L. Bøtter-Jensen

Developments in radiation, stimulation and observation facilities in luminescence measurements

Abstract This paper summarises recent developments in the Riso TL/OSL reader. These new developments include an elevated temperature irradiation facility that has been expanded to measure variable temperature radioluminescence (RL) during beta irradiation. This RL facility allows the measurement of TL, OSL and RL in the same software-controlled automatic sequence. To provide an alternative to the 90 Sr beta source, the performance of a software-controlled mini X-ray generator has been investigated. This has the advantage of providing a range of dose rates spanning more than three orders of magnitude up to more than 2 Gy / s . A separate bleaching facility has also been added to the reader, new powerful infrared (IR) light-emitting diodes have replaced the IR laser diode in the general OSL unit, and an IR laser has been included in the single-grain OSL attachment. This paper describes these new facilities, and illustrates their applications with representative measurements.

Equivalent dose estimation using a single aliquot of polymineral fine grains

We have tested the suitability of a new single-aliquot regenerative-dose protocol for estimating the equivalent dose (De) in polymineral fine grains extracted from colluvia from various sites in Germany. First, we report the behaviour of three OSL signals: (i) blue-stimulated, (ii) infrared-stimulated luminescence, and (iii) blue-stimulated luminescence following infrared (IR) stimulation, using a near-UV (290–380 nm) detection window in each case. For these three signals, there is a significant change in sensitivity with regeneration cycle; this change can be compensated for using the response to a fixed test dose after each natural or regenerated measurement. The source of the three luminescence signals is then investigated using pulse-anneal and elevated-temperature experiments. Fading tests on laboratory-induced signals show that although the IR signals fade by up to 23% in 15 days at 100°C, the post-IR blue signals are stable. The preheat dependence of estimates of De obtained using fine grains is presented for the first time, for both blue- and IR-derived signals. Our results are compared with De estimates derived from multiple-aliquot additive-dose IR luminescence data, obtained using a blue detection window, and also with expected values of De based on independent age estimates and measured dose rates. We conclude that post-IR blue-stimulated luminescence provides reliable estimates of De, and that these are probably superior to the IRSL estimates obtained using both near-UV and blue detection windows.

Characterisation of blue-light stimulated luminescence components in different quartz samples: implications for dose measurement

Abstract Over the last few years, we have become increasingly confident that quartz is a reliable natural dosimeter for sediment dating. Nevertheless, there is only a limited understanding of the behaviour of the different components of optically stimulated luminescence (OSL) from quartz. Recent single-aliquot dose-evaluation protocols seem to be relatively free of complications when applied to quartz dominated by the fast OSL component coming from 325°C TL region, but this may not be true for quartz in which other components are more significant. An adequate understanding of how different OSL components behave during various measurement cycles is critical to ensuring that our dose evaluation protocols are robust and it is also important to our interpretation of the variation of apparent dose with optical stimulation time for identification of partial bleaching. We report here the principal results from measurement of (a) sensitisation, (b) thermal stability, (c) recuperation, and (d) infrared response as a function of stimulation temperature from 3 different samples of sedimentary quartz selected on the basis of relative OSL contribution from different blue light stimulated linearly modulated (LM-OSL) components. We then discuss the implications of these characteristics for dose assessment using the single aliquot regeneration dose protocol.

Luminescence techniques : Instrumentation and methods

Abstract This paper describes techniques, instruments and methods used in luminescence dating and environmental dosimetry in many laboratories around the world. These techniques are based on two phenomena – thermally stimulated luminescence and optically stimulated luminescence. The most commonly used luminescence stimulation and detection techniques are reviewed and information is given on recent developments in instrument design and on the state of the art in luminescence measurements and analysis.

A new system for measuring optically stimulated luminescence from quartz samples

Abstract An optically stimulated luminescence (OSL) unit based on a halogen lamp light source has been developed for the stimulation of quartz samples. In addition to the halogen light source, the OSL unit incorporates an infra-red diode array for infra-red-stimulated luminescence (IRSL) measurements of feldspar samples and for screening the purity of quartz. The new OSL unit can be fitted directly onto the automated Riso TL dating apparatus. The stimulation spectrum filtered from the halogen lamp delivers about 16mW cm -2 at the sample. Preliminary OSL studies of quartz samples are described, and dating results are presented.

