Mysteries of LiF TLD response following high ionization density irradiation: Glow curve shapes, dose response, the unified interaction model and modified track structure theory
Three outstanding effects of ionization density on the thermoluminescence (TL) mechanisms giving rise to the glow peaks of LiF:Mg,Ti (TLD-100) are currently under investigation: (i) the dependence of the heavy charged particle (HCP) relative efficiency on ionization density and the effectiveness of its modeling by track structure theory (TST) (ii) the behavior of the TL efficiency, f(D), as a function of photon energy and dose and (iii) the shape of composite peak 5 in the glow curve for various HCP types and energies and following high dose electron irradiation. It is concluded that (i) The predictions of TST are very strongly dependent on the choice of photon energy used in the determination of f(D), (ii) Modified TST employing calculated values of f(D) at 2 keV is in agreement with 5 MeV alpha particle experimental results for composite peak 5 but underestimates the 1.5 MeV proton relative efficiencies. Both the proton and alpha particle relative TL efficiencies of the high temperature TL (HTTL) peaks 7 and 8 are underestimated by an order of magnitude suggesting that the HTTL efficiencies are affected by other factors in addition to radial electron dose. (iii) The dose response supralinearity of peaks 7 and 8 change rapidly with photon energy: this behavior is explained in the framework of the Unified Interaction Model as due to a very strong dependence on photon energy of the relative intensity of localized recombination, (iv) The increased width and decrease in Tmax of composite peak 5 as a function of ionization density is due to the greater relative intensity of peak 5a (a low temperature component of peak 5 arising from two-energy-transfer events which leads to localized recombination).