Study on the energy and angular distribution of secondary electron beams emitted from water vapor generated by the impact of 6 MeV/nα particles

At HIMAC, deep-seated cancer therapy is being performed through heavy ion irradiation. Here, the feature of charged particle beams called Bragg peak formation is utilized. This study measures the energy and angular distribution of secondary electron beams emitted from water vapor target through irradiation with heavy ions with the energy near Bragg peak. Then, it obtains a high-grade doubly differential cross-section area comparable with the existing theory. By incorporating the obtained data into the existing trajectory code, it analyzes the structure of the track and calculates radial dose distribution --- this is the purpose of this study. The authors calculated differential cross-section area by integrating the obtained doubly differential cross-section area with the solid angle, and compared it with the existing theoretical formula. The obtained differential cross-sectional area had lower value than that of the Rutherford cross-section area. Agreement with the Rudd model was good, but still a difference of about 20% was seen. As a possible factor, there is a multiple ionization process, so this is a future study subject. Furthermore, it is necessary to improve the detection efficiency of secondary electrons. The development of the measuring device for this purpose has been completed, and measurement in the future is planned. The authors will obtain new data and recalculate radial dose distribution and so on. (A.O.)