Study of Positronium by High Resolution AgeーMomentum Correlation Measurements

正电子是一种独特的“原子”，它的信息可以通过研究其湮没伽马射线的角关联或多普勒展宽的能谱来获得。在这项工作中，我们将正电子寿命方法与角相关或能谱方法相结合，以获得更详细的正电子信息。采用平面型高纯Ge探测器获得高分辨能谱。使用位置灵敏光电倍增管来提高角相关系统的效率。获得了以下新结果：(1)首次测定了正电子与氧碰撞的拾取湮没截面：由于氧分子的基态为三重态，正电子与氧的碰撞以极高的速率转化为副电子。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。(2)建立了用能谱和角关联测量正电子自旋极化的方法：利用磁场中正负电子形成几率的不对称性进行了正电子自旋极化的高精度测量。(3)首次确定了正电子-氦碰撞的动量转移截面：这是通过测量正电子在氦中的动量分布与时间的关系来实现的。

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.