Positronium fine-structure intervalΔνin oxide powders

The fine-structure interval $\Delta \nu$ of ground-state positronium formed in Si${\mathrm{O}}_2$ and MgO powder pellets has been determined through measurements of microwave-induced transitions between Zeeman substates of orthopositronium. The $\Delta \nu$ values for Ps in the powders are less than $\Delta \nu$ for free Ps by the order of 1 part in ${10}^4$, suggesting an attractive interaction between Ps and the powder grain surfaces within the pellet. No Zeeman transition was found for positrons stopping in a silicon single crystal or in fused quartz.