Observation of a Positronium Zeeman Transition inγ-Al2O3

The Zeeman transition between the $M=\pm 1$ and $M=0\mathrm{}$ states of orthopositronium in its ground state has been observed in a solid for the first time, when positrons were stopped in a $\gamma$-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ powder pellet, thus providing strong evidence that essentially free orthopositronium exists in $\gamma$-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$. The value for the positronium fine-structure interval is $\Delta \nu (\gamma -{\mathrm{Al}}_2{\mathrm{O}}_3)=203.308\mathrm{}\pm 0.024$ GHz (120 ppm). The difference $\Delta \nu \left(\mathrm{free}\mathrm{Ps}\right)-\Delta \nu (\gamma -{\mathrm{Al}}_2{\mathrm{O}}_3)$ is 88 ± 25 MHz.