Polarized Electron Beam Experiments at Bonn and Mainz Universities

德国波恩大学的ELSA （0.5 ~ 0.5 GeV）和美因茨大学的MAMI （0.3 ~ 0.9 GeV）两台中能量电子束加速器在德国运行，并组织了合作，以促进这两台机器上的极化电子束项目。近三年来，这一合作取得了三大成果。(1) ELSAA 120 keV电子枪的120 keV极化电子源结构是新设计和建造的，其负电子亲和阴极的寿命比Bonn小组先前建造的电子枪要长。最严重的问题是所谓的表面变化极限现象，这是观察到的大块砷化镓光电阴极。利用名古屋研究组开发的GaAs/AlGaAs超晶格光电阴极解决了这一问题，可以稳定产生强度大于70 mA、极化大于65%的光束，持续时间超过50小时。这些性能可以满足源端极化电子束的要求，使实验验证Gersimov-Drell-Hearn总结成为可能。(2)克服ELSA加速过程中的退极化原因，预计在0.0 ~ 0.5GeV的ELSA能量范围内存在11个退极化原因（DR）。利用极化电子束和简单的闭式轨道校正仪器，研究了“快通过”和“慢通过”两种方法来克服这些缺陷。结果表明，在0.32 GeV时，“慢速”方法对DR有效，偏振矢量完全翻转，但在0.76 GeV时，偏振度有所降低。这种现象最早是在ELSA中观察到的，并根据K. Yokoya等人在1983年预测的自旋扩散过程确定为DR。这一事实表明，“慢速”方法对ELSA中高于0.7 GeV的能量区域无效。“快速”方法对0.32和0.5 GeV DR有效。在0.0 GeV时极化完全消失，因为该DR强度中等，两种方法都无效。然而，KEK小组设计的一种新的调跃法仪器克服了这一障碍，它有望在不久的将来将极化光束加速到最大能量0.5给出，并通过充分的能量可用于极化电子实验。名古屋组为了实现更高的极化和量子效率，开发和研究了各种超晶格，但“哪种机制决定最大电子极化”仍然是一个根本性的问题。一个有趣的实验给出了这个问题的答案，该实验使用美因茨炮系统，该系统可以从名古屋超晶格光电阴极产生皮秒脉冲极化光束。结果表明，最大极化高度依赖于传导电子的初始极化，而对于厚度为100 nm的薄层超晶格，退极化效应很小且被忽略。这是第一个实验证明，在确定各种超晶格材料组合的最大极化时，带混合机制是最主要的。少

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