Measurement of the radiative electron polarization in a 2.4-GeV storage ring

We have observed the existence of transverse polarization of the electron beam in the SPEAR storage ring resulting from the emission of synchrotron radiation in the magnetic field. The time constant for polarization build-up has been measured for a single circulating electron beam to be ${90}_{-35\mathrm{}}^{+90\mathrm{}}$ min and the corresponding equilibrium polarization to be ${0.85}_{-0.25\mathrm{}}^{+0.15\mathrm{}}$. These values are to be compared with the theoretical predictions of 124 min and 0.924, respectively.