Internal α activity: localisation, compositional associations and effects on OSL signals in quartz approaching β saturation

Abstract

Luminescence signals from hydrofluoric acid etched grains of quartz from Mozambican dunes were investigated in terms of elemental impurities, structural defects, and their relationship to internal α activity, to examine the potential for this to cause differences in signal levels obtainable from natural and laboratory irradiated samples. Optical and scanning electron microscopy (SEM), X-ray fluorescence (XRF) and instrumental neutron activation analysis (INAA) indicated the presence of various types of quartz and mineralogical inclusions. A spatial association of Th and U with Fe in structural defects was observed. Fe concentrations and inclusion sizes indicated that internal α dose rate would affect the defects that contained these impurities, but would be insignificant to the bulk quartz. A broad range of optically sensitive thermoluminescence (TL) peaks were observed from this material, and indicated a preheating regime of 260 °C for 30 s to minimise effects of the observed inclusions and defects on absorbed dose determinations by optically stimulated luminescence (OSL). Growth in OSL with dose from etched coarse grains preheated in this way approached saturation by 332 Gy of β irradiation ( 90Sr/ 90Y) and by 4 kGy of α irradiation ( E ≈ 3.5 MeV: approximate natural soil spectrum average, using converted dose rate from 241Am). This indicated α-efficiency ( k eff) at saturation of less than 0.08. However, the OSL decay curves contained a small ‘medium’ component. Structural defects introduced by milling the grains produced a larger ‘medium’ component with a similar decay rate under optical stimulation, which exhibited high saturation doses (>32 kGyα, >8.4 kGyβ) and α-efficiency ( k eff = 0.34). Maximum dose normalised OSL signals from the milled material greatly exceeded those obtainable from the whole etched grains. It is inferred that the presence of structural defects within a quartz crystal, similar to those produced by milling, can produce OSL components that grow to signal levels and doses exceeding those from the crystalline quartz. In the present study such defects were found to host α-emitting impurities. High localised dose rates in nature could enhance signals from these locations without affecting the bulk crystal, producing high natural signal levels that may be difficult to reproduce in the laboratory. Association of internal α activity with Fe indicates potential for removal of the most α affected grains by magnetic separation. With or without associated α activity, signals from such defects may contribute to observed variability in the dose response of quartz OSL.