L. Oster

Optical absorption in LiF : Mg,Ti and its relationship to thermoluminescence and thermoluminescence dose response

Abstract The optical absorption (OA) spectrum of LiF : Mg,Ti has been investigated by optical bleaching and I m – T stop pulsed annealing in steps of 3–4°C over the temperature range from 100°C to 300°C. In agreement with previous investigations, the 5.45 eV OA band is identified as a likely candidate for the recombination stage competitor responsible for composite peak 5 supralinearity. An additional OA band at 5.7 eV is also identified as a possible recombination stage competitor. First-order kinetic analysis yields an activation energy of 0.8±0.13 eV for the 4.0 eV band usually associated with peak 5, an energy which is similar in value to the binding energy of the Mg–Li vac trimer believed to form the peak 5 trapping structure. Dose response measurements result in dose filling constants of β TC =0.9±0.1×10 −3 Gy −1 and β CC =3.8±0.8×10 −4 Gy −1 for the 4.0 and 5.45 eV OA bands, respectively. These dose filling constants deduced from OA measurements can be used in the framework of the unified interaction model (UNIM) to study the dose response supralinearity of composite dosimetric peak 5.

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.

Delocalized recombination kinetic modelling of the LiF : Mg,Ti glow peak 5 thermoluminescence system

TL kinetic theory based on delocalized recombination mechanisms is successful in predicting many characteristics of TL mechanisms when these are considered singly. However, the greater challenge of predicting a set of measured TL characteristics while using the same, or very nearly the same, parameter values has yet to be adequately addressed. In this work we apply delocalized kinetic modelling to describe four experimentally measured characteristics of the TL behaviour of peak 5 in LiF : Mg,Ti (TLD-100). These include: (i) optical absorption linear/exponentially saturating dose response of the appropriate OA bands with the experimentally measured dose filling constants; (ii) linear/supralinear TL dose response with f(D)max = 4–6 at a dose level of ~400–600 Gy; (iii) constancy of Tmax with dose; (iv) agreement with the experimentally measured behaviour of Tmax and f(D)max with heating rate. We are successful in describing most of the details of these characteristics; however, we have failed to find parameter values which also correctly describe the dose response behaviour at low dose levels (preceding the linear, then the supralinear region) and the behaviour of f(D)max with heating rate. It is suggested that kinetic modelling based on a mixture of localized and delocalized recombination could successfully describe all the characteristics involved.

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.

Ionization density effects following F-centre optical excitation in LiF : Mg, Ti (TLD-100): analysis via track structure theory

In track structure theory (TST), the calculation of the relative thermoluminescence (TL) efficiency of heavy charged particles (HCPs) is carried out by integration of radial dose profiles weighted with the TL efficiency measured by a low ionization density dose response. The reliability of these calculations is reviewed herein with special emphasis on the composite glow peak 5 system in LiF?:?Mg, Ti (TLD-100). Possible weaknesses in the basic assumptions and in the details of the experimental and calculational methods are discussed. The influence of HCP-enhanced production of atomic displacements/defects on the TL efficiency is investigated via an experiment sensitive to F-centre concentrations which may influence the luminescence mechanism/efficiency. In this experiment, the sample is irradiated by 5.3?MeV alpha particles to a low fluence of ~108?cm?2 in order to minimize track overlap and then read out to a maximum temperature of 350??C. A second readout is carried out following irradiation by 250?nm photons corresponding to the 5?eV optical excitation energy of the F centre. The intensity of glow peak 5 in the second readout relative to the first readout is calculated theoretically using the first-readout and second-readout dose responses of peak 5 measured with both 100?keV x-rays and 90Sr/90Y beta rays and found to lie outside the predictive capability of TST.

