Abstract
Purpose: To identify the chemical and or physical cause of a threshold dose in the low dose response of FXG gel dosimeters. Methods:Gels were prepared by mixing stock solutions of ferrous‐ammonium‐sulphate and different chemical supplies of xylenol‐orange (XO) together with sulfuric acid into a solution of gelatin and distilled water. Concentrations of ferrous‐ammonium‐sulphate, XO and sulfuric acid were varied to identify one of these chemicals as a source of impurities potentially causing the threshold dose effect. Samples in 1 or 10cm acrylic cuvettes were irradiated to doses between 0–5Gy. Optical transmissions were measured through the samples and referenced to water at 589 and 543nm, respectively, using an absorption spectrophotometer. This procedure was repeated for gelatin‐free FX solutions and with gel containing varying concentrations of ferric‐ammonium‐sulphate. Results: The threshold dose decreased to 0.20±0.05 Gy using reagent grade XO. There was no correlation between threshold dose and varying concentrations of ferrous and sulphuric acid although decreasing the XO concentration from 0.05 to 0.025mM lowered the threshold dose by a factor of two. Gelatin‐free FX solutions showed a threshold dose. Using a ferric concentration of 7.5uM decreased the threshold dose to 0.04±0.03 Gy. The threshold dose effect was not present at 543nm. Conclusions: Chemical impurities are not responsible for the threshold dose. The “apparent” threshold dose is caused by the production of different Fe(III)i:XOj species (each with their own respective spectral absorptivity). These species are formed at different dose levels and sensitivity to detecting these species requires optimization of the readout wavelength.
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