Maeyama Takuya, Fukunishi Nobuhisa, Ishikawa Kenichi L, Fukasaku Kazuaki, Fukuda Shigekazu
RIKEN Nishina Center for Accelerator-Based Science , 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
Department of Chemistry, School of Science, Kitasato University , 1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa 252-0373, Japan.
J Phys Chem B. 2017 Apr 27;121(16):4238-4246. doi: 10.1021/acs.jpcb.6b11936. Epub 2017 Apr 10.
We report a nanocomposite Fricke gel (NC-FG) dosimeter prepared using only Fe 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 kGy], which is three times higher than that reported previously (0.6 [s kGy]) for a similar dosimeter containing gelatin. The Fe yield was determined to be 0.19 μmol/J by evaluating the difference in spin-lattice relaxivity between Fe and Fe. A further increase in the radiation sensitivity was observed upon addition of the hydrated electron scavenger NO, suggesting the reduction of Fe 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(Fe) can be described by the stoichiometric equation {G(Fe) = G(OH) - G(e) + 2G(HO) + 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.
我们报道了一种仅使用铁和纳米粘土在水中制备的纳米复合弗里克凝胶（NC-FG）剂量计，无需任何有机胶凝剂。这种剂量计因其触变性而形成凝胶，并在碳离子束辐照下表现出与线性能量转移（LET）无关的放射学特性。该剂量计的辐射灵敏度为1.8 [s kGy]，比之前报道的含明胶的类似剂量计（0.6 [s kGy]）高三倍。通过评估Fe和Fe之间自旋晶格弛豫率的差异，确定铁产率为0.19 μmol/J。添加水合电子清除剂NO后，观察到辐射灵敏度进一步提高，这表明水合电子使Fe还原。在NC-FG的氧化产率中，OH自由基和水合电子贡献的LET依赖性变化相互补偿。这是与传统弗里克剂量计相比，在布拉格峰中抑制LET效应的主要机制。辐射诱导的氧化产率G(Fe)可以用化学计量方程{G(Fe) = G(OH) - G(e) + 2G(HO) + G(H)}来描述，该方程具有报道的水分解自由基初级产率的LET依赖性。计算结果与NC-FG实验氧化产率的绝对值大致相符。还评估了添加少量自由基清除剂（硝酸盐、硒酸盐或镉）的影响。灵敏度分为两种类型，并指出了清除反应后中间自由基的影响。
