Moritake, T., Tsuboi, K., Anzai, K., Ozawa, T., Ando, K. and Nose, T. ESR Spin Trapping of Hydroxyl Radicals in Aqueous Solution Irradiated with High-LET Carbon-Ion Beams. Radiat. Res. 159, 670–675 (2003).

The aim of this study was to quantify the hydroxyl radicals (^·OH) produced when aqueous solutions are decomposed by high-linear energy transfer (LET) 290 MeV/nucleon carbon-ion beams using an electron spin resonance (ESR) spectrometer. Aerated cell culture medium containing 200 mM 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was irradiated with doses of 0 to 20 Gy with an LET of 20 to 90 keV/μm. We were able to obtain ESR spectra 10 min after irradiation, and the formation of ^·OH and hydrogen atoms was confirmed by radiolysis of deuterium oxide and ethanol containing DMPO. Our results showed that the yield of ^·OH by carbon-ion radiolysis increased in proportion to the absorbed dose over the range of 0 to 20 Gy. Furthermore, we discovered that the yield of ^·OH decreased linearity as LET increased logarithmically from 20 to 90 keV/μm. The generation of ^·OH by carbon-ion radiolysis at LETs of 20, 40, 60, 80 and 90 keV/μm was 64, 58, 52, 49 and 50%, respectively, of that for low-LET X radiolysis. These unique findings provide a further understanding of the indirect effect of high-LET radiation.