Measurements of lineal energy distributions of various energetic heavy ion beams using a wall-less tissue equivalent proportional counter

Tsuda, Shuichi  ; Sato, Tatsuhiko   ; Takahashi, Fumiaki  ; Satoh, Daiki   ; Sasaki, Shinichi*; Namito, Yoshihito*; Iwase, Hiroshi*; Ban, Shuichi*; Takada, Masashi*

Deposit energy distribution in microscopic site is basic information for understanding of biological effects of energetic heavy ion beams. To estimate RBE, lineal energy, y, can be an appropriate physical index. In this work, a wall-less tissue equivalent proportional counter has been designed and used for the measurement of y distributions, yf(y), for 160 MeV H, 150 MeV/u He, 290 MeV/u C, 490 MeV/u Si and 500 MeV/u Ar. Data of yf(y) were also obtained in the wide range of LET. The dose-means of y, yD, were compared with those calculated by the microdosimetric function of PHITS. It is found that the calculated yf(y) and yD agree fairly well with those measured. The values of yD are larger than those of LET less than 10 keV/um because of the discrete energy deposition by delta rays, while the relation is reversed above 10 keV/um. The results indicate that care should be taken in the difference between yD and LET when the values of RBE of energetic heavy ions are estimated.