RECENT DEVELOPMENTS IN COMPUTERISED ANALYSIS OF THERMOLUMINESCENCE GLOW CURVES: SOFTWARE CODES, MECHANISMS AND DOSIMETRIC APPLICATIONS.

The computerised deconvolution of thermoluminescence glow curves into component glow peaks is discussed in detail with special emphasis on advances of the subject post 2013. A plethora of computer codes have been developed using models based on first-order kinetics, second-orders kinetics, interactive traps and continuous distributions of activation energies. The glow curves of several materials are displayed and discussed along with new and improved dosimetric applications:precision effects of heating rate, heavy charged particles, mixed field α/ϒ dosimetry, fading and dose-response linearity. Finally recommendations are made for future efforts.

RELATIVE HCP THERMOLUMINESCENCE AND OPTICAL ABSORPTION EFFICIENCIES: THE DEMISE OF TRACK STRUCTURE THEORY.

The thermoluminescence relative efficiency, ηTST, of LiF:Mg,Ti and LiF:Mg,Cu,P following heavy charged particle irradiation is calculated using track structure theory and compared with experimental measurements. The calculations use both 60Co generated values of secondary electron dose response and values of the dose response at lower photon energies. In both materials there is poor agreement with experiment. Optical absorption relative efficiencies are also in disagreement. For the F band, ηexpt'l/ηTST = 2.0 and 2.6 for He ions and protons, respectively. The values of ηexpt'l/ηTST for the 4.0-eV band, resulted in 0.18 (protons) and <0.12 (He ions). An indication that the 4.0-eV trapping structure is either destroyed or de-populated during the heavy charged particle (HCP) slowing down. The large deviations of ηexpt'l/ηHCP from unity demonstrate that TST, which predicts HCP induced radiation effects from the exclusive action of the released secondary electrons, is woefully inadequate.

DOSE DEPENDENCE OF RADIATION INDUCED DAMAGE IN THE THERMOLUMINESCENT RESPONSE OF LIF:Mg,Ti (TLD-100).

The effect of previous irradiation on the sensitivity of the glow peaks of LiF:Mg,Ti (TLD-100) is investigated up to levels of dose of 400 Gy in both slow-cooled and naturally cooled materials following the 400°C/1 hour pre-irradiation anneal. It is demonstrated that the naturally cooled samples can be re-used up to accumulated levels of dose of 50 Gy without recalibration. At 400 Gy a significant decrease in sensitivity of approximately 25% is observed for all the glow peaks (excluding peak 3). In slow-cooled materials even 100 Gy does not alter the sensitivity of the material.

DEMONSTRATION OF THE POTENTIAL AND DIFFICULTIES OF COMBINED TL AND OSL MEASUREMENTS OF TLD-600 AND TLD-700 FOR THE DETERMINATION OF THE DOSE COMPONENTS IN COMPLEX NEUTRON-GAMMA RADIATION FIELDS.

The results reported herein demonstrate the potential application of combined optically stimulated luminescence/thermoluminescent (OSL/TL) measurements in neutron-gamma discrimination dosimetry. The advantages of OSL/TL are two-fold: (i) The OSL and TL readout can be carried out on the same sample and (ii) the greater efficiency of OSL to high ionization density radiation due to F 2 and F3 excitation. The gamma/electron calibration coefficients for LiF:Mg, Ti (TLD-600 and TLD-700) were measured using a 90Sr/90Y source calibrated at the SARAF-SSDL nuclear facility. The estimation of the neutron calibration coefficients was carried out by irradiation with broad-spectrum beam of fast neutrons with median energy 5 MeV at the Radiological Research Accelerator Facility (RARAF) of Columbia University. Naturally cooled samples of TLD-600 and TLD-700 were dosed to levels of 29.8 Gy neutrons and 6.1 Gy gammas in air and KERMA calculations employed to transfer the levels of dose to6,7LiF. A figure of merit for fast-neutron/gamma ray discrimination was determined at 10.6 for TLD-700 in the current measurements. The use of combined TLD-600/TLD-700 allowed, as well, the determination of a considerable and somewhat unexpected thermal neutron component of 116 Gy in TLD-600.

