A Spectrometer for the Study of Angular Correlations in Polarized-Orthopositronium Decay

We describe a setup for experimental testing of the CPT-invariance by observing the angular correlations in the decay of polarized orthopositronium atoms. Annihilation events are detected by five NaI(T1) sensors of 150-mm diameter and 100-mm thickness, located at the vertices of a regular pentagon. The sensor signals are detected in the double-coincidence circuits gated by signals with different time delays from the positron escape. The setup has a time resolution of 8.0 ns. In order to reduce the effect of the long-term instability and the instrumental asymmetry on the experimental results, the direction of positron incidence on a SiO2 target was reversed with a program-driven period during data acquisition. The value of the asymmetry, measured with due account of statistical and systematic errors, is 0.0008±0.00091, which is twice better as compared with the previous experiment.