Author/Authors :
Aulenbacher، نويسنده , , K and Nachtigall، نويسنده , , Ch and Andresen، نويسنده , , H.G and Bermuth، نويسنده , , J and Dombo، نويسنده , , Th and Drescher، نويسنده , , P and Euteneuer، نويسنده , , H and Fischer، نويسنده , , H and Harrach، نويسنده , , D.v and Hartmann، نويسنده , , P and Hoffmann، نويسنده , , J and Jennewein، نويسنده , , P and Kaiser، نويسنده , , K.H and Kِbis، نويسنده , , K.-H. and Kreidel، نويسنده , , H.J and Langbein، نويسنده ,
Abstract :
The present work describes the source of polarized electrons that is run at the 855 MeV race track microtron MAMI at the Johannes Gutenberg Universität 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 100h. The source was mainly applied in measurements of nucleon form factors via the reactions 1H(ē, e′ p̄), 2D(ē, e′ p̄), 2D(ē, e′ n̄), and 3H̄e(ē, e′ n). More than 1600 h beamtime have been accomplished in physics experiments with polarized electron beams at MAMI up to now.