KINETIC SIMULATIONS OF THE THERMOLUMINESCENCE CHARACTERISTICS OF LIF:MG, TI INCORPORATING LOCALISED AND DELOCALISED RECOMBINATION.

Kinetic simulations of the thermoluminescence characteristics of LiF:Mg, Ti are reviewed in the framework of conduction band/valence band models. Delocalised recombination models have been mainly applied to the simulation of glow peak shapes, although comparison with experimental data has proven difficult if not impossible due to the scarcity of materials with demonstrably proven 'single-peak' glow curves. The delocalised models are incapable of the simulation of TL dose response linear/supralinear behaviour and the dependence of the supralinearity on particle energy. These characteristics require the incorporation of localised, nanodosimetric, recombination processes in the TL mechanisms. These investigations have simulated many of the TL characteristics of LiF:Mg, Ti in kinetic models based on a mixture of both delocalised and localised recombination.

MANIPULATION OF THE DOSE-RESPONSE OF COMPOSITE GLOW PEAK 5 IN THE THERMOLUMINESCENCE OF LiF:Mg,Ti (TLD-100) VIA OPTICAL EXCITATION POST-IRRADIATION: POTENTIAL FOR IMPROVED DOSE-RESPONSE LINEARITY BEYOND 1 Gy.

Many dosimetric applications and especially those involved in clinical dosimetry are hampered by the supralinearity of TLD-100 which begins at a level of dose of 1 Gy. This research investigates the effect of optical excitation following irradiation on the dose-response. It is expected that this will lead to a more linear dose-response, however, irrespective of the hoped-for linearity, the theoretical/kinetic simulations of the effect of optical excitation will further enhance our understanding of the thermoluminescence mechanisms, especially the role of spatially correlated trapping and luminescent centers. In the following, the various stages carried out in these investigations are discussed and preliminary results presented.

KINETIC SIMULATIONS OF THERMOLUMINESCENCE DOSE RESPONSE: LONG OVERDUE CONFRONTATION WITH THE EFFECTS OF IONISATION DENSITY.

The reader will time-travel through almost seven decades of kinetic models and mathematical simulations of thermoluminescence (TL) characteristics based on the band-gap theory of the solid state. From post-World-War II, ideas concerning electron trapping mechanisms to the highly idealised one trap-one recombination (OTOR) model first elaborated in 1956 but still in 'high gear' today. The review caresses but purposely avoids in-depth discussion of the endless stream of papers discussing the intricacies of glow peak shapes arising from first-order, second-order, mixed-order and general-order kinetics predominantly based on non-interacting systems, and then on to the more physically realistic scenarios that have attempted to analyse complex systems involving ever greater numbers of interacting trapping centres, luminescent centres and non-luminescent centres. The review emphasises the difficulty the band-gap models have in the simulation of dose response linear/supralinear behaviour and especially the dependence of the supralinearity on ionisation density. The significance of the non-observation of filling-rate supralinearity in the absorption stage is emphasised since it removes from consideration the possibility of TL supralinearity arising from irradiation stage supralinearity. The importance of the simultaneous action of both localised and delocalised transitions has gradually penetrated the mindset of the community of kinetic researchers, but most simulations have concentrated on the shape of glow peaks and the extraction of the glow peak parameters, E (the thermal activation energy) and s (the attempt-to-escape frequency). The simulation of linear/supralinear dose response and its dependence on ionisation density have been largely avoided until recently due to the fundamental schism between the effects of ionisation density and some basic assumptions of the band-gap model. The review finishes with an in-depth presentation and discussion of the most recent nanoscopic-localised/delocalised kinetic model that promotes an ice-breaking solution to bridge the schism.

The modified unified interaction model: incorporation of dose-dependent localised recombination.

