New Al2O3:C,Mg crystals for radiophotoluminescent dosimetry and optical imaging

Optical and dosimetric properties of a new radiophotoluminescent material based on aluminum oxide doped with carbon and magnesium (Al2O3:C,Mg) and having aggregate oxygen vacancy defects are presented. The Al2O3:C,Mg crystals are characterized by several new optical absorption and emission bands. It is suggested that the main optical properties of this material are due to the formation of aggregate defects composed of two oxygen vacancies and two Mg-impurity atoms. Radiation-induced optical absorption bands are centered at 335 and 620 nm and produce fluorescent emission at 750 nm with a 75 +/- 5 ns lifetime. The dose measurements are performed by illumination of the Al2O3:C,Mg crystal with 335 nm or 650 nm light and by measuring the intensity of the 750 nm fluorescence. The detector material is insensitive to room light before and after the irradiation and the traps are stable up to 600 degrees C. A dose measurement range between 5 mGy and 200 Gy, suitable for therapeutic radiology applications, was demonstrated. The short luminescent lifetime and nondestructive readout is favorable for imaging applications.     

An algorithm for real-time dosimetry in intensity-modulated radiation therapy using the radioluminescence signal from Al2O3:C

Although the radioluminescence (RL) signal from optical fibre Al(2)O(3):C dosemeters used in medical applications is essentially proportional to dose rate, the crystals used so far are imperfect in the sense that their RL sensitivity changes with accumulated dose. A computational algorithm has been developed that corrects for these sensitivity changes. We further report on a new system that effectively separates the RL signal generated in the crystal from fluorescence and Cerenkov emission generated in the optical fibre cable using a gating technique in connection with pulsed linear accelerator radiation beams. The dosimetry system has been used for dose measurements in a phantom during an intensity-modulated radiation therapy (IMRT) treatment with 6 MV photons. The RL measurement results are in excellent agreement (i.e. within 1%) with both the OSL results and the dose delivered according to the treatment planning system. RL signals from Al(2)O(3):C can be used for real-time dose rate measurements with a time resolution of approximately 0.1 s and a spatial resolution only limited by the size of the detector (<0.5 mm).     

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.     

Monte Carlo based voxel phantoms for in vivo internal dosimetry

The purpose of this communication is to describe briefly the computer programs developed to generate the MCNP input file corresponding to any segmented tomographic data and its application to the calibration procedures for in vivo internal dosimetry. The method has been applied to the determination of 241Am in bone by measurement in skull and knee using MCNP voxel models of a real human head and knee based on the tomographic Voxelman and Arms Down phantoms developed by Zubal et al. at Yale University.     

Recent advances in dosimetry using the optically stimulated luminescence of Al2O3:C

This paper presents an overview of some very recent developments in optically stimulated luminescence (OSL) dosimetry using aluminium oxide (Al(2)O(3):C), with special emphasis given to the work of the research group at Oklahoma State University. Some of the advances are: (i) the development of a real-time optical fibre system for in vivo dosimetry applied to radiotherapy; (ii) the development of a fibre dosimetry system for remote detection of radiological contaminants in soil; (iii) the characterisation of Al(2)O(3):C in heavy charged particle fields and the study of ionisation density dependence of the OSL from Al(2)O(3):C; and (iv) fast and separate assessment of beta and gamma components of the natural dose rate in natural sediments. These achievements highlight the versatility of the OSL technique associated with the high-sensitivity of Al(2)O(3):C for the development of new dosimetry applications.     

Comparative study of the luminescence of Al2O3:Ti and Al2O3 crystals under VUV synchrotron radiation excitation

The comparative study of the luminescent properties of Al₂O₃:Ti crystal in comparison with those for undoped Al₂O₃ crystal counterpart is performed under synchrotron radiation excitation with an energy of 3.7–25 eV. Apart from the main emission band peaked at 725 nm related to the ²E → ²T₂ radiative transitions of Ti³⁺ ions, the luminescence of excitons localized around Ti ions in the band peaked at 290 nm and the luminescence of F⁺–Ti and F–Ti centers in the bands peaked at 325 and 434 nm are also found in the emission spectra of Al₂O₃:Ti crystal. We show also that the luminescence of Ti³⁺ ions in Al₂O₃:Ti crystal can be effectively excited by the luminescence of excitons localized around Ti dopant as well as by the luminescence of F–Ti centers.     

