Background: The compositions of Ferrous sulphate, Agarose and Xylenol orange dye (FAX) and Ferrous sulphate, Gelatin and Xylenol orange dye (FGX) in solution of distilled water and sulphuric acid are two tissue-equivalent gel dosimeters. Ionizing radiation causes oxidation of Fe²⁺ ion to Fe³⁺ ions which diffuse through the gel matrix and blur the image of absorbed dose over a period of hours after irradiation.

Materials and methods: 25 mM sulphuric acid, 0.4 mM ferrous ammonium sulphate, 0.2 mM xylenol orange dye and 1% by weight agarose in distilled water named FAX and 0.1 mM ferrous ammonium sulphate, 0.1 mM xylenol orange dye, 50 mM sulphuric acid and 5% by weight gelatin in distilled water named FGX are used as two gel dosimeters. All chemicals were supplied by Sigma Aldridge Company, Germany . The gels were poured in Perspex casts and were irradiated to a beam of X ray from linear accelerators or x ray machine.

Results: In this study diffusion coefficients of FAX and FGX dosimeters have been measured through a computer program for different temperatures. The ferric ion diffusion coefficient (D) for the FAX and FGX dosimeters were measured as (1.19 ± 0.03) x 10^-2 cm².hr^-1 and (0.83 ± 0.03) x 10^-2 cm².hr^-1 respectively at room temperature.

Conclusion: For both dosimeters the diffusion coefficients decreased with gel storage temperatures down to 6^oC. FGX dosimeters have advantage of lower diffusion coefficient for a specified temperature. Iran . J. Radiat. Res.; 2003; 1(2): 79 – 86.