Alpha Particle Induced TL Supralinearity in TLD-100: Dependence on Vector Properties of the Radiation Field

Abstract

Abstract The linear/supralinear behaviour of the TL dose response in LiF:Mg,Ti (TLD-100) and its dependence on ionisation density is a fairly unique phenomenon which cannot be explained by conventional atomic 'conduction band/valence band' kinetic models. The Track Interaction Model (TIM) provides the microscopic framework which, when coupled with other appropriate physical mechanisms (spatial localisation of traps and recombination centres, competing centres, variations in the capture cross sections with temperature, etc.) can be used to describe all the dominant features of the TL supralinearity of LiF:Mg,Ti and similar TL systems. The unique feature of the TIM applied to alpha particles is that it is an integral approach with only one free parameter, the average charge carrier migration distance in the luminescence recombination stage. Although the TIM provides a comprehensive description of the mechanisms underlying supralinearity in TLD-100, definitive unambiguous proof of its validity has been lacking. The present paper reports on the experimentally measured dependence of 4 MeV alpha particle induced supralinearity of glow peaks 5, 7, 8, and 9 on the vector properties of the radiation field. The onset of supralinearity is shifted to lower fluence levels by a factor 3-4 in 'near-isotropic' geometry compared to 'near-parallel' irradiation geometry. No other experimental or radiation field parameter is changed. The only reasonable explanation is that the track interaction is enhanced in 'near-isotropic' geometry due to the approximately 50:1 aspect ratio of the alpha particle track. These results are considered 'proof-positive' or undeniable proof of the dominant importance of track interaction in TLD-100 supralinearity.