Novel Al2O3:C,Mg fluorescent nuclear track detectors for passive neutron dosimetry

The latest advances in the development of a fluorescent nuclear track detector (FNTD) for neutron and heavy charged particle dosimetry are described and compared with CR-39 plastic nuclear etched track detectors (PNTDs). The technique combines a new luminescent aluminium oxide single crystal detector (Al(2)O(3):C,Mg) with an imaging technique based on laser scanning and confocal fluorescence detection. Detection efficiency was obtained after irradiations with monoenergetic neutron and proton beams. Dose dependences were measured for different configurations of the detectors exposed in fast- and thermal-neutron fields. A specially developed image processing technique allows for fast fluorescent track identification and counting. The readout method is non-destructive, and detectors can be reused after thermal annealing.     

Superlinear dose dependence of high temperature thermoluminescence peaks in Al2O3:C

We report on strong superlinearity of high temperature (300-700 degrees C) thermoluminescence (TL) peaks in alpha-Al2O3:C powder. The main dosimetric peak at approximately 200 degrees C, previously found to exhibit slight superlinearity, is now shown to yield a slope of approximately 1.15 on a log(intensity) vs. log(dose) curve. Higher temperature peaks at approximately 320, 450 and 650 degrees C, previously reported following UV and X-ray excitation, have now been observed after beta irradiation and showed stronger superlinearity. Using a blue filter, the weak peak at approximately 320 degrees C started about quadratically and the slope on the log-log scale decreased gradually from 2 to about unity at approximately 100 Gy. The two higher temperature peaks at approximately 450 and approximately 650 degrees C also exhibited a strong superlinear dose dependence in this dose range, with an average slope of approximately 2. Roughly similar behaviour has been found when a UV filter was used. The possible explanation of the strong superlinearity is discussed.     

A Monte Carlo method for calculating Bayesian uncertainties in internal dosimetry

This paper presents a novel Monte Carlo method (WeLMoS, Weighted Likelihood Monte-Carlo sampling method) that has been developed to perform Bayesian analyses of monitoring data. The WeLMoS method randomly samples parameters from continuous prior probability distributions and then weights each vector by its likelihood (i.e. its goodness of fit to the measurement data). Furthermore, in order to quality assure the method, and assess its strengths and weaknesses, a second method (MCMC, Markov chain Monte Carlo) has also been developed. The MCMC method uses the Metropolis algorithm to sample directly from the posterior distribution of parameters. The methods are evaluated and compared using an artificially generated case involving an exposure to a plutonium nitrate aerosol. In addition to calculating the uncertainty on internal dose, the methods can also calculate the probability distribution of model parameter values given the observed data. In other words, the techniques provide a powerful tool to obtain the estimates of parameter values that best fit the data and the associated uncertainty on these estimates. Current applications of the methodology, including the determination of lung solubility parameters, from volunteer and cohort data, are also discussed.     

Fibre remote optoelectronic gamma dosimetry based on optically stimulated luminescence of Al2O3:C

Optically stimulated luminescence dosimetry (OSL-D) used in conjunction with fibre optics enables a remote measurement of dose, for the purpose of radioprotection in the nuclear industry and in medicine (radiology, radiotherapy). Alumina OSL crystals are used because of their low Z, low fading and optical transparency, which improves the sensitivity. An optoelectronic portable dosemeter has been designed and tested that shows a dose detection of 50 microGy with a 20 metre-long fibre. Following irradiation, all trapped electrons are released under light stimulation while the OSL is integrated to provide dose-equivalent measurements. A compensation technique is designed with the help of the MCNP4b code, so that both angular and photon energy characteristics comply with international standards (CEI 61066) for photon dose equivalent Hp(10). Two sensors are described that allow measurements over a wide solid angle (95% of 4piSr), for photon energies ranging from 15 keV to 3 MeV.     

