Letters to the Editor

Abstract

Sir, Comments on ‘Observation of enhanced efficiency in the excitation of ion-induced LiF:Mg,Ti thermoluminescent peaks’ G. Massillon., I. Gamboa-deBuen and M.E. Brandan, J. Appl. Phys. 100, 103521–1 (2006). In the above paper by Massillon et al., the authors have described experimental measurements of the heavy-ion relative TL efficiency of the high-temperature peaks in LiF:Mg,Ti (TLD-100) and, as well, their interpretation of the data in the framework of Track Structure Theory (TST)(1) and Modified Track Structure Theory (MTST)(2). In order to allow the reader an appreciation of the errors and conceptual failings involved in the efforts of Massillon et al. to interpret/analyse their data, a brief review of TST and MTST is required as well as a short description of what is commonly referred to as ‘high-temperature thermoluminescence’ or HTTL in LiF:Mg,Ti (TLD-100). The basic premise in the calculation of heavy charged particle (HCP) radiation effects in both TST and MTST is that the radiation effects of HCPs are due to the average dose delivered by the secondary electrons created by the HCP slowing down; and can therefore be predicted by the experimentally measured gamma-ray/electron dose–response folded into the radial dose distribution of the HCP. Radial dose profiles explicitly neglect the stochastic processes of the secondary electrons liberated by the HCP which, of course, are considered to be important in the microdosimetric treatment of radiobiological systems. In TL, these stochastic processes can be neglected since the TL signal is obtained or is measurable from the HCP interaction only at high levels of fluence, i.e. >105 cm−2 where the effects of stochastic processes which might arise in single tracks are completely averaged out. Of course, in solid-state systems, the radial dose distribution cannot be directly measured experimentally as in gaseous systems. These, therefore, have been calculated analytically(3) or obtained by track segment Monte Carlo calculations(4). In general, for low-energy proton and helium ions in the energy range 1–10 MeV, the calculated dose values start at MGy levels near the track axis, with a dependence on radial distance decreasing approximately as r−2 and reaching ∼Gy dose levels at distances of several hundreds to a few thousand Å from the track axis. More recently, full track radial dose profiles have been calculated for low-energy proton and helium ions using the Monte Carlo code FLUKA in nanometer-sized volumes(5). At these low proton and helium energies, the ions are stopped in the TL sample so that the nanometric FLUKA full track calculations describe the HCP slowing down and stopping process in a manner far more resembling the experimental situation than track segment calculations, and this particular point will be touched on later in this comment.