Fast decay of 2${}^3$S${}_1$ positronium atoms in an MgO lined cavity

We report experiments in which radiatively metastable 2 ${}^3$S${}_1$ positronium (Ps) atoms entered a waveguide whose internal surfaces were lined with smoked magnesium oxide (MgO) powder. The waveguide was built such that time-delayed microwave radiation pulses, tuned to drive 2 ${}^3$S${}_1$→2 ${}^3$P${}_2$ transitions, could be applied to the Ps ensemble. The lifetime of 2 ${}^3$S${}_1$ atoms was measured using time-delayed microwave induced decay, yielding ≈200 ns. This is considerably shorter than the Zeeman shifted lifetime of 1070 ns, indicating that Ps atoms in the 2 ${}^3$S${}_1$ state do not survive multiple collisions with MgO nanocrystals.