Study of Electro-magnetic Interaction and Search for Axion Using Decay Processes of Positronium.

为了有效探测静止电子-正电子湮灭产生的多种伽马射线，我们构造了“多伽马射线谱仪”。这个探测器是如此独特，在球体上放置了32块闪烁计数器，因此人们可以观察到来自中心正电子源的单个伽马射线发射。预计该探测器对多伽马射线的探测效率将比现有的探测器高出数百倍。此外，由于我们可以测量每条伽马射线的能量和发射角，因此可以利用能量动量守恒很好地区分背景。放射性同位素^<63>Ge提供正电子，产生正电子，衰变成许多伽马射线。为了验证QED的高阶（alpha^4）效应和探索轴子等新粒子，我们首先观测了四种伽马射线。巨大的背景可能是4个伽马事件的100倍。我们可以成功地提取四个过程的信号利用这个探测器的特殊方面它给了我们能量动量守恒的信息。在此基础上，我们首次测量了e^+e^- -> γ的衰减率和4gamma与2gamma的分支比为（1.48<正负>0.13 <正负>0.12）x10^<-6>，这与最低阶QED计算结果一致，但我们没有观察到任何表明轴子产生的显著信号。

In order to detect efficiently many gamma-rays emerging from electron-positron annihilation at rest, we constructed "Multi-gamma ray Spectrometer". This detector is so unique that 32 pieces of scintilation counters are placed on the sphere and thus one can observe individual gamma-ray emittel from the positron source at the center. It will be expected that this detector has the detection efficiency of multi-gamma rays hundred times better than any detectors available. Furthermore since we can measure the energy and emission angle of each gamma ray, background is well distinguished using energy-monentum conservation.The radio isotope ^<63>Ge provides positrons which creates an positronium decaying into many gamma rays. We first observed four gamma-rays so as to check higher order (alpha^4) effect of QED and to explore new particles such as an axion. The large background probably 100 times as much as four gamma events can be suppressed. We could successfully extract signals of four gamma processes using the particular aspect of this detector which gives us information on the energy-momentum conservation.Then we measured for the first time the decay rate for e^+e^- -> gamma and the branching ratio of 4gamma to 2gamma was obtained to be (1.48<plus-minus>0.13 <plus-minus>0.12) x10^<-6> being consistent with the lowest order QED calculation, However we could not observe anay significant signal indication the production of axion.