry of potassium feldspar extracts using imaging microprobe analysis and small sample gas flow beta counting. Atlantic Geology 32(1):72.
• 10. Huntley D.J. and Baril M.R., 1997: The K content of the K-feldspars being measured in optical dating or thermoluminescence dating. Ancient TL 15: 11-13.