Study of Positronium by High Resolution AgeーMomentum Correlation Measurements

正电子素是一种独特的“原子”，其信息可以通过研究其湮灭γ射线的角关联或多普勒展宽能谱来获得。在这项工作中，我们将正电子寿命方法与角关联或能谱方法相结合，以获得更详细的正电子偶素信息。采用平面型高纯锗探测器获得高分辨能谱。位置敏感光电倍增管用于提高角相关系统的效率。得到了以下新结果。(1)首次测定了正电子素-氧碰撞的拾取湮灭截面：由于氧分子的基态是三重态，正电子素与氧碰撞时以很高的速率转化为对位正电子素。正因为如此，人们认为不可能测量普通的传感器湮灭截面。这是可能的，在目前的工作中，通过使用的动量分布之间的差异的邻-对位转换和拾取湮灭。(2)建立了利用能谱和角关联测量正电子自旋极化的方法：利用磁场中正电子偶素和仲正电子偶素形成几率的不对称性，高精度地测量了正电子自旋极化。(3)首次测定了正电子素-氦碰撞的动量转移截面：这是通过测量氦中正电子素动量分布的时间依赖性来完成的。

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.