Demonstration of o-Ps detection with a cylindrical array of NaI detectors

Abstract Ortho-positronium (o-Ps), the triplet bound state of an electron and positron, is a promising system in which to search for new physics. O-Ps production and detection can be achieved with a tabletop setup, involving a 22 Na source, aerogel and a detector. We present our approach to o-Ps detection using the APEX array, which consists of 24 NaI(Tl) bars, arranged cylindrically. Our approach involves tagging on the 1.27 MeV gamma ray, a technique which is used in positron annihilation spectroscopy (PALS) Gidley et al. (2006). We demonstrate the ability to tag with any one of the bars in the array. Using a NaI(Tl) array of high angular coverage (75%) with this technique provides many benefits. This method provides some advantages over tagging on the positron directly insofar as it minimizes the amount of material inside the source holder and simplifies the design of the DAQ. This has potential applications to C P - and C P T -violation searches in o-Ps.