MODELING ACCUMULATION IN BETA AND GAMMA DOSE RATE FOR VERY THIN LAYERS: IMPLICATIONS FOR LUMINESCENCE DATING OF DUST DEPOSITS

Optically stimulated luminescence (OSL) dating has proven to be one of the most prominent methods in determining absolute age of sediments. For age calculations two parameters are needed, the equivalent dose and the dose rate. The dose rate is either measured directly (in the field) or indirectly (by means of decay counting or radioelement concentration methods). In the indirect method dose rate is usually derived by applying conversion factors assuming a homogeneous environment (infinite matrix dose rate). Nonetheless, we cannot keep this assumption of homogeneity in complex cases. DosiVox software is a powerful tool for dose rate simulations, based on particle-matter interactions, that allows the definition of a three dimensional space in which the radioactive elements and material composition can be selected.

We utilized DosiVox to model accumulative beta and gamma dose rates in attic dust collected from a building in Galena, KS, with thickness of 5.3 mm deposited on timber floorboards and surrounded by air from the top (inert layers). Fourteen components of the dust layer were chosen for simulation in DosiVox based on ICP -MS/-OES and XRD analysis data. Here we explore how accumulation of dust over time alters beta and gamma dose rate by modifying dust layer thickness. We divided the dust layer into 4 sublayers, each 1.325 mm thick. The dose rate was simulated by adding each sublayer in turn, and each spectrum (U,Th,K) separately (assuming that the chemical composition of dust had remained unchanged in time). Model results indicate there is a gradual increase in both beta and gamma dose rate with accretion of dust, while the rate of this accumulation is not the same in both cases. Depending on the rate of dust deposition, this result affects the equivalent dose distribution; the slower the dust is deposited the further the equivalent dose distribution broadens.