THE MAMI SOURCE OF POLARIZED ELECTRONS

The present work describes the source of polarized electrons that is r un at the 855 MeV race track microtron MAMI at the Johannes Gutenberg Universitat in Mainz. The source is based on photoelectron emission fr om (III-V)-semiconductors. Presently strained layer InGaP- or GaAsP-ca thodes are used, which are processed to negative electron affinity by coverage of the surface with a submonolayer of caesium and oxygen. Ele ctron beams spin-polarized up to a degree of P = 55% at a quantum effi ciency of QE = 2% or P = 75% at QE = 0.4%, respectively, are obtained. The well-known but hitherto unsolved problem of limited cathode lifet ime has been sidestepped by the attachment of an UHV load lock system to the source electron gun. It allows quick replacement of cathodes wi thout breaking the gun vacuum. Availabilities in excess of 85% are obt ained regularly in beamtimes longer than 100 h. The source was mainly applied in measurements of nucleon form factors via the reactions H-1( (e) over right arrow,e'(p) over right arrow), D-2((e) over right arrow ,e'(p) over right arrow), D-2((e) over right arrow,e'(n) over right ar row), and (3) (H) over right arrow e((e) over right arrow,e'n). More t han 1600h beamtime have been accomplished in physics experiments with polarized electron beams at MAMI tip to now.