Biomedical subjects

S W S McKeever

Publications and source records attributed to S W S McKeever.

At least 19 recordsLinked to original sources

PubMed2006

A system to irradiate and measure luminescence at low temperatures.

We have developed a system to irradiate samples and record radioluminescence (RL), optically stimulated luminescence (OSL), and thermoluminescence (TL) at temperatures ranging from -150 degrees C to 200 degrees C. The system consists of a cryostat, an irradiation/stimulation unit fitted with an X-ray tube (40 kV Moxtek) and a quartz window for optical stimulation, and a detection unit that utilises a photomultiplier tube and an interchangeable filter pack. Experiments have been conducted with quartz and albite (a feldspar). TL and OSL experiments show that several optically sensitive trapping states are stable below -50 degrees C. In addition, an increase in OSL is seen as the OSL stimulation temperature is lowered below -50 degrees C, and an increase in RL is apparent as the temperature is lowered during irradiation. This indicates that not only are optically sensitive low temperature traps present but that luminescence becomes more efficient at low temperatures.

Algorithms↗

PubMed2006

Low temperature radioluminescence of LiF:Mg,Cu,P.

Low temperature radioluminescence spectra of LiF, variously co-doped with Mg, Cu and P, show highly unusual temperature dependencies which resemble thermoluminescence data. The signals include intense peaks and a relatively weak continuous background. One peak occurs below 30 K, together with a major peak near 125 K. The signals are highly sensitive to the dopants and slightly sensitive to X-ray dose rate. The role of donor acceptor pairs and the perturbations from intrinsic defects formed by ionisation can be used to describe all the observations. The 290 nm emission band is linked to H center annealing.

Copper↗

PubMed2006

The use of optically stimulated luminescence from AL2O3:C in the dosimetry of high-energy heavy charged particle fields.

This paper presents two different approaches of quantifying the optically stimulated luminescence (OSL) response of Al(2)O(3):C to high-energy heavy charged particles (HCPs). The OSL efficiency of Al(2)O(3):C exposed to different HCPs is defined as the sensitivity of the material to HCPs normalised by the sensitivity to gamma. In this paper, we investigate the possibility of introducing a 'mean efficiency' eta(mean), which when used in conjunction with the total gamma dose D(gamma) measured for a mixed radiation exposure allows for the determination of the absorbed dose without the need to determine the individual contributions of different types of radiation to the OSL signal. We tested the hypothesis that information regarding the 'mean efficiency' eta(mean) is contained in the shape of the OSL decay curve, using several approaches in the analysis of the OSL data. This analysis was applied to various mixed field irradiations performed at the HIMAC facility, Chiba, Japan. The results of this analysis are discussed.

Aluminum Oxide↗

PubMed2006

A real-time, high-resolution optical fibre dosemeter based on optically stimulated luminescence (OSL) of KBr:Eu, for potential use during the radiotherapy of cancer.

A real-time optically stimulated luminescence (OSL) dosimetry system for potential in vivo use during radiotherapy treatments is proposed. Single-crystal europium-doped KBr samples were grown in a Bridgman furnace, and characterised using optical absorption techniques. An algorithm for the processing of the OSL signal was defined for use in real-time measurements, and its performance was studied on data obtained with a home-built reader, using optical-fibre-coupled dosemeters. OSL dose-response, fading properties and temperature dependence of the signal were investigated in correlation with the concentration of Eu(2+) dopant in the sample.

Bromides↗

PubMed2006

Radiation dosimetry for microbial experiments in the International Space Station using different etched track and luminescent detectors.

The laboratory of Microbiology at SCK.CEN, in collaboration with different universities, participates in several ESA programmes with bacterial experiments that are carried out in the International Space Station (ISS). The main objective of these programmes is to study the effects of space flight conditions such as microgravity and cosmic radiation on the general behaviour of model bacteria. To measure the radiation doses received by the bacteria, different detectors accompanied the microbiological experiments. The results obtained during two space flight missions are discussed. This dosimetry experiment was a collaboration between different institutes so that the doses could be estimated by different techniques. For measurement of the high linear energy transfer (LET) doses (>10 keV microm(-1)), two types of etched track detectors were used. The low LET part of the spectrum was measured by three types of thermoluminescent detectors ((7)LiF:Mg,Ti; (7)LiF:Mg,Cu,P; Al(2)O(3):C) and by the optically stimulated luminescence technique using Al(2)O(3):C detectors.

