COMPARISON BETWEEN PADC AND FNTD NEUTRON DETECTOR SYSTEMS IN BLIND TESTS.

The objective of this study was to compare a neutron dosimetry system based on polyallyl diglycol carbonate (PADC) detectors with a new system based on Al2O3:C,Mg fluorescence nuclear track detectors (FNTD). The irradiations, performed as part of an intercomparison organized by the Physikalisch-Technische Bundesanstalt (PTB), Germany, were on a PMMA phantom with 252Cf or 241Am-Be source, usually with the phantom surface perpendicular to the radiation beam (0° angle), and with Hp(10) values between 0.3 and 7 mSv. One 252Cf irradiation was performed at 30° angle, and one with an additional 1 mSv gamma irradiation. The results showed an agreement between the two techniques with an average and maximum difference between PADCs and FNTDs of 1.5 and 22%, respectively, if one compares only cases of doses >1 mSv. For one of the irradiation conditions with dose of 0.9 mSv, use of the incorrect calibration factor for the FNTD (252Cf instead of 241Am-Be) led to reported values ~×2 larger than the given doses, due to low statistics in the determination of the ratio between 6Li-doped glass and polyethylene neutron converters. Although the FNTD track analysis algorithm may need further development, the results presented here demonstrate the feasibility of the FNTD technology and indicate areas requiring improvements.

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.

CW-OSL measurement protocols using optical fibre Al2O3:C dosemeters.

A new system for in vivo dosimetry during radiotherapy has been introduced. Luminescence signals from a small crystal of carbon-doped aluminium oxide (Al2O3:C) are transmitted through an optical fibre cable to an instrument that contains optical filters, a photomultiplier tube and a green (532 nm) laser. The prime output is continuous wave optically stimulated luminescence (CW-OSL) used for the measurement of the integrated dose. We demonstrate a measurement protocol with high reproducibility and improved linearity, which is suitable for clinical dosimetry. A crystal-specific minimum pre-dose is necessary for signal stabilisation. Simple background subtraction only partially removes the residual signal present at long integration times. Instead, the measurement protocol separates the decay curve into three individual components and only the fast and medium components were used.

Fluence-based dosimetry of proton and heavier ion beams using single track detectors.

Due to their superior spatial resolution, small and biocompatible fluorescent nuclear track detectors (FNTDs) open up the possibility of characterizing swift heavy charged particle fields on a single track level. Permanently stored spectroscopic information such as energy deposition and particle field composition is of particular importance in heavy ion radiotherapy, since radiation quality is one of the decisive predictors for clinical outcome. Findings presented within this paper aim towards single track reconstruction and fluence-based dosimetry of proton and heavier ion fields. Three-dimensional information on individual ion trajectories through the detector volume is obtained using fully automated image processing software. Angular distributions of multidirectional fields can be measured accurately within ±2° uncertainty. This translates into less than 5% overall fluence deviation from the chosen irradiation reference. The combination of single ion tracking with an improved energy loss calibration curve based on 90 FNTD irradiations with protons as well as helium, carbon and oxygen ions enables spectroscopic analysis of a detector irradiated in Bragg peak proximity of a 270 MeV u(-1) carbon ion field. Fluence-based dosimetry results agree with treatment planning software reference.

Characterization of Al2O3 optically stimulated luminescence films for 2D dosimetry using a 6 MV photon beam.

This work evaluates the dosimetric properties of newly developed optically stimulated luminescence (OSL) films, fabricated with either Al2O3:C or Al2O3:C,Mg, using a prototype laser scanning reader, a developed image reconstruction algorithm, and a 6 MV therapeutic photon beam. Packages containing OSL films (Al2O3:C and Al2O3:C,Mg) and a radiochromic film (Gafchromic EBT3) were irradiated using a 6 MV photon beam using different doses, field sizes, with and without wedge filter. Dependence on film orientation of the OSL system was also tested. Diode-array (MapCHECK) and ionization chamber measurements were performed for comparison. The OSLD film doses agreed with the MapCHECK and ionization chamber data within the experimental uncertainties (<2% at 1.5 Gy). The system background and minimum detectable dose (MDD) were <0.5 mGy, and the dose response was approximately linear from the MDD up to a few grays (the linearity correction was <10% up to ~2-4 Gy), with no saturation up to 30 Gy. The dose profiles agreed with those obtained using EBT3 films (analyzed using the triple channel method) in the high dose regions of the images. In the low dose regions, the dose profiles from the OSLD films were more reproducible than those from the EBT3 films. We also demonstrated that the OSL film data are independent on scan orientation and field size over the investigated range. The results demonstrate the potential of OSLD films for 2D dosimetry, particularly for the characterization of small fields, due to their wide dynamic range, linear response, resolution and dosimetric properties. The negligible background and potential simple calibration make these OSLD films suitable for remote audits. The characterization presented here may motivate further commercial development of a 2D dosimetry system based on the OSL from Al2O3:C or Al2O3:C,Mg.

