1. Spin-Polarized Electron Sources

Abstract

This chapter focuses on the polarized electron sources that are used, based on photoemission from negative electron affinity (NEA) semiconductor photocathodes, such as GaAs and related compounds. In addition to producing spin-polarized beams, these photocathodes have other advantageous features, such as their high brightness, narrow energy spread of the emitted beam, and the possibility of modulating the beam intensity with an arbitrary time structure by controlling the photoexciting light. The chapter presents the experimental techniques for electron guns, with NEA photocathodes, are also relevant for such guns in their numerous applications beyond those involving spin polarizations. Because there is no simple polarization filter for electrons equivalent to a calcite prism for light or a Stern-Gerlach magnet for atoms, a number of spin-dependent processes have been tried in attempts to produce beams of spin-polarized electrons. For most applications, photoemission from NEA GaAs and related materials provides the most suitable source of polarized electrons. The source based on chemi-ionization of metastable He is competitive for applications that require high polarization in a continuous beam of moderate intensity and has the further advantage that ultrahigh vacuum is not required. The chapter discusses the advances in GaAs-polarized electron source technology, to present important information and considerations for someone building such a source, and to compare its performance with that of other polarized electron sources. A source of spin-polarized electrons can be characterized by a number of parameters that allow one to determine how well it will meet the requirements of a particular application.