Kinetic Modeling of the Lif:Mg,Ti TL System including Defect Creation: Implications to, and Development of Track Structure Theory Calculations of Heavy Charged Particle Radiation Effects

In this research, various kinetic models were developed for LiF:Mg,Ti crystals, both in the irradiation and recombination stages. The models were later used to improve on track structure theory, which attempts to describe radiation effects of Heavy charged particle. To achieve this goal, the research focused on three main areas of endeavor. 1. In the first experimental measurements of optical absorption on LiF:Mg,Ti following low ionization density radiation (photons) and high ionization density protons and He ions were carried out in order to investigate the degree of applicability of track structure theory to the prediction of heavy charged particle induced effects of radiation. These measurements are described below. a) Photon induced optical absorption (OA) dose response was measured over an extended dose-range from 10 Gy to 105 Gy for the main OA bands in LiF:Mg,Ti, i.e., the 4.0 eV band (trapping center associated with glow peak 5 in the thermoluminescence glow curve), 4.77 eV band , 5.08 eV (F band) and 5.45 eV band. The extended dose-range allowed the unambiguous determination of linear/exponentially saturation behavior for all the OA bands. For the two main OA bands of interest at 4.0 eV and 5.08 eV, the dose filling factor was determined to be 5 ± 0.6.10-4 Gy-1 and 6.1 ± 0.4 × 10-5 Gy-1 respectively. The surprising, previously unexplained, linear/exponentially saturating dose response of the F band even though vacancies/F centers are being created by the radiation was explained in a kinetic analysis also described in the following. b) Heavy charged particle (HCP) optical absorption was carried out for 1.4 MeV protons and 4 MeV He ions at the SARAF, RARAF and BINA accelerators. Fluence response was measured over the extended range from 1010 cm-2 to 2.1014 cm-2. The low fluence region from 1010 cm-2 to 1011 cm-2 in the no-track-overlap regime allows a comparison of the experimental measurements and the track structure theory (TST) evaluations of the relative OA band HCP induced efficiencies as described in the following. The high fluence region has allowed the determination of the saturation concentration of the F band and 4.77 eV band using the Beer-Lambert and Smakula formulas. The results indicate order of magnitude enhanced concentrations of vii these centers following the proton and He irradiations relative to photon irradiation. 2. Kinetic Analysis: The second focus of investigation was the development of a kinetic model to describe charge carrier transport in the LiF:Mg,Ti system including three features delineated in the following. The primary motivation for the kinetic analysis was the intention to modify track structure theory by taking into account enhanced vacancy/F center creation in the low ionization density kinetic simulations but was expanded in order to simulate the unique features of dose response in LiF:Mg,Ti. The kinetic model includes: c) Estimated electron-hole (e-h) and e-only population of the spatially correlated trapping center/luminescent center responsible for composite glow peak 5 in the glow curve of LiF:Mg,Ti. d) Combined localized and delocalized recombination of the e-h and e-only centers in the recombination stage. These features are shown to be capable of simulating both the linear/supralinear dose response and the dependence of the supralinearity on photon energy as is observed for composite peak 5. Both of these characteristics have previously eluded the predictive powers of kinetic theory based exclusively on delocalized recombination. e) Vacancy/F center creation in the irradiation stage including vacancyinterstitial recombination. The kinetic model with the latter mechanisms attempts to resolve a central question concerning the mechanisms leading to the linear/exponentially saturating dose response of the F band even though Fluorine vacancies are being continuously created during the irradiation. The electron-trapping characteristics of the created vacancies are assumed to differ somewhat from the vacancies originally present in un-irradiated samples due to differences in their immediate environment. The kinetic model accurately simulates the experimentally observed F center dose response over the entire investigated dose range of 10 Gy -105 Gy under the following conditions: (i) The concentration of vacancies initially present is unexpectedly high at ~ 1023 m-3, possibly due to the highly doped, noncrystalline and hot-pressed nature of the LiF:Mg,Ti samples. (ii) The transition probability, An4o, for electron capture into the initially-present vacancies is ~ 40 times greater than An4, the transition probability for the viii radiation-created vacancies. These two factors marginalize the effect of the created vacancies at low dose resulting in a linear dose response. 3. Track structure theory: The third focus of investigation concerned the ability of TST to accurately calculate HCP induced OA Relative HCP OA efficiencies. Values of ηTST based on LID dose response, were compared with experimentally measured relative efficiencies, ηm, at no-track-overlap fluence levels of 1010-1011 cm-2 for protons and He particles. The F band values of ηm/ηTST are 2.0 and 2.6 for the He ions and protons respectively. The deviation from unity is explained as due to the neglect of enhanced vacancy/F center creation in the conventional TST calculations which ignore defect creation. It is demonstrated that kinetic analysis simulating LID dose response with enhanced vacancy creation, and incorporated into the TST calculation, can lead to values of ηm=ηTST for the F band. At the other extreme, for the 4.0 eV band, the values of ηm/ηTST are much less than unity, equal to 0.18 for the protons and < 0.12 for the He ions. These very low values suggest that the 4.0 eV trapping structure is being either destroyed or de-populated (perhaps by local heating/thermal spike/Coulomb explosion) during the HCP slowing down. Processes which do not occur (or are greatly reduced) during LID irradiation. In any case, the large deviations of ηm/ηTST from unity for both the F band and the 4.0 eV band demonstrate that conventional TST, which attempts to predict HCP induced radiation effects from the exclusive action of the released secondary electrons, is woefully inadequate. On a more positive note it was demonstrated that inclusion of defect creation in the LID kinetic analysis could lead to agreement between calculated and experimentally measured HCP induced F band OA.