Polarized Electron Beam Experiments at Bonn and Mainz Universities

点击翻译按钮获取中文摘要

Two intermediate energy electron beam accelerators, ELSA (0.5〜0.5 GeV) in Bonn and MAMI (0.3〜0.9 GeV) in Mainz universities are operated in Germany and the collaborations were organized to promote the polarized electron beam projects in those machines. Three major achievements in recent three years are made by this collaboration.(1) Construction of 120 keV polarized electron source for ELSAA 120 keV gun was newly designed and constructed and it gave the better lifetime of NEA (Negative Electron Affinity) cathode than the gun previously build by Bonn group. The most serious problem was so-called surface change limit phenomenon which was observed for the bulk GaAs photocathode. This problem was solved by GaAs/AlGaAs superlattice photocathode developed by Nagoya group, and the beam with intensity more than 70 mA and polarization higher than 65% could be produced stably for more than 50 hours. These performances can satisfy the requirements for polarized electron beam at source which enabl … More es the experiment to check the Gersimov-Drell-Hearn sumrule.(2) Overcome the depolarization reasonances during the acceleration in ELSAIt was expected that there are eleven depolarization-reasonances (DR) in ELSA energy range of 0.0〜0.5GeV. Both methods of "fast passage" and "slow passage" were examined to overcome those DR's using the polarizes electron beam and simple instruments for closed orbit correction. It turned out that the "slow" method was effective for DR at 0.32 GeV and the polarization vector was completely flipped, but for 0.76 GeV DR, the polarization degree was decreased by some amounts. This phenomenon was first observed in ELSA, and is identified as the DR due to spin diffusion process predicted by K. Yokoya and others in 1983. This fact indicates the "slow" method is not effective for an energy region higher than 0.7 GeV in ELSA. The "fast" method was effective for 0.32 and 0.5 GeV DR's. The polarization was completely lost at 0.0 GeV, because this DR strength is medium and both methods are not effective. However, this DR was overcome by a new instrument of tune jump method which was designed by KEK group and it promised that polarized beam will be accelerated up to the maximum energy of 0.5 Gave in nearest future, and thru full energy can be used for polarized electron experiments.(3) Study of polarization mechanism by picosec-pulsed polarized beam at MainzVarious kinds of superlattice were developed and studied by Nagoya group to achieve higher polarization and quantum efficiency, but one fundamental question was still remained on "which mechanism dominates to determine the maximum electron polarization ?" The answer for this question was given by an interesting experiment which used a Mainz-gun-system which can produce the picosec-pulsed polarized beam form the Nagoya superlattice photocathodes. The results shows the maximum polarization is strongly dependent on the initial polarization of conduction electrons, while the depolarization effect is small and neglected for our thin-layer-superlattice with thickness of 100 nm. This is the first experimental demonstration that the band-mixing mechanism is most dominant for determination of the maximum polarization for various combinations of superlattice materials. Less