Polarized Electron Beam Experiments at Bonn and Mainz Universities

近年来，极化电子束被认为是粒子和核物理中不可缺少的工具，世界上大多数电子加速器都有极化电子源的设施。他们是波恩、CEBAF、MIT-Bates、Mainz、Nikhef、新西伯利亚和SLAC等。在这个项目中，主要是1993年和1991年分别在Mami/Mainz和Elsa/Bonn提出的GDH实验。从那时起，波恩和美因茨的极化电子源的发展一直在继续。名古屋大学已经开发出能够产生高极化和量子效率光束的光电阴极，如应变GaAs超晶格或AlGaAs/GaAs超晶格。这种光电阴极的应用有望为美因茨和波恩的实验做出重要贡献。在美因茨，超晶格阴极在100keV电子枪上进行了测试，获得了62%的极化，高的量子效率为2.0%。该光源可以产生50微安的直流束，其中10%的束流可以通过MAMI传输到靶上。在GDH实验中，已经准备了偏振束和靶、光谱仪和数据采集系统，以测量自旋平行和反平行的总截面，并计划于1996年年底第一次加工。在波恩，最近建造的120keV极化电子源可以产生设计值为100 mA的1微秒激光脉冲。这种枪的加速电极的设计和建造是在名古屋大学完成的。电极间的场发射电流被抑制在5nA以下，保证了长的阴极寿命。目前，0.2nA的极化光束可以从ELSA中提取出来，作为下一步，计划在1996年进行一项研究，以克服去极化问题，将极化电子加速到3.5GeV。

Recently polarized electron beam is considered as an indispensable tool in particle and nuclear physics, and most of electron accelerators in world have facilities of polarized electron source. They are Bonn, CEBAF,MIT-Bates, Mainz, NIKHEF,Novosibirsk and SLAC etc.In this program, the main subject is the GDH experiments at MAMI/Mainz and ELSA/Bonn which was proposed in 1993 and 1991, respectively. Since then, developments of polarized electron sources at Bonn and Mainz have been continued. The photocathodes which can produce high polarization and quantum efficiency beam, such as strained GaAs or AlGaAs/GaAs superlattice had been developed intensively at Nagoya university. The application of such photocathodes are expected to make important contributions for Mainz and Bonn experiments.At Mainz, the superlattice cathode was tested by a 100 KeV gun, and the polarization 62% was obtained with high quantam efficiency of 2.0%. The source could produce a 50 micro-A DC beam, and 10% of this beam could be delivered to the target by MAMI.For GDH experiment, the preparations of polarized beam and target, spectrometers, datataking system to measure the total cross-sections with spin paralell andanti-parallel have been made, and the first machine time is scheduled infall of 1996.At Bonn, a recently constructed 120 KeV polarized electron source could produce a design-valued intensity of 100 mA for 1 micro-sec laser pulse. The design and construction of accelerating electrodes for this gun were made at Nagoya university. The field emission current between electrodes was suppressed less than 5 nA and it assured the long cathode-lifetime. At moment, 0.2 nA of polarized beam could be extracted from ELSA,and as a next step, a study to overcome a depolarization problem is scheduled in 1996 to accelerate polarized electrons up to 3.5 GeV.