Angular dependence of the TL reading of thin alpha-Al2O3:C dosemeters exposed to different beta spectra

The angular dependence of the thermoluminescent (TL) signal of thin alpha-Al2O3:C dosemeters was investigated for a series of beta-emitting radionuclides commonly employed in nuclear medicine and characterised by different mean energies (99Tc, 177Lu, 90Sr/90Y and 90Y). Irradiations were performed in a controlled geometry, using a properly designed irradiator intended to realistically reproduce the situation of exposure of hospital personnel to beta-emitting pharmaceuticals. Under the conditions of extended source and short source to detector distance, the TL signal of thin alpha-Al2O3:C layers per unit irradiation time was observed to be independent on the angle of incidence within acceptable limits, particularly for those radionuclides with maximum energy >500 keV. This property may be easily explained by using simple physical considerations, such as the limited thickness of the dosemeters. The results confirm that these detectors are suitable for beta-ray extremity dose measurements, when the photon contribution is negligible, as in the case considered.     

A preliminary thermoluminescence and positron annihilation study of alpha-Al2O3:C

The induction of DNA double strand breaks (DSBs) by accelerated heavy ions was systematically measured in diploid yeast cells. Particles were provided by the accelerators at GSI, Darmstadt, and HMI, Berlin. DNA was separated using pulsed field gel electrophoresis and the intensity of the largest bands used to determine the loss of molecular weight. Since the DNA content of each chromosome is exactly known absolute values for DSB induction can be measured without calibration procedures. Ions used range from protons to uranium with LET values between 2 and about 15,000 keV.micron-1. Induction cross sections increase in the lower LET region approaching a plateau around 200 keV.micron-1. With higher LET values the dependence can no longer be described by a common curve with each ion showing a specific behaviour. With very heavy particles the influence of the penumbra becomes obvious: cross sections decrease with LET because of the reduced penumbra extensions. Classical target theory would predict cross sections to follow a simple saturation function which is not substantiated by the data. Track structure analysis as introduced by Butts and Katz in 1967 is also not able to predict the experimental results. A semi-empirical fit indicates a linear-quadratic dependence of induction cross sections on LET up to about 1000 keV.micron-1. RBE for DSB induction rises above unity reaching a maximum of about 2.5 around 200 keV.micron-1. This is different from many experiments in mammalian cells and is presumably due to differences in chromatin structure since yeast cells seem to lack a functional III histone.     

The sunlight OSL response of a commercially available alpha-Al2O3:C personnel dosimetry material

Carbon-doped, anion-defective aluminium oxide has become a widely used and effective medium for personnel dosimetry applications using optically stimulated luminescence (OSL) techniques. Though the commercial products currently using this material employ light-tight packaging to prevent light-induced effects on the OSL signal, the material could be employed in environments where package integrity cannot be assured. This paper reports on the results of an experiment performed to quantify the effects of sunlight exposure on alpha-Al2O3:C. Samples of commercially available Luxel material were exposed to carefully recorded levels of sunlight both before and after irradiations to determine the nature and magnitude of both activation and fading phenomena in this material. The results confirm that both fading and activation processes are seen in this material and indicate that the material reaches an equilibrium dose level in response to prolonged sunlight exposure equivalent to a dose of approximately 15 mGy under the experimental test conditions.     

Preparation of nanocrystalline alpha-Al2O3 using plant fiber

Nanocrystalline alpha-Al2O3 ceramic powders have been prepared from aqueous poly vinyl alcohol (PVA)-aluminum nitrate solution and jute (plant fiber). Soluble Al-ion-PVA solution formed a uniform coating on the surface of jute once it dried completely. Slow hydrolysis of aluminum ion with ammonium hydroxide deposited aluminum hydroxide on the jute surface. Decomposition of the dried aluminum hydroxide-coated jute at high temperature (1150 degrees C/2 hrs) resulted in the formation of single-phase, nanocrystalline alpha-Al2O3 with the corresponding average X-ray and TEM particle size approximately 50 nm. Precursor and heat-treated powders have been characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analysis. This method can be used to produce high-temperature as well as low-temperature nanocrystalline oxides.     