Ionisation density dependence of the optically and thermally stimulated luminescence from Al2O3:C

This paper presents an overview of recent results on ionisation density dependence of the thermally stimulated luminescence (TL) and optically stimulated luminescence (OSL) signals from Al2O3:C, with emphasis on the sensitivity, efficiency, shape of the TL/OSL curves and the emission spectrum. High-ionisation densities are created uniformly by accumulated high doses of low-linear energy transfer radiation (gamma, beta, X rays) or non-uniformly in heavy charged particle tracks, even at low fluences, as in the case of space radiation fields. Significant deep trap filling, which occurs at these high-ionisation densities, ultimately results in changes in the concentration of recombination centres (F+-centres) and, consequently, in sensitivity changes and other effects. An OSL emission band at 335 nm has been observed in addition to the main F-centre luminescence band, and the relative intensities of these bands have been observed to be dependent on the ionisation density. The implications of these results and open issues are discussed.     

Photoluminescence studies on energy transfer processes in cerium-doped Gd3Al2Ga3O12 crystals

We have measured photoluminescence properties of cerium-doped Gd₃Al₂Ga₃O₁₂ (Ce:GAGG) crystals at low temperatures with use of synchrotron radiation. Excitation spectra for the Ce³⁺ 5d–4f emission exhibit prominent peaks at Gd³⁺ intra-4f absorption bands. The Gd³⁺ intra-4f emission band is observed at 3.91 eV, but is not in resonance with the lowest energy Gd³⁺ intra-4f absorption band at 3.95 eV. The temperature dependence of the Gd³⁺ emission intensity is not correlated with that of the Ce³⁺ emission intensity. Decay curves of the Ce³⁺ emission were also measured at 9 K under excitation at various photon energies. The decay curve is remarkably changed, depending on the excitation photon energies. The present results give us hints to understand the whole of energy transfer processes in Ce:GAGG crystals.     

Uncertainties in personal dosimetry for external radiation: a Monte Carlo approach

This paper explores the possibilities of numerical methods for uncertainty analysis of personal dosimetry systems. Using a numerical method based on Monte Carlo sampling the probability density function (PDF) of the dose measured using a personal dosemeter can be calculated using type-test measurements. From this PDF the combined standard uncertainty in the measurements with the dosemeter and the confidence interval can be calculated. The method calculates the output PDF directly from the PDFs of the inputs of the system such as the spectral distribution of the radiation and distributions of detector parameters like sensitivity and zero signal. The method can be used not only in its own right but also for validating other methods because it is not limited by restrictions that apply to using the Law of Propagation of Uncertainty and the Central Limit Theorem. The use of the method is demonstrated using the type-test data of the NRG-TLD.     

EX vivo ESR measurements associated with Monte Carlo calculations for accident dosimetry: application to the 2001 Georgian accident

Electron spin resonance (ESR) spectroscopy and the Monte Carlo (MC) technique were jointly applied to the physical dose reconstruction of the accident that occurred on 2 December 2001 in Georgia. Three people were exposed to two very-high-activity (2.6 x 10(15) Bq) 90Sr sources. Following this exposure, the two most seriously affected victims exhibited severe radiological injuries localised in the back as well as a haematopoietic syndrome. The information concerning the circumstances of the accident in terms of sequences of irradiation and the exposure time was not clearly established. The physical dose reconstruction of the accident was performed for one victim, treated in France, using both MC simulations and ESR measurements made on one vertebra and two rib samples removed from the victim for medical reasons. The complementary nature of the two tools made it possible to estimate the dose distribution within the body with reasonable accuracy and helped to develop the treatment strategy.     

Personal dosimetry in terms of HP(3): Monte Carlo and experimental studies

H(p)(3) has been defined as the operational quantity for eye lens dosimetry. H(p)(3)/ka conversion coefficients were evaluated at the GSF (Germany) in a 30x30x15 cm(3) 4-elements ICRU slab phantom for various energies and incident angles through Monte Carlo. The ISO report 12,794 suggests to employ a PMMA water filled phantom, of the same dimensions, for dosemeter calibration in terms of H(p)(3). The present paper briefly summarises the main aspects of a study carried out at ENEA-Radiation Protection Institute (Bologna, Italy) to provide practical procedures for the calibration of dosemeters in terms of H(p)(3). Tabulations of a new set conversion coefficients and air kerma backscatter factors are provided as a function of energy and incident angle. The paper demonstrates that a more accurate approach to the dosimetric assessment in terms of H(p)(3) could be rather simply introduced employing a reduced phantom.     

