M.L. Chithambo

The analysis of time-resolved optically stimulated luminescence: II. Computer simulations and experimental results

This is the second in a set of paired papers on the analysis of time-resolved optically stimulated luminescence. This paper compares experimental results and computer simulations with corresponding conceptual models discussed in the first of this set of papers. The evaluation of principal and subsidiary luminescence lifetimes from time-resolved luminescence spectra measured during or after pulsed optical stimulation is illustrated using suitable analogues of exponential functions developed in the preceding paper. Various methods for calculating the activation energies for thermal quenching, thermal assistance and the frequency factor for the thermal quenching process using the temperature dependence of both luminescence lifetimes and luminescence intensity are described, as are techniques for dealing with cases not easily described by the standard Mott–Seitz model of luminescence. The paper then compares continuous and time-resolved optical stimulation methods, where it is shown that, dependent on suitable selection of the pulse-width, stimulation and measurement time, pulsing the stimulation light might give better accuracy in the determination of the luminescence.

Dependence of the thermal influence on luminescence lifetimes from quartz on the duration of optical stimulation

Abstract Time-resolved luminescence spectra may be measured from quartz at various stages of continuous optical stimulation in order to investigate properties of the spectra associated with the ‘fast’, ‘medium’, and ‘slow’ components of continuous optically stimulated luminescence (OSL). In this work, temperature related changes of luminescence lifetimes and luminescence intensity, both evaluated from time-resolved luminescence spectra recorded in the ‘fast’, ‘medium’ and ‘slow’ component regions of quartz OSL, have been investigated. The luminescence, stimulated at 525 nm , and measured at intervals between 20°C and 200°C reaches maximum intensity at 100°C and decreases thereafter up to 200°C, the maximum temperature of the investigations. Luminescence lifetimes, on the other hand, remain constant within 40±3 μs between 20°C and 100°C and then decrease down to about 8 μs at 200°C. The initial increase of luminescence intensity with temperature between 20°C and 100°C is discussed in terms of thermal assistance to luminescence stimulation. Beyond 100°C, radiative recombination is affected by quenching of luminescence and reduction in luminescence lifetimes. The activation energy for thermal quenching was evaluated to be in the range 0.63±0.07 eV at all stimulation times and that of thermal assistance was evaluated to be about 0.06 eV for the ‘fast’ and ‘medium’ component regions and about 0.1 eV for the ‘slow’ component region of the OSL.

The analysis of time-resolved optically stimulated luminescence: I. Theoretical considerations

This is the first of two linked papers on the analysis of time-resolved optically stimulated luminescence. This paper focusses on a theoretical basis of analytical methods and on methods for interpretation of time-resolved luminescence spectra and calculation of luminescence throughput. Using a comparative analysis of the principal features of time-resolved luminescence and relevant analogues from steady state optical stimulation, formulae for configuring a measurement system for optimum performance are presented. We also examine the possible use of stretched-exponential functions for analysis of time-resolved optically stimulated luminescence spectra.

The influence of annealing and partial bleaching on luminescence lifetimes in quartz

Abstract Some features of luminescence lifetimes related to annealing and depletion of trapped charge in quartz are reported. Time-resolved luminescence spectra were recorded following pulsing by blue (470 nm ) light-emitting diodes from sets of granular quartz annealed at temperatures between 600°C and 1000°C, daylight-bleached quartz, and high-purity synthetic quartz. Measurements of temperature-induced changes of lifetimes were made either immediately after dosing and pre-heating, that is, in the ‘fast’ component region or after pre-measurement optical bleaching. Luminescence lifetimes in each of the ‘fast’, ‘medium’, and ‘slow’ component regions in quartz annealed at 600°C, 800°C, and 900°C are similar, whereas in quartz annealed at 1000°C, lifetimes associated with the ‘fast’ component region are larger. The initial value of lifetime measured at 20°C is slightly dependent on annealing temperature. There are no systematic differences in the values of the activation energy for thermal quenching, Δ E , that can be associated with either annealing temperature, partial bleaching, or wavelength of stimulation in any of the ‘fast’, ‘medium’ or ‘slow’ component regions; all values are within 0.60±0.20 eV .

Temperature dependence of luminescence time-resolved spectra from quartz

Abstract Pulsed optical stimulation of luminescence has been used to study the thermal dependence of luminescence lifetimes in quartz over the temperature range 20–200°C. Time-resolved spectra for lifetime analysis were recorded from samples of quartz over a dynamic range of 64 μs following stimulation of luminescence by pulsed 525 nm green light emitting diodes (LEDs) using an 11 μs pulse and 12% duty cycle. It has been demonstrated that an increase in measurement temperature generally leads to a decrease in lifetimes from about 30 μs at 20°C to about 7 μs at 200°C. The form of the decrease is influenced by the initial optical or thermal pre-treatment of samples.

On luminescence lifetimes in quartz

In this paper we present results of investigations concerning the time dependence of luminescence emission relative to the time of stimulation in quartz. Measurements of time-resolved spectra were performed on a new versatile pulsed light emitting diode system using 525 nm stimulation, an 11 μs duration pulse, a repetition rate of 11 kHz and a 64 μs dynamic range. Effects on luminescence lifetime resulting from sample treatments such as optical stimulation, irradiation, and preheating, are reported.

