Improving the accuracy and precision of equivalent doses determined using the optically stimulated luminescence signal from single grains of quartz

Optically stimulated luminescence (OSL) measurements of quartz are widely used to measure equivalent dose (D_e). At radiation doses above ～100 Gy, saturation of traps results in a decrease in the rate of growth of the OSL signal, and this makes calculation of D_e increasingly difficult. A series of dose recovery experiments was undertaken using single grains of quartz from Kalambo Falls, Zambia to explore saturation of single grains. When the OSL signal from many grains is averaged, the characteristic dose (D₀) is 47 Gy, typical of published values for quartz. However, D₀ for individual grains varies from ～10 to 100 Gy. Doses up to two times the average D₀ could be accurately recovered, but above this dose the D_e became increasingly underestimated. Overdispersion for this type of experiment should be zero, but was observed in all data sets; furthermore the value of overdispersion increased with D_e. An additional acceptance criterion, the Fast Ratio, is suggested for single grain OSL analysis. This criterion assesses the relative contribution of the fast component of the OSL signal. Including this as an additional acceptance criterion leads to an improved precision, with overdispersion reduced to zero, and improved accuracy in dose recovery at high doses.