Measurement of the Half-Life of Orthopositronium

Abstract

Until recently, measurements of the orthopositronium decay rate disagreed with the value predicted by quantum electrodynamics. This poster describes a novel attempt to measure this decay rate. Positrons from the decay of 22Na were slowed in a vacuum chamber containing a sample of aerogel. The release of a positron by the 22Na source was signaled by the detection of a 1.27 MeV gamma ray by a plastic scintillation detector. Orthopositronium typically decays into three gamma rays, which were detected by three NaI detectors. By detecting the annihilation gamma rays in coincidence and using energy information from the events, systematic effects due to contamination by parapositronium decay, which plagued previous measurements, may be reduced. Preliminary analysis of data taken over 200 hours has yielded a value for the decay rate of 7.83±0.5 μs-1. Currently, problems with Compton scattering are preventing the use of the energy information in analysis.