Development of a position-sensitive γ-ray detector to measure annihilation points of ortho-positronium

A new detector system using multi-anode photomultiplier tubes has been developed for measurement of an annihilation point of ortho-positronium (oPs). Position resolutions of the detector with YAP:Ce and CsI(Tl) are determined to be 4.0 and 5.7 mm, respectively. A recently developed YAP:Ce scintillator might be a good candidate for our experimental purpose with respect to high light-output, short decay time and excellent stopping power. Extensive Monte Carlo simulation studies have been performed to understand utility of this system for measuring the annihilation points of oPs.