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

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Abstract
Two new Fricke dosimeter gel systems with low diffusion rates have been developed for 3D radiation dosimetry purposes. Both systems consist of a solution of 20% (by weight) polyvinyl alcohol (PVA) in a 50 mM H2 SO4 solution with 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 °C or (b) -20 °C before 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. The cryogel is rubbery in texture but opaque, so its internal Fe3+ concentration can only be measured with MRI. The hydrogel's optical attenuation coefficient 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 dose 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 coefficient (at 20 °C) was measured to be 0.14 mm2 h-1 , which is significantly lower than similar preparations reported for porcine gelatin or agarose. The PVA-FX gels can be stored for 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 significant improvement on previous Fricke gel systems.