1. Introduction Following Gore’s observation [1] that the well known method of Fricke dosimetry [2] was ideally suited to a “readout” by Nuclear Magnetic Resonance (NMR) methods, radiation dosimetry using volumes of gel as the detector material has been in development since the early 1990’s. Two principal types of gel system have been used and these have become known as Fricke and polymer gels respectively. Both methods have their disadvantages: Fricke gels suffere from the diffusion of the ferric ions that record the radiation dose, which leads to a blurring of the absorbed dose map; polymer gels are sensitivite to the effects of dissolved oxygen during the preparation process [3] and to other aspects of their manufacture, making inter-batch repeatability a problem This poster proposes a novel type of gel dosimeter that combines advantages of both the current methods. It is based on a simple, but fundamental, principle: The functions of recording a dose distribution and of fixing that distribution in space may be separated (i.e., performed by two different components of the gel). Materials, methods and results In this study, we illustrate the principles by investigating one of the simplest examples, that of a standard Fricke solution absorbed into granules of a hydrogel. Small pieces of three proprietary hydrogels (cross-linked co-polymers supplied by D. H. Research Ltd., Surrey, UK) with nominal water uptakes of 60, 75 and 85% by wet weight were soaked for approximately 24 hours in solutions of ferric chloride and ferrous sulphate, both in a range of concentrations from 0 to 2 mM. R 2 was then measured at 20 MHz on a Maran 20 benchtop spectrometer (ResonanceInstruments, Witney, UK) and the results are shown in Figure 2. Next, pieces of the 85% hydrogel were infused with a standard Fricke solution (0.4Msulphuric acid, 1 mM iron (II) sulphate, 1mM sodium chloride) for approximately 24 hours. They were then irradiated using a 60 Co “Hotspot” irradiator (Nuclear Chemical Plant Ltd.) to a range of doses between 0 and 100 Gy. Except at small doses, where the higher intrinsic R 2 of the new dosimeter dominates, Fig. 3 shows very good agreement between the dose recorded by the new medium and the standard Fricke solution. A new family of gel dosimeters, with improved physical properties, that avoids diffusion-related blurring of the derived dose map A new family of dosimeters Improved gel dosimeters may be obtained by separating physically, at a mesoscopic level, the radiation detector from the matrix that provides spatial rigidity. Such a scheme, illustrated schematically in Fig. 1, has a number of potential advantages: References [1] Gore et al. Phys. Med. Biol. 29, 1189–1197 (1984) [2] Fricke and Morse Am. J. Roent. Rad. Ther. 18, 430–432 (1927) [3] Hepworth et al. Phys. Med. Biol. (in press) Possible realisation of the new dosimeters A number of possibilities exist for creating dosimeters of the general type outlined above that are also compatible with readout using Magnetic Resonance Imaging (MRI): the detector phase may be an encapsulated liquid, the droplet phase in an emulsion, a solution absorbed in the granules of a hydrogel, or located in the interstitial spaces of a foam or sponge. Many different types of porous media would be suitable. Conclusions A new family of NMR-based radiation dosimeters has been introduced based upon a detector phase supported within a matrix phase of different composition. A preliminary approach to manufacturing such a system has been demonstrated. The properties of the new system have not yet been fully investigated and research is ongoing S S. J. Doran, R. Patel, D. Highgate ‡ and P. M. Glover ‡ Visiting fellow Department of Physics, University of Surrey, Guildford, GU2 5XH, UK (a) Figure 1: Schematic representation of the new type of gel dosimeter. Open circles represent the radiation-sensitive portion of the gel, whilst the shaded background represents the matrix fixing the measured dose distribution in place. Ideally, no contribution to the measured NMR signal will come from the matrix. Figure 3: Dose response curve for the hydrogel system compared with the conventional Fricke solution dosimeter (a)(b) 1.Since the detector material is fixed spatially, there is no opportunity for diffusion post-irradiation to occur. 2.Since the detector material is separate from the matrix, it may be chosen solely with regard to its performance as an absolute dosimeter. For example, one might choose to use a substance as simple as Fricke solution itself, whose properties are well known. 3.Since the matrix material is ideally isolated from the detector, its properties may be arbitrarily chosen, for example to create a dosimeter that is robust, has an increased melting point, or is non-toxic (all of which would solve potential problems with the current poly(acrylamide) dosimeters) Figure 2: Behaviour of the transverse relaxation rate of water absorbed into three different hydrogels in the presence of varying concentrations of (a) ferric and (b) ferrous ions. The errors represent an estimate of 10%, based on the known variation of R 2 with temperature. In this preliminary study, we were unable to control the temperature accurately for all samples.