The unified interaction model (UNIM) was developed to simulate thermoluminescence (TL) linear/supralinear dose-response and the dependence of the supralinearity on ionisation density, i.e. particle type and energy. Before the development of the UNIM, this behaviour had eluded all types of TL modelling including conduction band/valence band (CB/VB) kinetic models. The dependence of the supralinearity on photon energy was explained in the UNIM as due to the increasing role of geminate (localised recombination) with decreasing photon/electron energy. Recently, the Ben Gurion University group has incorporated the concept of trapping centre/luminescent centre (TC/LC) spatially correlated complexes and localised/delocalised recombination into the CB/VB kinetic modelling of the LiF:Mg,Ti system. Track structure considerations are used to describe the relative population of the TC/LC complexes by an electron-hole or by an electron-only as a function of both photon/electron energy and dose. The latter dependence was not included in the original UNIM formulation, a significant over-simplification that is herein corrected. The modified version, the M-UNIM, is then applied to the simulation of the linear/supralinear dose-response characteristics of composite peak 5 in the TL glow curve of LiF:Mg,Ti at two representative average photon/electron energies of 500 and 8 keV.

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

Low-energy alpha particle and proton heavy charged particle (HCP) relative thermoluminescence (TL) efficiencies are calculated for the major dosimetric glow peak in LiF:Mg,Cu,P (MCP-N) in the framework of track structure theory (TST). The calculations employ previously published TRIPOS-E Monte Carlo track segment values of the radial dose in condensed phase LiF calculated at the Instituto National de Investigaciones Nucleares (Mexico) and experimentally measured normalised (60)Co gamma-induced TL dose-response functions, f(D), carried out at the Institute of Nuclear Physics (Poland). The motivation for the calculations is to test the validity of TST in a TL system in which f(D) is not supralinear (f(D) >1) and is not significantly dependent on photon energy contrary to the behaviour of the dose-response of composite peak 5 in the glow curve of LiF:Mg,Ti (TLD-100). The calculated HCP relative efficiencies in LiF:MCP-N are 23-87% lower than the experimentally measured values, indicating a weakness in the major premise of TST which exclusively relates HCP effects to the radiation action of the secondary electrons liberated by the HCP slowing down. However, an analysis of the uncertainties involved in the TST calculations and experiments (i.e. experimental measurement of f(D) at high levels of dose, sample light self-absorption and accuracy in the estimation of D(r), especially towards the end of the HCP track) indicate that these may be too large to enable a definite conclusion. More accurate estimation of sample light self-absorption, improved measurements of f(D) and full-track Monte Carlo calculations of D(r) incorporating improvements of the low-energy electron transport are indicated in order to reduce uncertainties and enable a final conclusion.

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

The shape of composite peak 5 in the glow curve of LiF:Mg,Ti (TLD-100) following (90)Sr/(90)Y beta irradiation, previously demonstrated to be dependent on the cooling rate used in the 400°C pre-irradiation anneal, is shown to be dependent on ionisation density in both naturally cooled and slow-cooled samples. Following heavy-charged particle high-ionisation density (HID) irradiation, the temperature of composite peak 5 decreases by ∼5°C and the peak becomes broader. This behaviour is attributed to an increase in the relative intensity of peak 5a (a low-temperature satellite of peak 5). The relative intensity of peak 5a is estimated using a computerised glow curve deconvolution code based on first-order kinetics. The analysis uses kinetic parameters for peaks 4 and 5 determined from ancillary measurements resulting in nearly 'single-glow peak' curves for both the peaks. In the slow-cooled samples, owing to the increased relative intensity of peak 5a compared with the naturally cooled samples, the precision of the measurement of the 5a/5 intensity ratio is found to be ∼15% (1 SD) compared with ∼25% for the naturally cooled samples. The ratio of peak 5a/5 in the slow-cooled samples is found to increase systematically and gradually through a variety of radiation fields from a minimum value of 0.13±0.02 for (90)Sr/(90)Y low-ionisation density irradiations to a maximum value of ∼0.8 for 20 MeV Cu and I ion HID irradiations. Irradiation by low-energy electrons of energy 0.1-1.5 keV results in values between 1.27 and 0.95, respectively. The increasing values of the ratio of peak 5a/5 with increasing ionisation density demonstrate the viability of the concept of the peak 5a/5 nanodosemeter and its potential in the measurement of average ionisation density in a 'nanoscopic' mass containing the trapping centre/luminescent centre spatially correlated molecule giving rise to composite peak 5.

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

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).

Experimental investigation of the 100 keV X-ray dose response of the high-temperature thermoluminescence in LiF:Mg,Ti (TLD-100): theoretical interpretation using the unified interaction model.