Experimental and modelling study of pulsed optically stimulated luminescence in quartz, marble and beta irradiated salt

Optical stimulation luminescence (OSL) signals can be obtained using continuous-wave optical stimulation (CW-OSL), the linear modulation optical stimulation method (LM-OSL) and the time-resolved optical stimulation (TR-OSL) method. During TR-OSL measurements, the stimulation and emission of luminescence are experimentally separated in time by using short light pulses. This paper presents new TR-OSL data for annealed high purity synthetic quartz, for marble and for commercially available iodized salt. A new type of behaviour for TR-OSL signals for quartz and iodized salt is presented, in which the OSL signal exhibits a nonmonotonic behaviour during optical stimulation; this type of behaviour has not been reported previously in the literature for quartz. Furthermore, a luminescence component with very long luminescence lifetime is reported for some quartz aliquots, which may be due to the presence of a delayed-OSL (DOSL) mechanism in quartz. A new kinetic model for TR-OSL in quartz is presented, which is based on a main electron trap and on several luminescence centres. The model is used to quantitatively fit several sets of experimental data of pulsed optically stimulated luminescence from quartz.

On the dose-dependence of luminescence lifetimes in natural quartz

The influence of irradiation dose on luminescence lifetimes has been studied in suites of natural quartz from Brazil, Nigeria and South Korea. The lifetimes were evaluated from time-resolved luminescence spectra measured from quartz pulse-stimulated at 470 nm and detected between 360 and 380 nm. The effect of irradiation varied between materials, decreasing with dose in the Brazilian quartz, independent of dose in the samples from South Korea, and increasing with dose in the quartz from Nigeria. The dose-related effects are accounted for with reference to an energy band scheme for quartz in which the luminescence lifetime is associated with a primary radiative recombination centre at which most capture of holes produced during irradiation and associated hole re-trapping takes place. The measurements on the effect of irradiation dose on luminescence lifetimes were augmented by investigations concerned with the influence of measurement temperature between 20 and 200°C on luminescence lifetimes in the same materials. It was observed that the luminescence lifetimes decrease continuously with temperature of stimulation and in a manner suggestive of increased thermal quenching of the corresponding luminescence. However, better consistency with the Mott–Seitz configurational coordinate model of thermal quenching is only apparent at temperatures greater than 120°C. The discrepancy is attributed to the model used to derive the formulae for luminescence lifetimes in which any re-trapping of stimulated charge is assumed negligible. However, it is argued that if re-trapping were in fact not negligible then the decrease of luminescence lifetimes with measurement temperature would be evidence of the progressive reduction of the re-trapping as the measurement temperature were increased with the overall effect being the monotonic decrease in luminescence lifetimes as observed.

Influence of nitrogen implantation on thermoluminescence of synthetic quartz

Thermoluminescence (TL) of synthetic quartz exposed to beta irradiation following implantation with 60 keV N+ ions at fluences ranging between 1 × 1014 and 5 × 1015 ions/cm2 is reported. The glow curve measured at 5°C/s typically consists of a prominent peak near 110°C, studied in this work, and minor glow peaks at around 130°C and 190°C. The TL intensity of the main peak increased both with implantation and with fluence of implantation. The dependence of the intensity on heating rate and fluence suggests that the implantation introduces new defects that may possibly act as recombination centres. The increase in TL intensity with the heating rate exhibited by implanted samples has been observed in other luminescence materials. This anti-quenching phenomenon has been described as a competition effect between multiple luminescence pathways in luminescence materials. Kinetic analysis of the main glow peak using the initial rise, various heating rate and glow curve deconvolution methods shows that the activation energy of the main peak is about 0.7 eV with no systematic change due to ion fluence.

Thermoluminescence kinetic analysis of quartz with a glow peak that shifts in an unusual manner with irradiation dose

Thermoluminescence kinetic analysis using Tm–Tstop, glow curve deconvolution, variable heating rate and repeated initial rise methods have been carried out on the glow curves of a quartz with a glow peak whose peak temperature goes through a peak as a function of irradiation dose, an unusual feature for quartz. The glow curve of the sample showed four clear glow peaks around 82 °C (peak I), 148 °C (peak II), 200 °C (peak III) and 292 °C (peak IV) when irradiated to 1.8 Gy and read out at 1 °C s−1. The peak temperature of peak IV first increases and later decreases with dose whereas the peak temperatures of peaks I, II and III are independent of dose. Tm–Tstop and glow curve deconvolution (GCD) analyses indicated the presence of at least five peaks at about 80, 146, 199, 295 and 350 °C meaning that the apparently single peak IV may consist of at least two overlapping glow peaks. The activation energy, E, and frequency factor, S, associated with each of the glow peaks are presented. E and Tm values for all the glow peaks in the quartz sample were found to be dose-independent. It was also found, during the use of GCD, that as the irradiation dose increases, the number of thermoluminescence (TL) peaks required to produce the best fit to the glow curve increases although the location of the TL peaks themselves does not change with dose. This effect is explained in terms of the relative intensities of the glow peaks. We have also noted that the ability of GCD to completely isolate all the glow peaks in a glow curve may depend on irradiation dose.