Study of Positronium by High Resolution AgeーMomentum Correlation Measurements

正电子是一种独特的“原子”，其信息可以通过研究其湮灭伽马射线的角相关性或多普勒加宽能谱来获得。本文将正电子寿命法与角相关法或能谱法相结合，以获得正电子更详细的信息。采用刨床型高纯锗探测器获得高分辨率能谱。采用位置敏感光电倍增管提高了角相关系统的效率。得到了以下新的结果。(1)首次确定了正电子-氧碰撞的接收湮灭截面：由于氧分子的基态为三重态，与氧碰撞的正电子以非常高的速率转化为对正电子。正因为如此，人们一直认为不可能测量普通的拾取湮灭截面。这在目前的工作中是可能的，通过使用的动量分布之间的差异，为正交对偶转换和拾取湮灭。(2)建立了利用能谱和角相关性测量正电子自旋极化的方法：利用正正电子和准正电子在磁场中形成概率的不对称性，实现了正电子自旋极化的高精度测量。(3)首次确定了正电子-氦碰撞粒子的动量传递截面：这是通过测量正电子在氦中的动量分布的时间依赖性来实现的。

The positronium is a unique "atom" whose information can be obtained by investigating the angular correlation or the Doppler broadened energy spectrum of its annihilation gamma rays. In this work, we combine the positron lifetime method with the angular-correlation or energy-spectrum method to get more detailed information on positronium. A planer-type high purity Ge detector is used to get high resolution energy spectrum. Position sensitive photomultilipers are used to increase efficiency of the angular correlation system. The following new results are obtained.(1) The pickoff annihilation cross section of positronium-oxygen collisions has been determined for the first time : Because the ground state of the oxygen molecule is triplet, the ortho-positronium colliding with oxygen is converted into para-positronium at a very high rate. Because of this, it has been believed impossible to measure the ordinary pickoff annihilation cross section. This is made possible in the present work by using the difference between the momentum distributions for the ortho-para conversion and the pickoff annihilation.(2) Methods to measure the positron spin polarization by the energy spectrum and the angular correlation are established : High precision measurements of positron spin polarization are made by using the asymmetry of the ortho- and para-positronium formation probabilities in a magnetic field.(3) The momentum transfer crosssection for the positronium-helium collisionin is determined for the first time : This is performed by measuring the time dependence of the momentum distribution of positronium in helium.