Heavy charged particle thermoluminescence dosimetry: Track structure theory and experiments

The factors that influence the thermoluminescence (TL) yield of "tissue-equivalent" TL dosimeters (TLDs) following heavy charged particle (HCP) and neutron irradiation are evaluated. The relative TL response, η, the dose-TL response, f(D), and the relative glow peak intensities (in LiF) appear to be significantly dependent on the HCP charge, mass and energy and not only on the LET_∞ as is widely believed. Previous studies, which have established the batch dependence of η following 81-meV neutron irradiation and 4-MeV alpha-particle irradiation, have indicated the importance of material characteristics (impurity and defect composition). This finding is now supported by additional studies illustrating a further dependence of the relative TL properties on details of the high-temperature annealing procedure. These HCP-TL properties can be described in the framework of a modified track structure theory (TST) via their relationship to low energy electron TL response. The major premise of TST is that the concentration of liberated charge carriers around the path of the HCP is the only parameter that governs the dependence of the relative TL properties on the type of HCP radiation. In conventional TST the bulk response of the system following γ-irradiation (usually ⁶⁰Co or ¹³⁷Cs) is folded into the radial distribution of absorbed dose D(r, HCP, E) around the axis of the HCP track to determine the relative TL response. This approach has achieved considerable success in HCP radiation yield calculations but the choice of high energy γ-rays or electrons is particularly unfortunate in TL yield calculations because of the well-known dependence of f(D) on electron energy. This latter dependence therefore requires that the test electron spectrum be matched as closely as possible to the initial energy spectrum of the electrons ejected during the HCP slowing down E_max≅ a few keV for ∼ MeV/a.m.u. HCP). Further refinements incorporated into our modified TST approach are careful matching of all the relevant experimental parameters in the measurement of η and f(D) and simulation of the density of "low-energy" charge carriers around the HCP path in "tissue-equivalent" dosimeters using published experimental data for "tissue-equivalent" gas rather than the use of approximate analytic calculations. Our experimental measurements of η have encompassed a variety of radiation fields (alpha particles, meV neutrons, fission fragments, etc.), and f(D) has been measured for X-rays, electrons of various energies and ⁶⁰Co γ-rays. The overall agreement between theory and experiment is excellent, but considering our inadequate understanding of electron-induced TL it would be premature to conclude that TST is the whole truth underlying HCP-induced TL. TST does provide a convincing and convenient framework to understand HCP- and the neutron-induced TL.