Development of wide view gamma-ray Imaging Detector

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We have proposed the new imaging method of MeV gamma rays. By measuring the direction and energy of not only a scattered gamma ray but also a scattered electron, the direction of an incident gamma ray would be principally reconstructed event by event, and a full imaging would be realized. Furthermore, one of two measured (zenith and azimuthal) angles of a scattered electron gives co an additional redundancy which enable us to reject almost all the background events by kinematic fitting. In order to detect such a fine track of the recoil electron, a fine imaging gas detector made by the micro printed board technology was proposed here.In order to obtain 3-D track of a scattered electron, the micro Time Projection Chamber(TPC) has been developed, which measures the position of the track in a few hundred micron meter pitch successively. This TPC consists of the new type of a gas proportional chamber : Micro Pixel Gas Chamber (MPGC) which is a one of wireless gas chamber and expected to be more robust and reliable than Micro Strip Gas Chamber. Here 10cm×10cm 2D-MPGC with a 400 μm pitch pixel was developed, and the high gain of 〜 15000 and the stable operation as a micro TPC with a moderated gain of 〜 5000 during one month have been achieved. Also a compact Anguar camera consisting of a 10cm×10cm×2.5cm NaI scintillator and a 5×5 one-inch phototube array was constructed to detect a recoil gamma ray. Using the micro TPC and the Anguar camera, we successfully obtained the first gamma-ray image by the full reconstruction of the direction of gamma rays event by event. Although obtained angular resolution of 〜 30°is not so good due to the inefficiency of the MPGC, it is the first time in the world to demonstrate the possibility of full gamma ray imaging.