Further consideration of the track−interaction model for thermoluminescence in LiF(TLD−100)

Abstract

At the Second International Conference on Luminescence Dosimetry in Gatlinburg in 1968 Claffy, Klick, and Attix introduced the track−interaction model in an attempt to explain the phenomena of supralinearity and sensitization generally observed in the thermoluminescence of LiF(TLD−100). Their proposal in general terms was that the electron−hole traps are located near the paths of the energetic charged particles (primary or secondary) which traversed the crystal during ionizing irradiation, and that this nonrandom spatial distribution affects the thermoluminescence process both with respect to its dependence upon dose and upon linear energy transfer. Moreover the observed sensitization of the phosphor, after receiving a large radiation dose followed by a partial annealing, was attributed to the survival of luminescence centers. In the present work a very simplistic model has been adopted for purposes of estimating how the average distance between luminescence centers along a charged−particle track compares with the average distance between tracks. Magnesium centers, occurring at ?68−Å intervals, are assumed to provide trapping sites both for electrons and holes. The luminescence centers are taken to be F centers associated in some way with the titanium centers which occur at ?127−Å intervals. For the Compton secondary electrons resulting from ⁶⁰Co γ−ray interactions, the average separation of ionizing events along the track is ?350 Å, and the average separation of F centers is ?2300 Å, resulting from about every seventh ionizing event. The average separation of tracks for a γ−ray exposure of 10³ R is ?660 Å, which is roughly the same as the average separation of trapped charge carriers along a given track from their nearest F center in the same track. This is the same exposure level at which supralinearity of TL response vs exposure begins to appear for ⁶⁰Co γ irradiation. The more densely ionizing tracks of heavy charged particles are shown to be much more widely separated relative to the luminescence center intervals in each track; thus supralinearity should not be expected in that case, in agreement with observations. An important feature of this paper is the estimation of an energy budget for LiF(TLD−100), beginning with the deposition of ⁶⁰Co γ−ray dose in the crystal, and terminating in the emission of 0.04% of that energy as thermoluminescence. Reasonable avenues are suggested by which the energy losses may occur, and estimates are made of the energy fraction in each category, based on available evidence.