Mysteries of LiF TLD response following high ionisation density irradiation: nanodosimetry and track structure theory, dose response and glow curve shapes

Abstract

Three outstanding effects of ionisation density on the thermoluminescence (TL) mechanisms giving rise to the glow peaks of LiF:Mg,Ti (TLD-100) are currently under investigation: (1) the dependence of the heavy charged particle (HCP) relative efficiency with increasing ionisation density and the effectiveness of its modelling by track structure theory (TST), (2) the behaviour of the TL efficiency, f(D), as a function of photon energy and dose. These studies are intended to promote the development of a firm theoretical basis for the evaluation of relative TL efficiencies to assist in their application in mixed radiation fields. And (3) the shape of composite peak 5 in the glow curve for various HCP types and energies and following high-dose electron irradiation, i.e. the ratio of the intensity of peak 5a to peak 5. Peak 5a is a low-temperature satellite of peak 5 arising from electron-hole capture in a spatially correlated trapping centre/luminescent centre (TC/LC) complex that has been suggested to possess a potential as a solid-state nanodosemeter due to the preferential electron/hole population of the TC/LC at high ionisation density. It is concluded that (1) the predictions of TST are very strongly dependent on the choice of photon energy used in the determination of f(D); (2) modified TST employing calculated values of f(D) at 2 keV is in agreement with 5-MeV alpha particle experimental results for composite peak 5 but underestimates the 1.5-MeV proton relative efficiencies. Both the proton and alpha particle relative TL efficiencies of the high-temperature TL (HTTL) peaks 7 and 8 are underestimated by an order of magnitude suggesting that the HTTL efficiencies are affected by other factors in addition to radial electron dose; (3) the dose-response supralinearity of peaks 7 and 8 change rapidly with photon energy: this behaviour is explained in the framework of the unified interaction model as due to a very strong dependence on photon energy of the relative intensity of localised recombination and (4) the increased width and decrease in T(max) of composite peak 5 as a function of ionisation density is due to the greater relative intensity of peak 5a (a low-temperature component of peak 5 arising from two-energy transfer events, which leads to localised recombination).

Alpha particle and proton relative thermoluminescence efficiencies in LiF:Mg,Cu,P:is track structure theory up to the task?

“…It was found that the re-population of peak 5 TCs via F band optical excitation results in a second readout alpha-particle relative TL efficiency 14 times greater than the first readout alpha-particle relative TL efficiency and beyond the ability of TST which predicts a value of 1.5. The second analysis (33) reaches the conclusion that for composite peak 5, the predictions of MTST using UNIM-calculated values of f(D) for 2 keV electrons are in agreement with 5-MeV alpha-particle relative TL efficiencies but underestimate 1.5-MeV relative proton efficiencies by 50 %. Moreover, both the proton Figure 10.…”

“…R max and r max are the maximum axial and radial distances of penetration of the charge carriers from the HCP path (in the radiation absorption stage); f d (D) is the TL dose-response measured with a test electron radiation chosen to mimic the radiation action of the HCP as closely as possible. Further information is available in other publications (4,14,15,33) . Due to the great variability of even relative quantities in TL characteristics, it is necessary to emphasise the importance of measuring f d (D) under experimental conditions and protocol identical to the experimental measurement of h HCP,g (34,35) .…”

“…When applied to low-energy protons and alpha particles ( 1 MeV amu 21 ), the maximum photon energy required to experimentally generate f (D) is 2 -3 keV (Figure 10). One approach (33) to estimate f (D), as mentioned above, has been to extrapolate the values of f (D) from higher photon/electron energies extending over the range of 8 keV to 1.25 MeV using the UNIM (4) . Such an extrapolation, of course, introduces a certain uncertainty in the conclusions regarding the possible failure of MTST to accurately predict the proton relative efficiencies of peak 5 as well as the relative efficiencies of the high temperature thermoluminescence.…”

Thermoluminescence solid-state nanodosimetry--the peak 5A/5 dosemeter

“…Ionisation density-dependent phenomena in the thermoluminescence (TL) characteristics of LiF:Mg,Ti (TLD-100) are well known (1,2) . The most intensively investigated include: (i) the linear/ supralinear dose response following photon/electron irradiation; (ii) the decrease of the dose response supralinearity (decreasing TL efficiency) with decreasing electron/photon energy below 100 keV, (iii) the decreasing heavy-charged particle (HCP) relative efficiency at high-ionisation density (HID) and (iv) the 'over-response' in the TL efficiency with decreasing X-ray energy appearing again below 100 keV.…”

“…The most intensively investigated include: (i) the linear/ supralinear dose response following photon/electron irradiation; (ii) the decrease of the dose response supralinearity (decreasing TL efficiency) with decreasing electron/photon energy below 100 keV, (iii) the decreasing heavy-charged particle (HCP) relative efficiency at high-ionisation density (HID) and (iv) the 'over-response' in the TL efficiency with decreasing X-ray energy appearing again below 100 keV. All of these phenomena have been successfully modelled using microdosimetric theory (1,2) in the framework of the Unified Interaction Model (UNIM), Track Structure Theory (TST) and microdosimetric 'target' theory, respectively. Recently, however, some reservations have been voiced concerning the basic premise of TST which ignores the effects of nuclear scattering on the relative HCP TL efficiency (3) .…”