Design of a finger ring extremity dosemeter based on OSL readout of alpha-Al2O3:C

A finger-ring dosemeter and reader has been designed that uses OSL readout of alpha-Al2O3:C (aluminium oxide). The use of aluminium oxide is important because it allows the sensitive element of the dosemeter to be a very thin layer that reduces the beta and gamma energy dependence to acceptable levels without compromising the required sensitivity for dose measurement. OSL readout allows the ring dosemeter to be interrogated with minimal disassembly. The ring dosemeter consists of three components: aluminium oxide powder for measurement of dose, an aluminium substrate that gives structure to the ring, and an aluminised Mylar cover to prevent the aluminium oxide from exposure to light. The thicknesses of the three components have been optimised for beta response using the Monte Carlo computer code FLUKA. A reader was also designed and developed that allows the dosemeter to be read after removing the Mylar. Future efforts are discussed.     

Radiation damage induced by krypton ions in sintered alpha-Al2O3

Alpha-alumina is a useful thermoluminescence (TL) dosemeter. The knowledge of its behaviour under irradiation is thus of primary importance. The purpose of this paper is to characterise the radiation damage produced by swift krypton ions using various experimental methods, namely TL, optical absorption, fluorescence and electron paramagnetic resonance (EPR). After ion irradiation, the TL intensity is shown to decrease, whereas the optical absorption rises in the whole studied wavelength range. These two phenomena seem to be related to one another. Furthermore, optical absorption measurements highlight the appearance of new absorption bands probably owing to oxygen vacancies. Induced defects are also observed in the EPR spectra of irradiated pellets. They are likely related to electronic holes trapped on oxygen ions. The concentration of these defects increases with ion fluence and fluorescence measurements indicate that some pre-existing defects such as F2(2+) centres follow the same trend up to approximately 4.1 x 10(13) ions cm(-2).     

A new model for numerical analysis of simultaneously thermally stimulated luminescence and exoelectron emission phenomena of alpha-Al2O3 single crystals

An efficient model to analyse numerically the simultaneously detected thermally stimulated luminescence (TL) and exoelectron emission (TSEE) dosimetric peak D' (450 K) of a-alumina is proposed. In this model, the TSEE obeys a thermionic mechanism. To take into account the fact that TL is a bulk phenomenon and that TSEE results from surface processes, a multilayer structure of the sample is adopted. In this work, two different approaches are discussed and compared. The first implies that the charge neutrality condition is applied to the sample alone, whereas in the second, the condition concerns the sample added to the detection equipment. The two models are applied to the dosimetric peak D' (at about 450 K) of alpha alumina and give very good results in both cases.     

Thermoluminescence glow curves and optical stimulated luminescence of undoped alpha-Al2O3 crystals

The characteristics of thermoluminescence (TL) and optical stimulated luminescence (OSL) in undoped alpha-Al2O3 single crystals were studied. The TL glow curves of the crystal samples irradiated at various dose levels were measured by Ris? TL/OSL-DA-15B/C reader with U-340 or 7-59 filters at different heating rates. The glow peaks measured with U-340 at approximately 210 degrees C of the undoped alpha-Al2O3 can be well fitted by first-order kinetic equation whereas the glow peaks measured with 7-59 filters are a composite of two first-order glow peaks. It indicates that the TL glow curves are dependent upon the filter used in the reader that is related to the emission spectra of luminescence materials. The OSL were also measured and fitted by two exponential functions to get the luminescence intensities. The TL and OSL dose responses of the undoped alpha-Al2O3 crystal were obtained in the dose range of 0.12-248 Gy and fitted by the composite action dose-response function to get nonlinear characteristic parameters. The TL and OSL dose responses are linear-sublinear.     

Influence of the irradiation temperature on the dosimetric and high temperature TL peaks of Al2O3:C

The TL glow curves of Al2O3:C crystals have been investigated as a function of the irradiation temperature. The nature of the observed TL peaks has been studied by optical annealing. The filling of traps was found strongly dependent on the irradiation temperature in the case of UV exposure, which has been explained by the temperature dependence of the photoionisation of F centres. This latter phenomenon could have a part in the luminescence quenching and UV bleaching of F centres.     

