Radial dependence of lineal energy distribution of 290-MeV/u carbon and 500-MeV/u iron ion beams using a wall-less tissue-equivalent proportional counter

Using a wall-less tissue-equivalent proportional counter for a 0.72-μm site in tissue, we measured the radial dependence of the lineal energy distribution, yf</it>(y</it>), of 290-MeV/u carbon ions and 500-MeV/u iron ion beams. The measured yf</it>(y</it>) distributions and the dose-mean of y</it>, <f>$${\overline{y}}_{D}$$</f>, were compared with calculations performed with the track structure simulation code TRACION and the microdosimetric function of the Particle and Heavy Ion Transport code System (PHITS). The values of the measured <f>$${\overline{y}}_{D}$$</f> were consistent with calculated results within an error of 2%, but differences in the shape of yf</it>(y</it>) were observed for iron ion irradiation. This result indicates that further improvement of the calculation model for yf</it>(y</it>) distribution in PHITS is needed for the analytical function that describes energy deposition by delta rays, particularly for primary ions having linear energy transfer in excess of a few hundred keV μm−1.