A feasibility test of a new method to polarize beams of strongly interacting charged particles circulating in a storage ring is described. The stored particles, here protons, pass through a polarized hydrogen gas target (thickness 6×${10}^{13}$ H/${\mathrm{cm}}^2$) in the ring some ${10}^{10}$ times and become partially polarized because one spin state is attenuated faster than the other. The polarization buildup is clearly demonstrated in the present experiment.

• Received 14 June 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.1379