Life-time measurement of the para-positronium

正电子Ps是电子和正电子的结合态，是量子电动力学的良好试验场。自旋为0的Ps，即对正电子子（p-Ps）的寿命至今从未被直接测量过。这是因为p-Ps的寿命很短，约为0.1 ns，无法用传统的测量方法获得。在这项研究中，我们建议基于一个全新的想法来衡量它；使用高能正电子束。主要观点如下。(a) Ps的动能越大，寿命越长。(b)理论预测可以从晶体靶产生高能量单色Ps。(c)由于相对论效应，可以用场电离方法在磁场中探测高能Ps。为了实现基于上述项目的全寿命测量系统，我们进行了初步的实验和施工工作。现将迄今取得的发现和成果总结如下。(a)在使用350MeV光子束的实验中，我们发现在预测光子能量下单晶上的对产生截面显著增强。(b)当发射角限制在较小的角锥内时，发现增强量更大更窄，从而支持了晶体产生Ps的理论预测。(c)在广岛大学的150兆电子伏电子设施中，已经建造并建立了相干正电子产生和探测正电子的实验装置，用于今后的长波时间。

The positronium Ps is a bound state of an electron and a positron and is a good test ground of the quantum electro dynamics. The life time of the spin-0 Ps, the para-positronium (p-Ps), has never been measured directly upto date. This is because the life time of p-Ps is very short, about 0.1 ns and is not accessible with conventional measuring method. In this research we propose to measure it based on a completely new idea; use of high-energy positronium beam. The main points are as follows.(a) The life time is longer when the Ps has higher kinetic energy.(b) Theoretical prediction exists that a high energy monochromatic Ps can be produced from a crystal-target.(c) Detection of high energy Ps with the field-ionization method is possible in the magnetic field is possible owing to the relativistic effect.For realizing an life-time measuring system based on the above items, we carried out preliminary experiments and construction works. The finding and achievement so far made are summarized as follows.(a) In an experiment using a 350MeV photon beam, we found a significant enhancement of pair-production cross section on a single crystal at a predicted photon energy.(b) The amount of enhancement is found to be the larger and the narrower when the emission angle is restricted to the smaller angular cone, thus supporting the theoretical prediction of Ps production from a crystal.(c) The experimental apparatus for coherent production of positronia and their detection has bee constructed and set up at the 150MeV electron facility REFER in Hiroshima University to be used in the long beam time to come.