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 23S1 positronium atoms, generated by passing the beam through a gas cell. Our method employs circularly polarized microwave radiation to drive 23S1 -> 23P1 transitions, for which either Delta mJ 1/4 & thorn;1 or Delta mJ 1/4 -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%.