Static Magnetic Field Quenching of the Orthopositronium Decay: Angular Distribution Effect

An effect of the angular dependence of the annihilation radiation from the magnetic substates of ortho-positronium has been observed in an experiment similar to that of Deutsch and Dulit. Quanta due to positron annihilations were observed at a direction of 90° to a magnetic field. The fraction, $f$, of ortho-positronium decaying by three-quantum annihilation can then be computed, and at high values of the field $f$ was found to be less than $\frac{2}{3}$. The value $\frac{2}{3}$ is expected statistically since only annihilation radiation of the $M=0\mathrm{}$ substate is affected by the field. Our observation is understandable if Drisko's formulas for the angular distribution of the annihilation radiation from the different magnetic substates are considered.