Optical dating of single sand-sized grains of quartz: sources of variability

Optically stimulated luminescence (OSL) measurements have been made of over 3000 sand-sized grains of quartz. Analysis at this scale highlights the variability in the luminescence sensitivity and the dose saturation characteristics of individual quartz grains. Using a new instrument capable of measuring single grains it is feasible to routinely measure the equivalent dose from many hundreds of grains from each sample. Analysis of such datasets requires assessment of the uncertainties on each equivalent dose since these may vary significantly. This paper assesses the significance of signal intensity, dose saturation characteristics and instrument uncertainty in equivalent dose calculation.

Real-time optical-fibre luminescence dosimetry for radiotherapy: physical characteristics and applications in photon beams

A new optical-fibre radiation dosimeter system, based on radioluminescence and optically stimulated luminescence from carbon-doped aluminium oxide, was developed and tested in clinical photon beams. This prototype offers several features, such as a small detector (1 x 1 x 2 mm3), high sensitivity, real-time read-out and the ability to measure both dose rate and absorbed dose. The measurements describing reproducibility and output dependence on dose rate, field size and energy all had standard deviations smaller than 1%. The signal variation with the angle of incidence was smaller than 2% (1 SD). Measurements performed in clinical situations suggest the potential of using this real-time system for in vivo dosimetry in radiotherapy.

Temperature dependence of OSL decay curves : Experimental and theoretical aspects

The factors which affect the shape of Optically Stimulated Luminescence (OSL) decay curves are examined, both experimentally and theoretically, in an effort to understand and describe the behavior of OSL as a function of temperature. The processes considered include: (1) retrapping by shallow traps; (2) thermally assisted optical stimulation; (3) thermal quenching; and (4) localized donor-acceptor type recombination. Experimental OSL data from natural quartz and feldspars, stimulated with both green and infra-red light, are examined in the light of the theoretical considerations.

Luminescence sensitivity changes in quartz as a result of annealing

Abstract Retrospective dosimetry using optically stimulated luminescence (OSL) on quartz extracted from (for example) bricks needs to account for strong OSL sensitivity changes that are known to occur depending on the previous thermal treatment of the sample. Non-heated quartz exhibits OSL orders of magnitude less per unit radiation than that for heated material. The reason these temperature-induced sensitivity changes occur in quartz is presently not well understood. This phenomenon is also seen in the related area of luminescence dating in which sedimentary quartz and quartz from heated archaeological samples show very different OSL sensitivities. In this paper we report on studies of the effects of high temperature annealing on the OSL and phototransferred TL (PTTL) signals from sedimentary and synthetic quartz. A dramatic enhancement of both OSL and PTTL sensitivity was found especially in the temperature range 500–800°C. Computer simulations of the possible effects are shown to produce data that agree in all essential details with the experimental observations. It is further demonstrated that the enhanced OSL sensitivity as a function of annealing temperature is not a pre-dose effect.

New light on OSL

Abstract We have tested recently released green and blue light emitting diodes (LEDs) for the optical stimulation of quartz, with a view to using these in routine optically stimulated luminescence (OSL) dating and retrospective dosimetry. We compare results obtained with blue LEDs with those obtained using broad-band blue/green light filtered from a halogen lamp. We describe two practical blue LED OSL configurations and conclude that a small number of blue LEDs provide the same stimulation efficiency for quartz as can be obtained from filtered halogen lamps and other commonly used sources. From the direct comparison of OSL decay curves produced by the two light sources, and by examination of the dependence of blue LED OSL on preheat temperature, we conclude that there is no evidence that the blue LED light stimulates deep traps in a different manner from broad-band halogen light. Preliminary intercomparisons, using 34 quartz samples taken from both heated and unheated materials, between equivalent doses obtained using blue LEDs and using broad-band blue/green light from a halogen lamp show that, on average, the two data sets are indistinguishable (blue/halogen=0.98±0.02). We conclude that blue LEDs offer a practical alternative to existing stimulation sources. They have the significant advantage that the output can be controlled electronically; thus the power can be readily controlled by software.