Direct measurement of the decay rate of parapositronium using high power milli-wave radiation source

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Positronium is a bound state of an electron and a positron. Two ground states, orthopositronium and parapositronium, have energy level difference called as the hyperfine structure. A high power milli-wave (203GHz) radiation source corresponding to the hyperfine structure is developed. With this source, the direct transition of positronium hyperfine splitting is observed for the first time. The observed event rate is in good agreement with the theoretical value. This source is improved to be tunable in frequency to measure the decay rate of parapositronium from the resonance width of the transition.