Study of low energy positron-helium scattering : Precise measurements of total cross sections from 0.6 to 22 eV 低エネルギー陽電子-ヘリウム散乱の研究 : 0.6から22eVでの全散乱断面積の高精度測定

博士論文

Precise measurements for positron-helium absolute total cross sections (TCS's) have been performed over the energy range from 0.6 to 22 eV with a high resolution time-of-flight spectrometer. Since the Ramsauer effect occurs around 2 eV, discrimination against forward scattering is of special importance for precise determination of the TCS's. The spectrometer, characterized by a straight flight tube and weak axial magnetic field, has been designed to achieve effective reduction of the errors due to forward scattering. The results around the Ramsauer minimum are close to the theoretical results by Campeanu and Humberston, and remaining slight discrepancies are fully explained by taking account of imcomplete discrimination against forward scattering. In the energy region excluding the vicinity of the minimum, the overall error is a few percents. Above ~5 eV, an excellent agreement between our results and some theoretical works has been found. Above 17.8 eV (the positronium-formation threshold), our results reasonably agree with other experimental TCS's.