Determination of diffusion coefficient in Fricke Xylenol gel dosimeter after electron beam bombardment

The Fricke Xylenol Gel (FXG) dosimeter is a ferrous sulfate aqueous solution that, when irradiated or subject to heat, oxygen and light, oxidizes the Fe2+ ions to Fe3+ and this new type of ion, together with xylenol orange corant form the Fe3+-Xylenol complex. This new concentration, generally determined through spectrophotometry, is directly proportional to absorbed dose [M.A. Bero, W.B. Gilboy, P.M. Glover, H.M. El-masri, Nucl. Instrum. Methods Phys. Res. B 166–167 (2000) 820]. Several applications of this dosimeter in radiotherapy are found in the literature such as: dosimetric parameters for photons [L.N. de Oliveira, C.S.G. Calcina, M.A. Parada, C.E. de Almeida A. de Almeida, Braz. Phys. J. 37 (3B), (2007) 1141] and electrons beams [P.C.D. Petchevist, FXGB Dosimeter of characteristics for electron beam in the radiotherapy, Master Science dissertation (University of São Paulo) 2006], radiosurgery [C.S.G. Calcina, L.N. de Oliveira, C.E. de Almeida, A. de Almeida, Phys. Med. Biol. 52, (2007) 1431], photoacustic [A.M. Caldeira, A.M. Neto, A.C. Bento, M.L. Baesso, M.A. Silva, A. Almeida, Appl. Radiat. Isotopes 65 (2007) 605] and FXG mass attenuation coefficient determination [M.V. Moreira, A. Almeida, R.T. Costa, L.A. Perles, J. Phys. D: Appl. Phys. 3 (2004) 146]. Although in the Fricke dosimetry, optical density measurements are made normally right after the irradiation, the knowledge of the Fe+3 diffusion coefficient of this dosimeter is interesting, in order to control the spatial absorbed dose distribution, inferred from the absorbances. This fact reveals the real signal value useful in radiotherapy. In this work the ferric ions diffusion coefficient was determinate, together with their activation energy for two different temperatures in the FXG gel, through a new method not reported in the literature. From the experimental data and innovative analysis, we obtained diffusion coefficients of 0.20 ± 0.05 and 0.40 ± 0.05 mm2/h for T1 = 280.2 ± 0.3 K and T2 = 297.8 ± 0.3 K, respectively. From these two values we could infer the Fe+3 activation energy as 0.30 ± 0.02 meV.