The dose response of LiF:Mg,Ti (TLD-100) chips was measured from 1 to 50,000 Gy using 100 keV X rays at the European Synchroton Radiation Facility. Glow curves were deconvoluted into component glow peaks using a computerised glow curve deconvolution (CGCD) code based on first-order kinetics. The normalised dose response, f(D), of glow peaks 4 and 5 and 5b (the major components of composite peak 5), as well as peaks 7 and 8 (two of the major components of the high-temperature thermoluminescence (HTTL) at high levels of dose) was separately determined and theoretically interpreted using the unified interaction model (UNIM). The UNIM is a nine-parameter model encompassing both the irradiation/absorption stage and the thermally induced relaxation/recombination stage with an admixture of both localised and delocalised recombination mechanisms. The effects of radiation damage are included in the present modelling via the exponential removal of luminescent centres (LCs) at high dose levels. The main features of the experimentally measured dose response are: (i) increase in f(D)(max) with glow peak temperature, (ii) increase in D(max) (the dose level at which f(D)(max) occurs) with increasing glow peak temperature, and (iii) decreased effects of radiation damage with increasing glow peak temperature. The UNIM interpretation of this behaviour requires both strongly decreasing values of ks (the relative contribution of localised recombination) as a function of glow peak temperature and, as well, significantly different values of the dose-filling constants of the trapping centre (TC) and LC for peaks 7 and 8 than those used for peaks 4 and 5. This suggests that different TC/LC configurations are responsible for HTTL. The relative intensity of peak 5a (a low-temperature satellite of peak 5 arising from localised recombination) was found to significantly increase at higher dose levels due to preferential electron and hole population of the trapping/recombination complex giving rise to composite glow peak 5. It is also demonstrated that possible changes in the trapping cross section of the LC and the competitive centres due to increasing sample/glow peak temperature do not significantly influence these observations/conclusions.

Investigation of the ionisation density dependence of the glow curve characteristics of LIF:MG,TI (TLD-100).

The dependence of the shape of the glow curve of LiF:Mg,Ti (TLD-100) on ionisation density was investigated using irradiation with (90)Sr/(90)Y beta rays, 60 and 250 kVp X rays, various heavy-charged particles and 0.2 and 14 MeV neutrons. Special attention is focused on the properties of high-temperature thermoluminescence; specifically, the behaviour of the high-temperature ratio (HTR) of Peaks 7 and 8 as a function of batch and annealing protocol. The correlation of Peaks 7 and 8 with average linear-energy-transfer (LET) is also investigated. The HTR of Peak 7 is found to be independent of LET for values of LET approximately >30 keV microm(-1). The behaviour of the HTR of Peak 8 with LET is observed to be erratic, which suggests that applications using the HTR should separate the contributions of Peaks 7 and 8 using computerised glow curve deconvolution. The behaviour of the HTR following neutron irradiation is complex and not fully understood. The shape of composite Peak 5 is observed to be broader following high ionisation alpha particle irradiation, suggesting that the combined use of the HTR and the shape of Peak 5 could lead to improved ionisation density discrimination for particles of high LET.

OSL and TL in LiF:Mg,Ti following alpha particle and beta ray irradiation: application to mixed-field radiation dosimetry.

The optically stimulated luminescence (OSL) of LiF:Mg,Ti (TLD-100) following irradiation by beta and alpha particles was investigated by the measurement of the excitation and emission spectra of OSL and comparison with thermoluminescence (TL) characteristics. Measurements were also carried out on nominally pure LiF monocrystals. The preferential excitation of OSL compared to TL following high-ionisation density (HID) alpha irradiation is naturally explained via the identification of OSL with the 'two-hit' F2 or F3+ centre, whereas the major component of composite TL glow peak 5 is believed to arise from a 'one-hit' complex defect. This discovery allows near-total discrimination between HID radiation and low-ionisation density radiation and may have significant potential in mixed-field radiation dosimetry.

The thermoluminescence dose-response and other characteristics of the high-temperature TL in LiF:Mg,Ti (TLD-100).