Thermoluminescent and dosimetric properties of anion-defective alpha-Al2O3 single crystals with filled deep traps

Some new experimental results illustrating the effect of deep traps on luminescent and dosimetric properties of anion-defective single crystals of alpha-Al2O3 have been described. It was found that deep traps had an electronic origin. They were filled thanks to the photoionisation of F-centres and their filling was accompanied by the conversion of F-->F+ centres. The experiments revealed an interactive interaction of deep trapping centres. A model taking into account the thermal ionisation of excited states of F-centres was proposed. This model describes the trap filling process and mechanisms of the radio-, photo- and thermoluminescence, TSC and TSEE of the crystals under study. The sensitivity of TLD-500 detectors based on anion-defective alpha-Al2O3 equalised when deep trapping centres were filled.     

Thin layer alpha-Al2O3:C beta dosemeters for the assessment of current dose rate in teeth due to 90Sr intake,and comparison with electron paramagnetic resonance dosimetry

The use of thin-layer alpha-Al2O3:C thermoluminescent detectors (TLDs) for the assessment of current beta dose rate in human teeth due to 90Sr intake is investigated. The teeth used in this study were collected from members of the Techa river population who were exposed to radiation as a result of releases of the Mayak plutonium production facilities (Southern Urals-Russia) between 1949 and 1956. The beta dose rates from different parts of the tooth (enamel, crown dentine, and root) were determined by storing the detectors over the samples in a shielded environment. The cumulative dose measured by electron paramagnetic resonance (EPR) in different dental tissues is found to be proportional to current dose rate obtained from alpha-Al2O3:C thermoluminescence dosemeters. The retention of 90Sr in various parts of the teeth is discussed.     

New model for the In2O3-SnO2 system

The In₂O₃-SnO₂ (ITO) solid solution system was formulated and a model for the ITO material derived. The model starts from the formation of a solid solution — giving rise to a doping effect up to the solid solubility of tin in the In₂O₃ lattice, together with the formation of a new associated vacancy (V_InV_o)^x. The mechanism is discussed by which the (V_InV_o)^x dissociates, reducing the number of free carriers through trapping, which in turn affects the electrical characteristics.Seconded to the University of Salford.     

Influence of thermal quenching on the thermostimulated processes in alpha-Al2O3. Role of F and F+ centres

Anion deficient alpha-Al2O3 is highly sensitive to ionising radiations and is widely used as a thermoluminescence and optically stimulated luminescence dosemeter in environmental monitoring. Two types of alpha alumina were studied and it was observed that both were affected by thermal quenching of luminescence. This effect, which manifests itself by the decay of the TL response when the heating rate increases, can be described by the Mott-Seitz theory. It was observed that thermostimulated exoemission response increased when the heating rate increased, whereas thermostimulated conductivity remained constant. However, none of the available theories could explain the dependence of the F- centre emission on the heating rate. A model is proposed to describe simultaneously the various thermally stimulated processes.     

Quenched metastable phases and their transformations on heating in the Bi2O3-WO3 system

A single-roller apparatus was used to quench 16 melts in the Bi₂O₃-WO₃ system. Metastable polycrystalline phases were obtained in all compositions. Quenched specimens and their phase transformations on heating were examined by X-ray diffraction, differential thermal analysis, density measurement, electrical measurement and observation of optical anisotropy. F c c solid solutions of the type, 2WO₃ 2W ^Bi + 30 ^pI +30 _X ^PO occurred in the quenched samples for 18 to 32 mol % WO₃. Quenched phases remained unchanged with only a slight structural relaxation at temperatures lower than the first exotherm. Near the first exotherm, most samples showed an irregular change in the lattice parameter(s).     

Thermal neutron response of the Li2B4O7:Cu TL dosimeter

A measurement of the thermal neutron response of the Li₂B₄O₇:Cu TLD has been conducted. The results obtained using the Panasonic UD-806 dosimeter and UD-854A holder yield a free-in-air response of 3.3±0.1 R ⁶⁰Co equivalent per mSv of thermal neutrons. A thermal neutron response of over 7 R ⁶⁰Co equivalent per mSv was observed when the dosimeter was irradiated on water phantoms. The high sensitivity may result in a substantial overestimate of the gamma dose equivalent if the TLD is used in a mixed neutron and gamma environment of unknown ratio. Measurements of the Li₂B₄O₇:Cu glow curve, TL saturation curve and the thermal neutron response dependence of the dosimeter filtration thickness are also presented.