1. Introduction A Fricke gel was the first dosimetric system to offer the possibility of making 3-D radiation dose measurements in a totally non-destructive and non-invasive fashion [1, 2]. The gel is a tissue-equivalent medium based on the most commonly used chemical dosimeter, in which the radiation energy deposited causes ferrous ions to oxidise to the ferric form. These changes can be quantified by measuring the related changes in the Magnetic Resonance (MR) spin relaxation times of the hydrogen nuclei that abound in the gel water. MR imaging is then used to obtain the spatial radiation dose distribution by mapping the concentration of ferric ions, which is proportional to the radiation dose. The Fricke gel formulation has been extensively studied and problems associated with this method have been identified. Among these is low sensitivity, which the early gels have in common with the basic Fricke solution. Appleby and Leghrouz [3] produced a more sensitive ferrous agarose xylenol (FAX) gel in which radiation induced colour changes can be used to measure the absorbed dose. In this work we studied in detail a related ferrous xylenol gelatin (FXG) system, and the role of each of its components was determined. An optimal formulation for making a gel appropriate for 3D dosimetry using an optical readout is proposed. post-irradiation. It is well known that the ferrous ions undergo a constant oxidation in the presence of oxygen in the aerated aqueous systems and this process was found to be directly proportional to the square of the ferrous ion concentration in the system [9]. We studied the response of FXG loaded with different ferrous ion concentrations (Figure 3(a)) and found the best concentration to be 0.5mM at which the system showed maximum response with a self-oxidation four times lower than the more usual 1 mM. Xylenol orange: The indicator has a very limited role in the basic chemical reaction caused by radiation. On the other hand the consumption of ion indicator molecules and the formation of purple complex control the range over which the system response is linear. Figure 3(a) shows the colour saturation effects, and also indicates that xylenol concentration does not affects the system sensitivity because all dose response curves in the pre-saturation region have similar slopes. The FXG stability is found to be independent of the concentration of xylenol: the differences in FXG dose response curves for samples measured immediately and after two days of irradiation are very small and are mostly due to the ferrous self-oxidation. A coloured gel system for optical three-dimensional dosimetry Components of the new gel system The manufacture of the FXG system is similar to that of the conventional Fricke gel with the addition of the metal ion indicator, xylenol orange, to the chemical constituents. The use of a gelling agent is primarily to solidify the material and stabilise the dose patterns but the addition of a large amount of organic substance plays another role by enhancing the chemical yield. References [1] M.J. Day, Phys. Med. Biol. 35, 1605, 1990 [2] J.C. Gore et al., Phys. Med. Biol. 41, 2695, 1996 [3] M.J. Maryanski et al., Magn. Res. Im. 11, 253, 1993 [4] M.A. Bero et al., Nucl. Inst. Meth. A422, 617,1999 Ferrous sulphate: The concentration commonly used in Fricke solution and gel systems is 1mM [1,9]. Although the concentration of ferrous ions does not strongly affect the sensitivity of the various forms of this chemical system, it has a significant effect on the system stability, in pre-irradiation storage and also Conclusions We have described the general characteristics of the FXG system, demonstrated the effect of each chemical constituent and suggested an optimal composition. FXG is a promising system for 3D dose measurements and is tissue-equivalent for X-rays, γ-rays and neutrons. It has the capability to measure doses as low as 1cGy and is suitable for both optical and MRI readout techniques. S M. A. Bero, W. B. Gilboy, P.M. Glover and S.J. Doran Department of Physics, University of Surrey, Guildford, GU2 5XH, UK (a) Figure 1: The effects of sulphuric acid on FXG sensitivity: measurements made immediately (solid line), after 24 hours (dotted line) and after 48 hours (dashed line) for ( )50mM, ( )25mM and (  )10mM sulphuric acid concentration. 25mM H 2 SO 4 0.05mM Fe 2 (NH 4 ) 2 SO 4.6H 2 O 0.01mM xylenol orange sodium salt 5% gelatin powder by weight Recipe for an FXG gel Sulphuric acid: It is generally accepted that the concentration of sulphuric acid may range from 0.4 M down to as low as 50mM with very limited effects on the Fricke system sensitivity. However, in the coloured gels, the acid’s effects are more complex and less well understood. The FXG system does not show any response to the radiation field when the concentration of acid is higher than 0.1M and the system sensitivity starts to increase steadily with decreasing acid concentration. On the other hand, at the same time, the system stability worsens, as shown in Figure 1. The optimal concentration of acid that gives the best compromise between sensitivity and stability of the FXG system was found to be 25mM. Note that an aerated acidic medium is essential in the early stages of the reaction that follows the radiolysis of the water and formation of the chemically active free radicals. (a) (b) Figure 2: (a) The new FXG gels compared with standard BANG gels. From left to right: unexposed FXG, exposed FXG, unexposed BANG, exposed BANG with artifact at top due to oxygen spoilage of the gel. Notice how the exposed BANG gel is a cloudy and hence scattering medium. This means that it is unsuitable for use with a parallel beam optical tomography scanner. (b) Dose response characteristics for (  ) the new FXG gel, (  ) FBX gel and ( ) xylenol Fricke solution all at 585 nm Figure 3: (a) Ferrous ion conc- entration ( ) 2mM, ( ) 1mM, (  ) 0.5mM effects on FXG sensitivity, and stability — measurements made immediately (solid line), 24 hours after (dotted line) and 48 hours after (dashed line) irradiation. (b) Effects of indicator concentration on FXG dose response: ( )0.01mM, (  ) 0.025mM,(  ) 0.05mM (  ) 0.1mM. The dashed lines show corresponding measurements 48 hours afterwards to measure the sample stability. (a) (b)