Polyvinyl alcohol-Fricke hydrogel and cryogel: two new gel dosimetry systems with low Fe3+ diffusion

Two new Fricke dosimeter gel systems with low diffusion rates have been dev eloped fro 3D radiation dosimetry purposes. Both systems consist of a solut ion of 20% (by weight) polyvinyl alcohol (PVA) in a 50 mM H2SO4 solution wi th 0.4 mM ferrous ammonium sulphate and xylenol orange (FX). The difference in the two gels is the way that the gelation process was initiated: either by bringing the temperature to (a) +5 degrees C or (b) -20 degrees C befor e returning them to room temperature. These gels are termed 'hydrogel' and 'cryogel', respectively. The hydrogel is optically transparent, and can be used with either optical or MRI detection methods for dosimetric imaging. T he cryogel is rubbery in texture but opaque, so its internal Fe3+ concentra tion can only be measured wit MRI. The hydrogel's optical attenuation coeff icient is linear (r(2) = 0.99) with dose from 0 to 20 Gy with a sensitivity of 0.106 cm(-1) Gy(-1) (at 543 nm). In terms of MR relaxation rate, the do se response for both the hydrogel and cryogel was linear (r(2) = 0.99) with a sensitivity of 0.020 s(-1) Gy(-1) (at 1.5 T). The Fe3+ diffusion coeffic ient (at 20 degrees C) was measured to be 0.14 mm(2) h(-1), which is signif icantly lower than similar preparations reported fro porcine gelatin or aga rose. The PVA-FX gel can be stored fro long periods of time before exposure to radiation since the auto- oxidation rate was 10 times less than that of gelatin-Fricke recipes. The new gels developed in this work are a signific ant improvement on previous Fricke gel systems.