Studies of the test stand of the polarized electron cathode at the Pohang Accelerator Laboratory

Abstract

Spin-polarized electrons have been generated from a GaAs crystal whose surface was coated with a NEA (negative electron affinity) layer. The electron polarization was measured using the Mott polarimeter. The work was done utilizing the photocathode test-stand in the Pohang Accelerator Laboratory. That stand was constructed to characterize GaAs photocathodes for use in PES (polarized slectron source) development. The test-stand consists of a GaAs crystal, devices for NEA surface production, an electrostatic bend, and a Mott polarimeter. All components, except the polarimeter are contained in a extremely high vacuum (XHV) chamber that can provide a vacuum better than 5 x 10(-10) Torr. Circularly polarized photons are provided by a multi-color laser with quarter-wavelength plates. In order to have a clean surface on the GaAs crystal, which is essential for successful production of a NEA layer, we have adopted the method of hydrogen glow-discharge cleaning. With careful cathode cleaning and activation (NEA surface production), spin-polarized electrons are routinely generated with a quantum efficiency of 3.01% and a polarization of 30.4%.