The yield of OH radical induced by ionizing radiation was estimated by an empirical model; a prescribed diffusion model for a spur of single size applying to neutral water. Two representative spur distances were introduced, one for an incident primary charged particle and one for a representative secondary electron, to calculate chemical yields among active species in a spur. The total yield from the track was a combination of these primary and secondary yields. Two coefficients of this combination were the parameters of the present model. Based on an optimization of these parameters by existing experimental Fricke G-values, the present model estimates the yields of OH at the microsecond timescale after an irradiation, in a unified manner from electrons to heavy ions. The predicted yields of OH around the nanosecond timescale after an irradiation may be a relevant basis for a study on the mechanisms of radiation effects. This prediction by the present model was exemplified for electrons, photons and heavy ions (proton, helium, carbon, neon, argon and iron).
The frequency distributions of the lineal energy, y, of 160 MeV proton, 150 MeV/u helium, and 490 MeV/u silicon ion beams were measured using a wall-less tissue equivalent proportional counter (TEPC) with a site size of 0.72 µm. The measured frequency distributions of y as well as the dose-mean values, y(D), agree with the corresponding data calculated using the microdosimetric function of the particle and heavy ion transport code system PHITS. The values of y(D) increase in the range of LET below ~10 keV µm(-1) because of discrete energy deposition by delta rays, while the relation is reversed above ~10 keV µm(-1) as the amount of energy escaping via delta rays increases. These results indicate that care should be taken with the difference between y(D) and LET when estimating the ionization density that usually relates to relative biological effectiveness (RBE) of energetic heavy ions.
The frequency distribution of the lineal energy, y, of a 290 MeV/u carbon beam was measured to obtain the dose-weighted mean of y and compare it with the linear energy transfer (LET). In the experiment, a wall-less tissue-equivalent proportional counter (TEPC) in a cylindrical volume with a simulated diameter of 0.72 microm was used. The measured frequency distribution of y as well as its dose-mean value agrees within 10% uncertainty with the corresponding data from microdosimetric calculations using the PHITS code. The ratio of the measured dose-mean lineal energy to the LET of the 290 MeV/u carbon beam is 0.73, which is much smaller than the corresponding data obtained by a wall TEPC. This result demonstrates that a wall-less TEPC is necessary to precisely measure the dose-mean of y for energetic heavy ion beams.
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