Life-time measurement of the para-positronium

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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.