Cosmic Radiation↗

PubMed2006

Experimental determination of the dose deposition profile of a 90Sr beta source.

Three different methods for characterising the dose deposition profile of a (90)Sr/(90)Y radioactive source are described: GAFChromic film dosimetry, Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL). For the film measurements, GAFChromic film samples were stacked at different depths between polyethylene terephthalate (PET) foils. For TL, the thickness of a TLD-500 dosemeter was gradually reduced by polishing and the TL from chips of different thickness was used in conjunction with a mathematical model based on the exponential attenuation of dose inside the crystal to determine the decay constant for the dose-depth profile. Finally, an OSL reader with confocal stimulation / detection capabilities was used to map the two-dimensional dose distribution in TLD-500 dosemeters as a function of depth. The shapes of the dose deposition profiles obtained from all the investigated methods are in good agreement.

Beta Particles↗

PubMed2006

In situ long-term monitoring system for radioactive contaminants.

A long-term in situ subsurface instrument for monitoring radioactive contaminant plumes, as an alternative to soil analysis, is described. A portable, laser-based reader optically stimulates luminescence from sensors, each containing an Al2O3:C dosemeter. The sensors, designed for placement at various subsurface locations around a waste site, are allowed to accumulate dose for a predetermined time that is based on the instrument's minimum detectable dose (MDD). The reader is then attached to the sensor by fibre optic cable to read the accumulated dose; an increase above natural background levels indicating the presence of leaked radioactivity. Based on an MDD of 5 microGy, it is shown that the sensor can measure soil concentrations of 1.85 Bq cm(-3) after an exposure time of 50 h for 137Cs and 67 h for 90Sr/90Y. Discrimination between beta and gamma radiation is possible using an end cap placed over one of the two paired sensors, allowing simultaneous measurement of 137Cs and 90Sr/90Y in a mixed field. The monitor system represents a substantial improvement over quarterly soil sampling because of a greatly increased measurement frequency and the ability to perform measurements reproducibly.

Dose-Response Relationship, Radiation↗

PubMed2006

Progress towards a polymineral single-aliquot OSL dating procedure.

Published single-aliquot regenerative-dose (SAR) procedures for quartz use a 'cutheat' different from the pre-heat. In contrast, previous work has shown that the SAR procedure can be used to correct for sensitivity changes exhibited by feldspars if the cutheat is equal to the pre-heat. In this paper, a procedure that corrects for sensitivity changes in both quartz and feldspar is tested. The results indicate that for sedimentary quartz the cutheat can be equal to the pre-heat. Thus, it may be possible to develop one procedure to measure equivalent doses in a polymineral sample. We also tested several different optical stimulation methods. Using the infrared (IR)-stimulated signal from a post-IR blue-stimulated optically stimulated luminescence sequence extends the potential age range when using feldpathic materials. In addition, since the post-IR blue-stimulated signal from feldspar is much smaller than the blue-stimulated signal from quartz, the possibility exists of extracting a 'quartz signal' from a mixed mineral sample.

Algorithms↗

PubMed2005

Near-real-time radiotherapy dosimetry using optically stimulated luminescence of Al2O3:C: mathematical models and preliminary results.

In this paper we report investigations aimed toward applying optically stimulated luminescence (OSL) of Al2O3:C for near-real-time medical dosimetry, especially in radiotherapy. The classical mathematical model normally used for the description of OSL phenomena was expanded to predict the behavior of the luminescence signal in the case when the OSL sample is simultaneously irradiated and optically stimulated. The predictions obtained were used to develop different measurement approaches and correction algorithms for the luminescence signals, thus enabling dose estimation from OSL during rather then after the irradiation procedure. Radiation probes with diameters of less than 1 mm, suitable for the envisioned in-vivo measurements were constructed by attaching small Al2O3:C crystals to optical fiber cables. The OSL fiber probes and a purpose-built, portable OSL stimulation and readout system were used to measure doses at speeds up to 1 data point every 3s, under irradiation at dose rates of the same order of magnitude as those found in conventional radiotherapy techniques. The corrected OSL signal was found to be proportional to the absorbed dose, and accurately followed sudden transitions in the irradiation dose rate.

