A microdosimetric track interaction model applied to alpha-particle-induced supralinearity and linearity in thermoluminescent LiF:Mg, Ti

A microdosimetric track interaction model for heavy charged particles has been developed which is capable of quantitatively predicting the experimentally observed supralinearity of the alpha particle TL dose response of peak 8 in LiF:Mg, Ti (TLD-100, Harshaw/Filtrol) as well as the linear behaviour of the lower temperature glow peaks. The linear behaviour at low dose is due to lack of track interaction arising from the highly localised nature of the alpha particle dose deposition profile (98% of the dose is deposited within 200 AA of the track axis). Track dose overlap in the radiation absorption stage is shown to be incapable of significantly increasing the thermoluminescence efficiency of intersecting tracks. The alpha-induced supralinearity of peak 8 can, therefore, be explained only via greatly increased charge carrier migration lengths in the glow curve heating stage which brings about significant nearest-neighbour track interactions above a fluence of approximately 10⁸ particles cm^-2 (approximately 10 Gy in LiF). The increase in TL efficiency arises from the increased population of luminescence recombination centres available to the migrating charge carriers released from the TL trapping centres. The linear behaviour of the lower temperature peaks (up to a fluence of approximately 10¹⁰ particles cm^-2 followed by exponential saturation) yields a charge carrier average migration distance of approximately 250AA (cf 5000 AA for peak 8) which implies that at low sample temperatures there is little inter-track migration of charge carriers in the luminescence recombination stage.