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

Abstract

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\ifmmode^\circ\else\textdegree\fi{} 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 $M=0$ 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.The ratio of the decay rates of parapositronium to orthopositronium can be computed from values of $f$ vs the field, after corrections are made for the angular distribution of the quanta. Our data yield $\frac{{\ensuremath{\lambda}}_{p}}{{\ensuremath{\lambda}}_{o}}=1302$ with a probable error of \ifmmode\pm\else\textpm\fi{}15 percent, in agreement with the theoretical value of 1114.