Probing the defect nanostructure of helium and proton tracks in LiF:Mg,Ti using optical absorption: Implications to track structure theory calculations of heavy charged particle relative efficiency

Abstract

A major objective of track structure theory (TST) is the calculation of heavy charged particle (HCP) induced effects. Previous calculations have been based exclusively on the radiation action/dose response of the released secondary electrons during the HCP slowing down. The validity of this presumption is investigated herein using optical absorption (OA) measurements on LiF:Mg,Ti (TLD-100) samples following irradiation with 1.4MeV protons and 4MeV He ions at levels of fluence from 1010cm−2 to 2×1014cm−2. The major bands in the OA spectrum are the 5.08eV (F band), 4.77eV, 5.45eV and the 4.0eV band (associated with the trapping structure leading to composite peak 5 in the thermoluminescence (TL) glow curve). The maximum intensity of composite peak 5 occurs at a temperature of ∼200°C in the glow curve and is the glow peak used for most dosimetric applications. The TST calculations use experimentally measured OA dose response following low ionization density (LID) 60Co photon irradiation over the dose-range 10–105Gy for the simulation of the radiation action of the HCP induced secondary electron spectrum. Following proton and He irradiation the saturation levels of concentration for the F band and the 4.77eV band are approximately one order of magnitude greater than following LID irradiation indicating enhanced HCP creation of the relevant defects. Relative HCP OA efficiencies, ηHCP, are calculated by TST and are compared with experimentally measured values, ηm, at levels of fluence from 1010cm−2 to 1011cm−2 where the response is linear due to negligible track overlap. For the F band, values of ηm/ηHCP=2.0 and 2.6 for the He ions and protons respectively arise from the neglect of enhanced Fluorine vacancy/F center creation by the HCPs in the TST calculations. It is demonstrated that kinetic analysis simulating LID F band dose response with enhanced Fluorine vacancy creation, and incorporated into the TST calculation, can lead to values of ηm=ηHCP. On the other hand, the values of ηm/ηHCP for the 4.0eV band are much less than unity at 0.18 for the protons and <0.12 for the He ions. These very low values suggest that the 4.0eV trapping structure is either destroyed or de-populated, perhaps by local heating/thermal spike/Coulomb explosion, during the HCP slowing down. These HCP induced processes are believed to be absent or greatly reduced during LID irradiation. The large deviations of ηm/ηHCP from unity for both the F band and especially the 4.0eV 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.