Polarized Electron Source Development in Japan

We have continued the basic research to develop the photocathodes for the polarized electron source and to construct the guns for the high energy accelerator , such as e + e ? linear colliders. Four subjects, (1) photocathode development of strained layer superlattice, (2) photoluminescence study of spin relaxation inside the cathode, (3) study of eld emission dark current, and (4) gun design and construction, are described in this progress report. 1. Strained-layer-superlattice cathode Two kinds of photocathode, strained GaAs and superlattice (SL) have been developed by Nagoya-KEK-Osaka Pref.-NEC-Daido collaboration, to provide a degree of electron spin polarization (ESP) much higher than 50% together with a quantum eeciency (QE) larger than 1%. The photocathodes which were already studied by Here, the strain layer SL (SLSL) is reported as the most recently developed material. As the rst SLSL sample, InGaAs-GaAs structure was studied. An Indium fraction of 15% was chosen to bring a strain of 1% into the InGaAs layer, and the energy splitting, deened as the diierence between the hh and lh mini-bands, was estimated to be 30 meV. 5] Four samples with diierent combinations of surface layer and Be doping density inside the SL structure were already tested, where the Be doping density in the surface layer was kept to be constant at the highest level such as 4 10 19 /cc.