Optically stimulated luminescence techniques in retrospective dosimetry

Optically stimulated luminescence (OSL) is observed during the stimulation of minerals that have been previously exposed to ionising radiation. Dosimetry techniques based on this phenomenon, and using widespread naturally occurring minerals such as quartz and feldspar, are now of major importance in a variety of environmental sciences (e.g. dating in geology and archaeology) and human dosimetry applications. The latter includes reconstruction of radiation doses following accidents involving radioactivity. The Risoe laboratories have been active in all these areas. Initial OSL systems used an argon ion laser (514 nm) for stimulation. Since then, we have developed several inexpensive alternative light sources, including broad-band (420 to 550 nm, about 25 mW/cm2 ) light filtered from a halogen or xenon lamp source, and intense blue (470±30 nm, approx. 30 mW/cm2 ) light-emitting diodes. The exponentially increasing effectiveness of the stimulating light as wavelength is decreased makes these sources effectively more efficient than a large, longer wavelength, laser installation. A solid state infrared laser (850 nm, 500 mW/cm2 ) is also employed to take advantage of an IR resonance specific to feldspars. The development of single aliquot measurement protocols has enabled the accurate and precise measurement of the very small doses (a few tens of mGy). These techniques allow all the measurements needed to estimate the dose to be made on a single subsample, or even a single grain. This has made possible the recent development of an automatic system for the measurement of luminescence signals from single sand-sized grains, using a steerable laser spot, from a compact variable power solid-state laser (0 to 10 mW/512 nm, approx. 20 μm spot size). The single-grain system has a measurement capacity more than two orders of magnitude higher than a conventional system, without compromising on sensitivity or reproducibility. Such capacity is needed to identify reliably the small fraction of grains in unheated materials that were fully reset by daylight exposure prior to receiving the dose of interest. Such materials include water-lain sediments, soil surfaces, mortar and concrete, which are all important in geological dating and retrospective accident dosimetry. (author)