Study on neutron yield and dose distributions around a water phantom bombarded by carbon ions with therapeutic energies

Satoh, Daiki   ; Kajimoto, Tsuyoshi*; Shigyo, Nobuhiro*; Itashiki, Yutaro*; Imabayashi, Yoichi*; Koba, Yusuke*; Matsufuji, Naruhiro*; Sanami, Toshiya*; Nakao, Noriaki*; Uozumi, Yusuke*

For reasonable design of shields at heavy-ion therapy facilities, it is important to understand neutron yields and doses around a patient bombarded by heavy-ion beams. In the present study, we measured the neutrons emitted from a water phantom, which mimics a body of patient, bombarded by carbon ions with therapeutic energy of 290 MeV/nucleon. The neutron detectors composed of liquid organic scintillator were placed at the directions of 15, 30, 45, 60, 75 and 90 with respect to the beam axis. By applying conversion coefficients of effective dose for AP irradiation to the measured double-differential neutron yields and integrating them above 2 MeV which is the minimum energy of the experimental data, we obtained the effective-dose distribution around the water phantom. The experimental data were compared with the results of the Monte-Carlo simulation code PHITS. While PHITS can reproduce the neutron spectra in higher energy region well, it overestimates the spectra below about 10 MeV. The effective dose calculated by PHITS with the same integration period also gave a larger value compared with the experimental data. We adjusted the absolute values of the PHITS results to agree with the experimental data at 90, and deduced the effective-dose distribution for neutrons above thermal energy in the angular region from 0 to 180. Furthermore, a simple analytic function was proposed to give the dose distribution around a patient. It makes possible to assess the dose distribution quickly and easily with sufficient precisions. These results contribute to a sophistication of shielding design at heavy-ion therapy facilities.