The MAMI source of polarized electrons

The present work describes the source of polarized electrons that is run at the 855 MeV race track microtron MAMI at the Johannes Gutenberg Universitaet in Mainz. The source is based on photoelectron emission from (III-V)-semiconductors. Presently strained layer InGaP- or GaAsP-cathodes are used, which are processed to negative electron affinity by coverage of the surface with a submonolayer of caesium and oxygen. Electron beams spin-polarized up to a degree of P=55% at a quantum efficiency of QE=2% or P=75% at QE=0.4%, respectively, are obtained. The well-known but hitherto unsolved problem of limited cathode lifetime has been sidestepped by the attachment of an UHV load lock system to the source electron gun. It allows quick replacement of cathodes without breaking the gun vacuum. Availabilities in excess of 85% are obtained regularly in beamtimes longer than 100 h. The source was mainly applied in measurements of nucleon form factors via the reactions 1H(vector e,e' vector p), 2D(vector e,e' vector p), 2D(vector e,e' vector n), and 3 vector He(vector e,e'n). More than 1600 h beamtime have been accomplished in physics experiments with polarized electron beams at MAMI up to now. (orig.)