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.

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.

Correlation between OSL and the distribution of TL traps in Al2O3:C.

Al2O3:C single crystals are known to exhibit some variation of their thermoluminescence (TL) glow curve shape from sample to sample. This variation is attributed to the fact that the TL peak in Al2O3:C is produced by the set of several traps. It is important to understand what part of the trap distribution correlates with the optically stimulated luminescence (OSL) process in this material. Crystals with a difference in the full-width, half-maximum (FWHM) of the TL peak as big as 30% and crystals having double-peak glow curves were used in this study. It was found that bleaching with a wavelength longer than 525 nm depletes mostly the low temperature part of the TL glow curve, leaving the high temperature peak unaffected. It was shown that when the pre-irradiation annealing temperature was increased from 500 to 1000 degrees C, the high temperature part of the abnormal double peak disappears. The phototransferred luminescence (PTTL) processes and conversion of F and F+ centres were also studied for samples having different glow curves shape. The practical aspect of this research is important for OSL dosimetry, because such crystals with wide or double TL peak can make it difficult to bleach OSL dosemeters for very low dose measurements.

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.

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.

Automatic neutron dosimetry system based on fluorescent nuclear track detector technology.

For the first time, the authors are describing an automatic fluorescent nuclear track detector (FNTD) reader for neutron dosimetry. FNTD is a luminescent integrating type of detector made of aluminium oxide crystals that does not require electronics or batteries during irradiation. Non-destructive optical readout of the detector is performed using a confocal laser scanning fluorescence imaging with near-diffraction limited resolution. The fully automatic table-top reader allows one to load up to 216 detectors on a tray, read their engraved IDs using a CCD camera and optical character recognition, scan and process simultaneously two types of images in fluorescent and reflected laser light contrast to eliminate false-positive tracks related to surface and volume crystal imperfections. The FNTD dosimetry system allows one to measure neutron doses from 0.1 mSv to 20 Sv and covers neutron energies from thermal to 20 MeV. The reader is characterised by a robust, compact optical design, fast data processing electronics and user-friendly software.

Ion track reconstruction in 3D using alumina-based fluorescent nuclear track detectors.

Fluorescent nuclear track detectors (FNTDs) based on Al2O3: C, Mg single crystal combined with confocal microscopy provide 3D information on ion tracks with a resolution only limited by light diffraction. FNTDs are also ideal substrates to be coated with cells to engineer cell-fluorescent ion track hybrid detectors (Cell-Fit-HD). This radiobiological tool enables a novel platform linking cell responses to physical dose deposition on a sub-cellular level in proton and heavy ion therapies. To achieve spatial correlation between single ion hits in the cell coating and its biological response the ion traversals have to be reconstructed in 3D using the depth information gained by the FNTD read-out. FNTDs were coated with a confluent human lung adenocarcinoma epithelial (A549) cell layer. Carbon ion irradiation of the hybrid detector was performed perpendicular and angular to the detector surface. In situ imaging of the fluorescently labeled cell layer and the FNTD was performed in a sequential read-out. Making use of the trajectory information provided by the FNTD the accuracy of 3D track reconstruction of single particles traversing the hybrid detector was studied. The accuracy is strongly influenced by the irradiation angle and therefore by complexity of the FNTD signal. Perpendicular irradiation results in highest accuracy with error of smaller than 0.10°. The ability of FNTD technology to provide accurate 3D ion track reconstruction makes it a powerful tool for radiobiological investigations in clinical ion beams, either being used as a substrate to be coated with living tissue or being implanted in vivo.

Neutron field characterisation at mixed oxide fuel plant.

A neutron field characterisation was conducted at the AREVA Melox Plant to determine the response of passive and active neutron dosemeters for several stages in the mixed oxide fuel manufacturing process. Landauer Europe provides radiation dosimetry to many contractors working at the Melox site. The studies were conducted to assist in determining the neutron radiation fields the workers are exposed to routinely, evaluate the need for specific neutron correction factors and to ensure that the most accurate neutron dose is reported for the Melox Plant workers.