Showing papers by "Stephen W.S. McKeever published in 1999"

A Radiation Dosimetry Method Using Pulsed Optically Stimulated Luminescence

Abstract: A method for the determination of absorbed radiation dose is described based on pulsed optically stimulated luminescence (POSL). The method relies upon the stimulation of an irradiated sample with a train of light pulses from a suitable light source (e.g. a laser using a wavelength which is within the range of wavelengths corresponding to the radiation-induced optical absorption in the irradiated sample. The subsequent emitted light, due to the detrapping of trapped charges and their subsequent recombination with charge of the opposite sign, is synchronously detected in the period between each stimulation pulse. The total luminescence is summed over the desired number of stimulation pulse and this forms the measured POSL signal. By monitoring the emitted light only in the period between stimulation pulses one can reduce the optical filtering required to discriminate between the stimulation light and the emission light: in this way a high measurement efficiency, and, therefore, a high radiation sensitivity (luminescence intensity per unit absorbed dose) is achieved. Key parameters in the method are the intrinsic luminescence lifetime for the material being used as the luminescent detector, the width of the optical stimulation pulse. and the period between pulses. For optimum operation the measurement parameters should be such that both the pulse width and the time between pulses are much less than the luminescence lifetime. By appropriate choice of the power of the optical stimulation, the frequency of the stimulation pulses, and the total stimulation period, one can also re-measure the absorbed dose several times. In this way, a re-read capability is available with the procedure. The method is illustrated using light from a 2nd-harmonic Nd:YAG laser, with irradiated, anion-deficient aluminium oxide as the luminescent detector material.

Characterisation of Al2O3 for Use in Thermally and Optically Stimulated Luminescence Dosimetry

Abstract: This paper discusses several materials aspects which are important in the development and optimisation of Al 2 O 3 :C for both thermoluminescence (TL) and optically stimulated luminescence (OSL) applications, including aspects of crystal growth, luminescence spectra, trap energy distributions and general dosimetric properties.

Radiation Dosimetry using Pulsed Optically Stimulated Luminescence of Al2O3:C

Abstract: A method of radiation dosimetry is described using pulsed optically stimulated luminescence (POSL). An irradiated sample is stimulated with a series of light pulses from a laser tuned to an appropriate wavelength. The emitted light, due to the detrapping of charges and their subsequent recombination with charges of the opposite sign, is synchronously detected in the period between each stimulation pulse. The total luminescence is summed over the desired number of stimulation pulses and this forms the measured POSL signal. Key parameters in the method are the intrinsic uminescence lifetime for the material being used as the luminescent detector, the width of the optical stimulation pulse, and the period between pulses. By appropriate choice of the power of the optical stimulation, the frequency of the stimulation pulses, and the total stimulation period, one can also re-measure the absorbed dose several times. The method is illustrated using a Nd.YAG laser and aluminium oxide as the luminescent material.

Thermally Stimulated Conductivity and Thermoluminescence from Al2O3:C

Abstract: Simultaneous measurements of thermoluminescence (TL) and thermally stimulated conductivity (TSC) are reported on single-crystal dosimetry-quality Al 2 O 3 :C following gamma irradiation at room temperature. Analysis of the data reveals a superposition of several first-order TL and TSC peaks caused by release of charge carriers from a distribution of trapping states. The TSC curves are analysed using a deconvolution routine based on the Fredholm equation and assumes a constant frequency factor.

Thermal Quenching of F-centre Luminescence in Al2O3:C

Abstract: Measurements of time-resolved photoluminescence reveal an F centre luminescence lifetime of 35 ms in Al 2 O 3 :C at room temperature, decreasing to less than 2 ms over the temperature range from 370 to 500 K. The decrease in the lifetime follows a classical Mott-Seitz dependence for thermal quenching of luminescence, with an activation energy W of 1.08 ± 0.03 eV and a frequency factor v of 10 14 s -1 . Similar values for the energy and frequency factor were also obtained from an analysis of thermoluminescence (TL) glow curves. The parameters obtained were independent of the glow curve shape, the degree of trap filling, and the specific conditions under which the crystals were grown. It is concluded that the heating rate dependence of the TL from Al 2 O 3 :C is a result of thermal quenching of the F centre emission.

