Systematic Measurement of Lineal Energy Distributions for Proton, He and Si Ion Beams Over a Wide Energy Range Using a Wall-less Tissue Equivalent Proportional Counter

Abstract

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.