Algorithms↗

PubMed2004

Effect of high-dose irradiation on the optically stimulated luminescence of Al2O3:C.

This paper examines the effect of high-dose irradiation on the optically stimulated luminescence (OSL) of Al2O3:C, principally on the shape of the OSL decay curve and on the OSL sensitivity. The effect of the degree of deep trap filling on the OSL was also studied by monitoring the sensitivity changes after doses of beta irradiation and after step-annealing of samples previously irradiated with high doses. The OSL response to dose shows a linear-supralinear-saturation behavior, with a decrease in the response for doses higher than those required for saturation. This behavior correlates with the sensitivity changes observed in the samples annealed only to 773 K, which show sensitization for doses up to 20-50 Gy and desensitization for higher doses. Data from the step-annealing study leads to the suggestion that the sensitization is caused by the filling of deep electron traps, which become thermally unstable at 1100-1200 K, whereas the desensitization is caused by the filling of deep hole traps, which become thermally unstable at 800-875 K, along with a concomitant decrease in the concentration of recombination centers (F+ -centers). Changes in the shape of the OSL decay curves are also observed at high doses, the decay becoming faster as the dose increases. These changes in the OSL decay curves are discussed in terms of multiple overlapping components, each characterized by different photoionization cross-sections. However, using numerical solutions of the rate equations for a simple model consisting of a main trap and a recombination center, it is shown that the kinetics of OSL process may also be partially responsible for the changes in the OSL curves at high doses in Al2O3:C. Finally, the implication of these results for the dosimetry of heavy charged particles is discussed.

Aluminum Oxide↗

PubMed2004

Optically stimulated luminescence and thermoluminescence efficiencies for high-energy heavy charged particle irradiation in Al2O3:C.

The thermally and optically stimulated luminescence (TL and OSL) response to high energy heavy-charged particles (HCPs) was investigated for two types of Al2O3:C luminescence dosimeters. The OSL signal was measured in both continuous-wave (CW) and pulsed mode. The efficiencies of the HCPs at producing TL or OSL, relative to gamma radiation, were obtained using four different HCPs beams (150 MeV/u 4He, 400 MeV/u 12C, 490 MeV/u 28Si, and 500 MeV/u 56Fe). The efficiencies were determined as a function of the HCP linear energy transfer (LET). It was observed that the efficiency depends on the type of detector, measurement technique, and the choice of signal. Additionally, it is shown that the shape of the CW-OSL decay curve from Al2O3:C depends on the type of radiation, and, in principle, this can be used to extract information concerning the LET of an unknown radiation field. The response of the dosimeters to low-LET radiation was also investigated for doses in the range from about 1-1000 Gy. These data were used to explain the different efficiency values obtained for the different materials and techniques, as well as the LET dependence of the CW-OSL decay curve shape.

Aluminum Oxide↗

PubMed2003

On the advantages and disadvantages of optically stimulated luminescence dosimetry and thermoluminescence dosimetry.

The use of thermoluminescence as a method for the dosimetry of ionising radiation has been established for many decades and has been unquestionably successful. It is therefore difficult to imagine how any new technique could easily supplant it. Perhaps optically stimulated luminescence dosimetry should not be characterised as an entirely new technique, but rather a development of the well established technology that may be considered superior in some respects. As is obvious from the tenor of this debate, our two participants are longtime colleagues who have had numerous discussions on this topic. No doubt, many of us will have similar discussions in our own institutions as we weigh the advantages and disadvantages of TLD versus OSL.

Optics and Photonics↗

PubMed2002

Determination of ionisation energies and attempt-to-escape factors using thermally stimulated conductivity.

A procedure for determining an arbitrary distribution of activation energies (E) and attempt-to-escape frequencies (s) from overlapping contributions to thermoluminescence (TL) or thermally stimulated conductivity (TSC) is described. For the case of no retrapping, i.e. first order kinetics, the glow curve can be described by a two-dimensional Fredholm equation representing a superposition of Randall-Wilkins first-order peak shapes. The solution to this equation gives the distribution of trapping energies and attempt-to-escape-frequency factors necessary to obtain the TL or TSC peak shape. Analysis of simulated TL/TSC data for trap distributions distributed in both E and s demonstrates that the arbitrary E and s values can be determined from the solution of the Fredholm equation. The procedure is demonstrated for experimental TSC data from gamma-irradiated Al2O3:C.