System and method for the detection of abnormal radiation exposures using pulsed optically stimulated luminescence

Abstract: This invention relates generally to luminescence techniques for imaging radiation fields and, more specifically, to the use of experimental and mathematical methods to distinguish between dynamic irradiation and static - or other abnormal radiation - exposure conditions for applications in personnel and environmental radiation dosimetry, or related fields. In more particular, the instant invention provides a rapid and reliable method of detecting abnormal dosimeter exposure conditions over a wide dynamic range of radiation doses, while avoiding significant background interference and stimulation light leakage. Additionally, the preferred embodiment of the instant invention uses a pulsed and synchronized luminescence detection scheme. Further, this invention teaches a complete method and system for abnormal exposure detection - including the use of a luminescent thin powder layer, the use of a periodic radiation absorbing filter, the pulsed stimulation and synchronized luminescence detection scheme, and the method of analyzing and interpreting the recorded images. Finally, the system provides a means of mathematically characterizing an image as containing either a normal or abnormal exposure.

Optically Stimulated Luminescence Response of Al2O3 to Beta Radiation

Abstract: High sensitivity dosemeters based on Al 2 O 3 :C have been prepared and tested for use as beta dosemeters using optically stimulated luminescence (OSL). Two types of sample were prepared and tested, namely unpolished thick, single crystal chips and thin powder layers on aluminium substrates. The samples were used in two OSL readout modes: (a) cw-OSL using a tungsten lamp, and (b) pulsed-OSL using a Nd:YAG laser. Comparisons were made with TL properties, where possible. The samples were irradiated using a variety of sources, including 147 Pm, 204 Tl, 90 Sr- 90 Y and 60 Co, and under a variety of cover thicknesses. The response per unit H p (0.07), normalised to 60 Co, is compared for each dosemeter type and discussed within the framework of DOELAP and EU recommended limits.

An Integrating UVB Dosemeter System

Abstract: Recent evidence of sharp increases in the number and severity of deformations in many amphibian species worldwide resulting from increased ultraviolet-B (UVB) exposure has prompted the development of a new integrating UVB dosemeter system. This dosemeter can be read out using either thermoluminescence (TL) or optically stimulated luminescence (OSL) techniques. The detection method takes advantage of the photqtransfer properties of Al 2 O 3 :C in the UVB region (280-320 nm). These UVB dosemeters have a linear dynamic range of several orders of magnitude and no significant temperature dependence. The inherent angular dependence of the interference filter has been flattened using diffusers and the increased phototransfer efficiency of Al 2 O 3 :C at shorter UVB wavelengths. The new UVB dosemeters can be used in air or in water and require no external power source. Their small, lightweight and watertight design make them ideally suited for measuring the UVB exposure of amphibian egg sacks in situ.

Method of preparing detection materials for use in UV detection using phototransferred thermoluminescence

Abstract: The instant invention teaches a method of preparing a luminescent detecting material for use in UV dosimetry which utilizes phototransferred luminescence. The detecting material has a set of shallow dosimetry traps for trapping electronic charge carriers, which are thermally released upon heating to a first temperature, and a set of deep traps for trapping electronic charge carriers, which charge carriers are released upon heating to a second temperature. The detecting material is prepared by irradiating the detecting material to fill the shallow and deep traps with charge carriers, heating the material to release charge carriers from the shallow traps, and then cooling the material. When the detecting material is subsequently exposed to ultraviolet light a proportion of charge carriers will be released from the deep traps to be retrapped in the shallow traps, thereby allowing for the measurement of phototransferred luminescence by thermal or optical stimulation.