Abstract
Understanding of water radiolysis is necessary not only in basic science but also in application field such as radiation therapy of cancer because water is a main component of human body. However, water decomposition yields are not known well for highly energetic ions. In addition, there are few data obtained under neutral pH in spite of their crucial importance.
Then, heavy ions having energies up to 28 GeV were taken for irradiation at HIMAC, NIRS to determine the primary yields (primary g values) of main 3 species in water radiolysis, e-aq, •OH and H2O2. Because pulse irradiation was not available in the present experiment, steady-state irradiation was applied. The scavenger method was used to extract information of water decomposition products, which have high reactivity and diminish within short time after their production. 1-cm width cells were used for these irradiations to neglect LET changes in the samples. In addition, energy absorber made of PMMA was used to decrease beam energy, leading to the increase of LET. In this case, 2-mm width cells were used to minimize large LET change in the region close to Bragg peak. Thus, LET was varied widely from 2 to 700 keV/μm and measured yields were regarded as track segment yields as opposed to track averaged yields.
Above measurement showed that radical yields decrease with increasing LET while molecular yields increase. This is resulted from the increase of intra-track reactions. And lighter ion irradiation gave smaller radical yields and larger molecular yields than heavier ion irradiation having comparable LET, indicating that lighter ion irradiation generates smaller but denser track closer to its trajectory.
Further investigation with microscopic viewpoint is now being conducted by using simulation of intra-track reactions to clarify the mechanism of distinctive features seen in heavy-ion irradiation.
