CsBr photocathode at 257nm: A rugged high current density electron source

There is a continuing need for high intensity electron sources that will operate in demountable vacuum and can be externally modulated. Materials with wide band gap, e.g., diamond, are rugged but need photon energies exceeding the band gap to emit efficiently and this rules out the use of cw lasers. The authors have found that a photocathode of CsBr is both adequately intense (>150A∕cm2) and rugged and can be excited with photons of energy of 4.8eV (257nm). This is below the energy gap of CsBr (7.3eV) but such operation can be explained by the presence of intraband states about 4eV below the conduction band minimum.