Monte Carlo study of MOSFET dosemeter characteristics: dose dependence on photon energy, direction and dosemeter composition

MOSFET dosemeters are emerging as a versatile tool in various medical physics and health physics dose measurements. It is an important but difficult task to understand their energy and directional dependences because of their unique features. This paper presents a study to characterise a MOSFET dosemeter using Monte Carlo simulation method. Monoenergetic photon beams ranging from 15 to 6 MeV were simulated to study the energy and angular dependences. The results were compared with published experimental data. The Monte Carlo model also provided insightful information on optimising the dosemeter design by examining how various regions of the dosemeter contributed to the dose. Detailed energy deposition processes were further analysed by tracking individual particles inside the dosemeter.     

Temperature dependence of the Henry's law constant for hydrogen storage in NaA zeolites: a Monte Carlo simulation study

Grand canonical Monte Carlo simulations of hydrogen adsorption in zeolites NaA were carried out for a wide range of temperatures between 77 and 300 K and pressures up to 180 MPa. A potential model was used that comprised of three main interactions: van der Waals, coulombic and induced polarization by the electric field in the system. The computed average number of adsorbed molecules per unit cell was compared with available results and found to be in agreement in the regime of moderate to high pressures. The particle insertion method was used to calculate the Henry coefficient for this model and its dependence on temperature.     

Confocal fluorescent imaging of tracks from heavy charged particles utilising new Al2O3:C,Mg crystals

A completely optical, non-destructive imaging of tracks in a fluorescent crystal provides a new way to detect and to assess doses from heavy charged particles and neutrons. The technique combines confocal fluorescent microscopy with a new radiation-sensitive, luminescent material based on aluminium oxide single crystals doped with carbon, magnesium and having aggregate oxygen vacancy defects (Al2O3:C,Mg). Radiation-induced colour centres in the new material have an absorption band at 620 nm and produce fluorescence at 750 nm with a high quantum yield and a short, 75 +/- 5 ns, fluorescence lifetime. Three-dimensional spatial distribution of fluorescent intensity allows one to obtain depth-dose distributions and to discriminate between high- and low-linear energy transfer radiations. Images of single tracks produced by different types of radiation have been obtained. Irradiations with a calibrated 241Am alpha source showed high efficiency for track detection. Thermal neutrons were detected using a nuclear reaction with a 6LiF radiator and production of alpha particles and tritium ions. Fast neutrons were detected using recoil protons produced in a polyethylene radiator installed in front of the crystalline detector. Three-dimensional reconstruction of a recoil proton propagating through the crystal was demonstrated.     

Plastic deformation of Al2O3 single crystals by indentation at temperatures up to 750° C

Vickers hardness measurements have been made on three differently oriented sapphire single crystals in the temperature range from room temperature to 750° C. The hardness generally decreased with increasing temperature but varied with specimen orientation. In the regions surrounding the indents, twinning lamellae and slip lines have been studied systematically and could be associated with dislocation systems. Rhombohedral {01¯1 2} and basal (0 0 0 1) twinning as well as prismatic {1 1¯ 2 0} and basal (0001) slip have been observed. Plastic deformation occurred during placement of Vickers microhardness indentations. The hardness was markedly influenced by the number and kind of activated systems. A change in the activated systems caused variation and inversion in hardness ratios. Investigating high-index planes gave much more information on anisotropy effects than would have been obtained had only basally and/or prismatically oriented specimens been used. The latter is commonly the case in the literature. It was shown that in brittle materials, such as ceramics, plastic deformation occurs at temperatures below 0.5T/T _m if a stress field with a large hydrostatic component is applied. It is suggested that this is also the case in abrasion.