Development of an MeV gamma-ray imaging detector

An MeV gamma-ray imaging detector which makes an event-by-event reconstruction possible has been developed. This MeV gamma-ray detector is a hybrid detector of a micro-TPC which is a time projection chamber (TPC) with a gaseous micro pixel chamber (μ-PIC) readout and an enclosing scintillation camera. We developed a prototype of the MeV gamma-ray imaging detector with the micro-TPC of 10 × 10 × 8 cm³ volume and an NaI scintillator of 10 × 10 × 2.5 cm³ size read by 25 single anode PMTS of 3/4" diameters, and succeeded in reconstructing the gamma-ray events with a radioactive source. Angular resolutions of the prototype detector were restricted mainly by the tracking sensitivity of the micro-TPC for minimum ionizing particles (MIPs) which is determined by the gas gain of theμ-PIC. We have, then, developed an electrode structure of theμ-PIC using a new manufacture technology in order to improve the gas gain. As a result, a gas gain three times higher than that of the formerμ-PIC has been obtained. This result is in good agreement with the simulation. Owing to this improvement of the electrode structure, not only the high gas gain but also a good gain uniformity has been achieved. In addition, a low ion-feedback and long term stability have been confirmed by the newμ-PIC as an X-ray detector.