On the internal radioactivity in quartz

Abstract

We report on our continuous investigations on the significance of the internal dose rate in the optical dating of sedimentary quartz. In the present study, we applied instrumental neutron activation analysis (INAA) to determine the U and Th concentrations in quartz fractions extracted from three coversand samples from the southern Netherlands. The quartz extracts were obtained from the bulk material using sample preparation procedures, which are routinely employed in any luminescence dating laboratory. These sample preparation procedures do not necessarily yield separates, which are exclusively composed of quartz, and it is demonstrated that coarser particle size fractions result in quartz extracts of a higher purity. After manual extraction of the non-quartz grains, INAA analysis of both these accessory minerals (such as zircon, rutile and tourmaline) and the resulting purified quartz separates shows that the majority of the radioactivity is located outside of the grains. The overall average U and Th concentrations ( ± 1 standard error) obtained for the manually purified coarse quartz extracts are 0.08 ± 0.02 and 0.18 ± 0.03 mg kg - 1 , respectively. Owing to the difficulty of obtaining an absolutely pure quartz extract, these U and Th concentrations are interpreted as upper limits to the internal radionuclide concentration. Assuming an a -value of 0.04, the corresponding upper limit to the internal alpha dose rate in our quartz is estimated at 0.010 ± 0.002 Gy ka - 1 . The implications of our findings for luminescence dating are considered.