Improvements in the detection accuracy of miniature optical fibre real time X-Ray radiation sensor based on plastic optical fibre coupled with a fast decay time (10s ns) trivalent cerium activated yttrium aluminum garnet Y3Al5O12 (YAG:Ce) scintillator material is reported. The YAG material is tested only in its powder form. The sensor was irradiated on a Siemens Artiste linear accelerator (linac) and tested for their percentage dose depth (PDD) response whilst being immersed in a standard water test tank in the clinic. The accumulated dose during each test was simultaneously recorded using a standard ionization chamber (PTW Semiflex) which allowed direct comparison and calibration of the PDD response as measured using the Optical Fibre Sensor. Initial experimental results corresponding to the PDD are reported and these demonstrate increased accuracy from previously reported data which is primarily achieved through improvement of exposure conditions for the multi-pixel photon counting (MPPC) detector and monitoring of its real time analogue voltage output.
This paper presents an initial investigation into the depth dependence of an inorganic optical fibre sensor (OFS) based on physical measurements and Monte Carlo (MC) simulations, using a 6 MV flattening filter free (FFF) beam. The OFS was fabricated using an inorganic scintillating material (Gd2O2S:Tb), which was embedded in a cavity of diameter 700 μm, in a 1mm plastic optical fibre. Percentage depth dose (PDD) profiles were measured in a solid water phantom for three field sizes: 10×10 cm2 , 4×4 cm2 and 2×2 cm2 . The OFS results were then compared to an ion chamber and the W1 plastic scintillator. A MC model of an Elekta Versa HD linear accelerator (linac) was developed using the MC software packages BEAMnrc and DOSXYZnrc and then used to simulate the Gd2O2S:Tb and polystyrene scintillators. The OFS measurements over-estimated the dose when compared to the ion chamber and the W1 measurements, across the investigated field sizes, by a maximum of 30%, 20% and 15% for 10×10 cm2 , 4×4 cm2 and 2×2 cm2 , respectively. The MC simulations of the Gd2O2S:Tb and polystyrene scintillators were in good agreement with the W1 and ion chamber measurements, however, the OFS measurements were found to differ across all field sizes. Our results therefore indicate the need for further investigation into the overall contribution of the stem effect to the discrepancy between the OFS physical measurements and the ion chamber and the W1 measurements.
The aim of this study was to investigate the over-response of an inorganic optical fibre sensor (OFS) when measuring percentage depth dose curves (PDDs) with respect to an ion chamber by means of physical measurements and Monte Carlo (MC) simulations. The sensor was constructed by filling a cavity (700 μm diameter and 7 mm deep), which was made in a PMMA (polymethyl methacrylate) plastic optical fibre, with an inorganic scintillating material: terbium doped gadolinium oxysulphide (Gd2O2S:Tb). The MC software packages BEAMnrc and DOSXYZnrc were used to develop a MC model of an Elekta Versa HD linear accelerator (linac), which was then used to simulate the Gd2O2S:Tb scintillator. The results of the PDD measurements showed a depth dependence of the OFS, however the percentage differences between the ion chamber and the OFS measurements showed that as the radiation field size decreases, the difference between the two measurements decreases from 16.5% to 5.1% for 10x10 cm2 and 2x2 cm2, respectively. The MC simulation of the sensor showed a good agreement compared to physical measurements at shallow depth in the phantom; however, discrepancies were observed at depth, which was less pronounced for 4x4 cm2 than for 10x10 cm2. The results of this study indicate that including Cerenkov radiation measurements is essential to accurately quantify the overresponse and the higher discrepancy between the measured and simulated PDD profiles of the OFS.
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