Aluminum Oxide↗

PubMed2002

New millennium frontiers of luminescence dosimetry.

What are the new frontiers' facing us in the new millennium with respect to luminescence dosimetry? I suggest that the first is in methodology. The fast, sensitive optically stimulated luminescence (OSL) techniques developed recently have yielded the potential for rapid environmental monitoring, multiple measurements, dose imaging, and fast readout. New vistas of applications in medical dosimetry and remote dosimetry have opened. A second frontier is literally 'out of this world'--namely, space dosimetry. Extended stays in low Earth orbit and the potential for a 1000-day mission to Mars emphasise the challenges of dosimetry in this unique radiation environment. What role does luminescence dosimetry play in this field? This talk explores the possibilities and the challenges as we seek to penetrate these new frontiers.

Environmental Monitoring↗

PubMed2002

A real-time, fibre optic dosimetry system using Al2O3 fibres.

Al2O3:C optical fibres were examined for potential use as real-time luminescence dosemeters for use in radiotherapy applications. The dosimetric properties of the fibres were studied in order to determine their usefulness as luminescence dosemeters. The measurements were performed by connecting the Al2O3:C fibres to a standard fused silica optical fibre and the optically stimulated luminescence (OSL) and radioluminescence (RL) were measured through the fibre. The OSL and RL responses of the Al2O3 fibre probes were measured during irradiation to determine the potential of the Al2O3:C fibres in a real-time fibre optic dosimetry system.

Aluminum Oxide↗

PubMed2002

Thermal quenching of thermoluminescence in natural quartz.

The thermal quenching properties of the thermoluminescence (TL) from several natural quartz specimens were examined, at two different wavelengths, using the heating rate dependence of the TL signal. As the heating rate varies the TL peaks shift to different temperatures and become affected by thermal quenching to different extents. In this work the heating rate was varied over several orders of magnitude and, through deconvolution of the TL glow curve the behaviour of the main TL peaks was followed as a function of the temperature at which the peak appeared in the glow curve. Through an analysis of the glow peak areas as a function of glow peak temperature the decrease in the efficiency of TL production with increasing temperature could be monitored, at the different emission wavelengths. The analysis supports the contention that the quenching phenomenon is a result of a Mott-Seitz mechanism.

Hot Temperature↗

PubMed2002

Phototransferred thermoluminescence for use in UVB dosimetry.

Results are reported of a study of phototransferred thermoluminescence (PTTL) as a potential method for ultraviolet (UV) radiation dosimetry using several different TL materials. The materials studied in depth include Al2O3:C, CaF2:Cu, MgO and MgO:Cu. The experimental features discussed in this paper include the PTTL response as a function of exposure time at 307 nm and the PTTL wavelength dependence over the range 250-450 nm, using a fixed photon flux at the sample. The study confirms the attractive properties of Al2O3:C as a UV dosimetry material, while CaF2:Cu is shown to have potential as a UVC dosemeter.

Aluminum Oxide↗

PubMed2002

Characterisation of the thermally stimulated conductivity and thermoluminescence of natural topaz.

Thermally stimulated conductivity (TSC) and thermoluminescence (TL) measurements were conducted to investigate the mechanisms of charge transfer and luminescence emission in natural samples of Brazilian topaz irradiated with beta particles from a 90Sr/90Y source or with a 1.75 MeV Van de Graaff electron beam. The luminescence and conductivity were simultaneously monitored during the heating of the samples, allowing direct comparison of the TL and TSC peaks. The results show that the three main TL peaks are accompanied by corresponding TSC peaks, usually shifted to higher temperatures. Comparison of the relative TL/TSC intensities of peaks 2 and 3 indicates that the process of thermal quenching of the luminescence is probably active, which is also supported by TL/TSC measurements at different heating rates. Results on the dose response of TL/TSC peaks also reveal an interesting feature: the TL intensity shows a monotonic increase with dose in the range of study (50 Gy-3 kGy) comprising a linear-supralinear-saturation characteristic, while the TSC peaks exhibit an increase from 50 Gy to 1 kGy, followed by a small decrease for doses greater than 1 kGy. This result is interpreted in terms of a model involving multiple traps and one recombination centre.

Aluminum Silicates↗