Development of an MeV gamma-ray imaging detector

An MeV gamma-ray imaging detector which makes a reconstruction of single photons possible has been developed. This MeV gamma-ray detector is a hybrid detector of a micro time projection chamber (micro-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(Tl) 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. The angular resolutions of the prototype detector were limited mainly by the tracking accuracy of the micro-TPC for minimum ionizing particles (MIPs) which is determined by the gas gain of the μ-PIC. We have, thus, developed an electrode structure of the μ-PIC using a new manufacture technology in order to improve the gas gain. As a result, a gas gain which is three-times higher than that of the previous μ-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.