Water radiolysis with therapeutic high-energy heavy-ion beams; Yield of OH radical near the Bragg peak

Yamashita, Shinichi; Funtowiez, D.*; Maeyama, Takuya*; Midorikawa, Masamichi*; Oka, Toshitaka   ; Baldacchino, G.*; Taguchi, Mitsumasa; Kimura, Atsushi; Kudo, Hisaaki*; Katsumura, Yosuke; Murakami, Takeshi*

Product yields of water radiolysis are inevitably important in understanding detailed mechanism of indirect action of ionizing radiations. We have conducted yield measurement of main products such as hydrated electron, hydroxyl radical (OH) and hydrogen peroxide. We have also developed a sensitive method to determine OH yield with a fluorescent probe. In the method, aqueous solution of coumarin-3-carboxylic acid (CCA) is irradiated and OH is scavenged by CCA, resulting in production of fluorescent product 7OH-CCA. Applying this method, OH yield near the Bragg peak has been measured in this study because the Bragg peak is overlapped cancer position in actual therapy. Beams of C 290 and 135 MeV/u and so on were taken for irradiation at the biological irradiation port at HIMAC facility of NIRS. It was found that OH yield show minimal value around the Bragg peak for every ion beam and it drastically jumps up several times at just downstream of the Bragg peak. It is already well known that contributions of fragmentation particles become non-negligible for high-energy heavy ions. Thus, further consideration including fragmentations is necessary to separate contributions of different fragmentation particles. Such consideration is being attempted by using simulations with HIBRAC and PHITS.