Abstract
It is inevitably important to estimate yield of hydroxyl radical (•OH) in radiolysis of neutral water with heavy ions because of the following two facts. One is that •OH is most responsible to indirect effect induced by radiation, and the other is that heavy ion irradiation shows highly effective cell-killing property. In the present work, it is aimed to apply fluorescence spectroscopy as a sensitive detection technique to such estimation.
It is known with 60Co γ ray that Coumarin scavenges •OH and produces fluorescent species, 7-hydroxy-coumarin, which can be detected with resolution around 10 nM in concentration. The solubility of Coumarin is not high enough to investigate early events like intra-track reaction because the time scale of scavenging reaction is 10-7 s at earliest. Therefore, a similar compound, Coumarin-3-carboxylic acid (CCA) was used in the present work because of its high solubility. Compared to Coumarin, there are little information about reactivity of CCA toward water decomposition radicals and that about reaction pathways from CCA to final, stable and fluorescent product, 7-hydroxy-CCA (7OH-CCA). Then, electron pulse radiolysis and product analysis with 60Co γ ray irradiation were preliminary done to check the reactivity and to discuss the reaction pathways, respectively.
Subsequently, beams of 12C6+, 28Si14+, 40Ar18+ and 56Fe26+ having energies of several hundred MeV were taken for irradiation at HIMAC, NIRS. Irradiated samples were analyzed with reversed phase HPLC coupled with fluorescence spectrometer. Excitation and detection were done at 365 and 445 nm, respectively. It was found that the yield of the fluorescent product, 7-OH-CCA, decreases not only with increasing LET but also increasing scavenging capacity, reflecting the increase and the development of intra-track reactions, respectively. It is concluded that 7OH-CCA produced in CCA solution can be a probe of •OH although further investigation is necessary.