STUDY OF THE SUITABILITY OF ISRAELI HOUSEHOLD SALT FOR RETROSPECTIVE DOSIMETRY

The first results of an in-depth evaluation of the practical potential of common household Israeli salt as a retrospective dosemeter in the event of a nuclear accident or terror attack are presented. Ten brands of salt were investigated with emphasis on four of the bestselling brands that constitute 76 % of the total consumer market. Eight of the ten brands show similar glow curves with two main glow peaks at maximum temperatures of ∼176°C and ∼225°C measured at a heating rate of 1°C s(-1) Chemical analysis of three major brands indicates substantial impurity levels of 200-500 ppm of Ca, K, Mg and S and significant differences of additional ppm trace impurities, which lead to an ∼50 % difference in the TL response of the three major brands. Fading in the dark is in significant but under room light is of the order of 35 % per day. The dose response is linear/supralinear with the threshold of supralinearity at ∼0.01 Gy reaching maximum value of ∼4 at 0.5-1 Gy for two of the major brands. The precision of repeated measurements is ∼10 % (1 SD), but the accuracy of dose assessment under field conditions requires further study.

Study of Combinations of TL/OSL Single Dosimeters for Mixed High/Low Ionization Density Radiation Fields

We have previously demonstrated that the increased response of optically stimulated luminescence (OSL) compared to thermoluminescence (TL) following high ionization density (HID) alpha irradiation is naturally explained via the identification of OSL with the ”two-hit” F2 or F\(_3^+\) center, whereas the major component of composite TL glow peak 5 is believed to arise from a ”one-hit” complex defect. This discovery suggested that near-total discrimination between HID radiation and low ionization density (LID) radiation using combined OSL and TL measurements may have significant potential in mixed-field radiation dosimetry In this paper we discuss and compare the potential application of combined OSL/TL measurements using 6LiF:Mg,Ti (TLD-600) or 7LiF:Mg,Ti (TLD-700) and TLD-100 natural isotopic composition) detectors.

Optically stimulated luminescence in anion-defective carbon doped α-Al2O3 crystals

Abstract Optically stimulated luminescence (OSL) was studied in C-doped α-Al2O3 crystals, which had previously been exposed to X or β irradiations. The OSL was excited by monochromatic UV light in 200–300 nm region. Emission maxima appeared at 330 nm and 410 nm which are attributed to F+ and F-luminescence respectively. This is supported by measurements of excitation spectra, which showed maxima at 260 and 215 nm. Results were compared to those obtained by measurements of the X-luminescence and phototransferred thermoluminescence in the same samples as well as in pure α-Al2O3 crystals. Effects of previous irradiations and of thermal treatment on the OSL intensities were also investigated.

The kinetics of peak 5 in LiF:Mg,Ti - dose dependence of the thermoluminescence isothermal decay at 165

The question of the order of kinetics of the thermoluminescence (TL) of peak 5 in LiF:Mg,Ti has been subject to considerable controversy. Recently, we have shown that peak shape analysis of post-irradiation annealed LiF:Mg,Ti (TLD-100) at 165 leads to non-first-order kinetics, reaching a value as high as following a 165 30 min post-irradiation anneal. When no post-irradiation anneal is applied, peak shape analysis almost always yields first-order behaviour. Isothermal decay data are equally ambiguous. To begin to resolve this discrepancy (at least at the experimental level) we have studied the isothermal decay of peak 5 at 165 (presumably cleansed of the low-temperature peaks 2 - 4 by a 165 15 min post-irradiation anneal). The isothermal decay was initiated at four dose levels of 10, 100, 300 and 1000 Gy, and these decays were not even vaguely similar. The initial rate of decay at Gy was at least five times greater than at 10 Gy and the decays were clearly non-exponential. Our data are not consistent with first-order decay (b = 1) and lead to using general-order (GO) model analysis.

IDENTIFICATION OF COMPETITIVE TRAPPING CENTERS IN IRRADIATED LIF:MG, TI VIA OPTICAL ABSORPTION STUDIES

Abstract The optical absorption (OA) and thermoluminescence (TL) of irradiated LiF: Mg, Ti (TLD-100) was studied as a function of the temperature of post-irradiation annealing using the Im-Tstop and Tm-Tstop techniques. Computerised deconvolution was used to resolve the OA spectra into Gaussian bands and the TL glow curve into glow peaks. The dose response of the OA bands was also investigated. The filling and emptying rate “anticorrelation” and the behaviour with dose of the 5.6 eV OA band on the one hand and 3.25 and 4.0 eV OA bands on the other lead us to propose that the 5.6eV band trapping structure actively competes with the 3.25 and 4.0eV OA bands (or TL peaks 2 and 5) trapping structures and may be the “deep” competitor responsible for the supralinearity of these peaks.