Measurement of the Spin Polarization of a Slow Positron Beam Using Circularly Polarized Microwave Radiation

We have measured the spin polarization of a slow positron beam via state-selective depopulation of 2^{3}S_{1} positronium atoms, generated by passing the beam through a gas cell. Our method employs circularly polarized microwave radiation to drive 2^{3}S_{1}→2^{3}P_{1} transitions, for which either Δm_{J}=+1 or Δm_{J}=-1, and relies on the fact that asymmetries between the two cases yield the underlying positron beam polarization. Using this technique we show that the polarization of a positron beam derived from a solid neon moderator may be increased from 30% to 90% by increasing the moderator thickness, with an associated reduction in beam intensity of 60%.