Organic-Gelatin-Free Nanocomposite Fricke Gel Dosimeter

We report a nanocomposite Fricke gel (NC-FG) dosimeter prepared using only Fe²⁺ and nanoclay in water, without any organic gelling agents. This dosimeter gels due to its thixotropic properties and exhibits linear energy transfer (LET)-independent radiological properties under carbon ion beam irradiation. The radiation sensitivity of this dosimeter was 1.8 [s^-1 kGy^-1], which is three times higher than that reported previously (0.6 [s^-1 kGy^-1]) for a similar dosimeter containing gelatin. The Fe³⁺ yield was determined to be 0.19 μmol/J by evaluating the difference in spin-lattice relaxivity between Fe³⁺ and Fe²⁺. A further increase in the radiation sensitivity was observed upon addition of the hydrated electron scavenger N₂O, suggesting the reduction of Fe³⁺ by a hydrated electron. LET-dependent variations of the contributions of OH radicals and hydrated electrons compensate each other in the oxidation yield of NC-FG. This is the main mechanism of the suppression of LET effects in the Bragg peak compared to conventional Fricke dosimeters. The radiation-induced oxidation yield G(Fe³⁺) can be described by the stoichiometric equation {G(Fe³⁺) = G(OH) - G(e_aq^-) + 2G(H₂O₂) + G(H)} with the reported LET dependence of the primary yield of water decomposition radicals. The calculated results are in approximate agreement with the absolute value of the experimental oxidation yield of NC-FG. The effects of the addition of small amounts of radical scavengers (nitrate, selenate, or cadmium) are also evaluated. The sensitivity was divided into two types, and influences of intermediate radicals after scavenging reaction are indicated.