Various characteristics of the high-temperature thermoluminescence (HTTL) in the glow curve of LiF:Mg,Ti (TLD-100) are reviewed. The proposed applications of the HTTL to mixed-field radiation dosimetry are outlined with special emphasis on the question of the linearity/supralinearity of the HTTL dose-response at low dose levels from 2.5 to 250 mGy. Recent measurements of the HTTL dose-response using non-linear hot-gas heating and linear planchet heating are discussed in detail. It appears that a mild HTTL supralinearity of approximately 15-50% for each dose decade may be present, followed by an abrupt and rapid increase in the supralinearity >250 mGy. However, difficulty in the estimation of background and the great variability in the protocols of measurement do not allow a definitive conclusion. There is much work to be done in the areas of protocol standardisation, materials selection, methods of data analysis and especially the details of background behaviour, and subtraction before the HTTL can become a reliable dosimetric tool.

The effects of ionisation density on the glow curve structure of LiF:Mg,Ti (TLD-100): the behaviour of composite glow peak 5 in 'slow-cooled' material.

The effects of ionisation density on the structure of the glow curve of LiF:Mg,Ti (TLD-100) are briefly reviewed and discussed within the framework of the spatially correlated TC/LC model and localised recombination. The effects of 'slow-cooling' on the structure of composite peak 5 following low-ionisation density beta/gamma irradiation are described and analysed in both 'slow-cooled' and 'normally cooled' samples. It is demonstrated that 'slow-cooling' using a cooling rate of 30 degrees C h(-1) increases the relative intensity of glow peak 5a to composite glow peak 5 from approximately 0.1 to approximately 2, thereby greatly improving the precision of measurement of the ratio 5a/5. The improved precision removes a hurdle impeding the development of the ionisation density discrimination properties of the peak 5a/5 nanodosimeter.

Advanced multistage deconvolution applied to composite glow peak 5 in LiF:Mg,Ti (TLD-100).

The effects of 'slow-cooling' on the structure of composite peak 5 following low-ionisation density beta/gamma irradiation are described and analysed in both 'slow-cooled' and 'normally-cooled' samples. Computerised glow curve deconvolution is employed with constrained 'peak-shape' parameters deduced from anciliary studies using 4 eV and 5 eV optical excitation.

Ionisation density effects following optical excitation in LiF:Mg, Ti (TLD-100).

The TL signal following 5 eV photon excitation of previously irradiated and readout material has been studied as a function of ionisation density and various experimental parameters: (i) maximum temperature of the first readout; (ii) photon fluence; (iii) photon energy and (iv) beta ray dose. Following alpha particle irradiation, the ratio of the second-readout to first-readout TL signal, epsilon(alpha,) has been found to be 10-20 times higher than that following beta irradiation, indicative of the possibility of using the double ratio epsilon(alpha)/epsilon(beta) as a mixed-field discriminator. The beginning of an attempt to explain this unusual effect is offered in the framework of the track structure theory and kinetic modelling of the beta ray dose-response of the first and second readouts.

Modelling the dose response of peaks 4, 5 and 5b, in TLD-100, as a function of recombination temperature.

This investigation focuses on whether the experimentally observed changes, in supralinearity f(D) as a function of recombination temperature, can be, successfully, predicted by the Unified Interaction Model (UNIM). To resolve this question, the measurement of f(D) has been carried out for glow peaks 4, 5 and 5b in LiF:Mg,Ti (TLD-100) at two different heating rates and UNIM analysis is used in an attempt to arrive at a consistent/physically reasonable explanation of all the f(D)/heating rate data for the three glow peaks. It is demonstrated that in addition to the variation in the luminescence and competitive centre capture cross section with temperature changes in the ks (the geminate recombination coefficient) parameter as a function of temperature is also required in order to predict the changes in f(D).

Theoretical and experimental investigation of the f(D)max-(S/S0)max anomaly in LiF:Mg,Ti.

Previous investigations in the framework of the Unified Interaction Model (UNIM) have demonstrated the difficulty in theoretically predicting both f(D)max and [S(Ds)/S(0)]max with the same set of UNIM parameters. This anomaly is re-investigated and an alternative explanation, based on experimental measurements combined with UNIM analysis, is given. This explanation is based on the assumption that N(LC), the total number of available luminescent centres is changed by the sensitisation dose or anneal or both. Allowing N(LC) to increase as the sensitisation dose is increased, a good fit to both f(D) and S(Ds)/S0 is obtained.