Berlin 2001 – wissenschaftliches Programm
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A: Atomphysik
A 10: Posters Wednesday (Cooling and Trapping, Electron Scattering and Recombination)
A 10.25: Poster
Mittwoch, 4. April 2001, 12:30–15:00, AT3
Cold electrons from cryogenic GaAs photocathodes: Energetic and angular distributions — •U. Weigel1, D. A. Orlov1, M. Hoppe1, A. S. Terekhov2, A. Wolf1, and D. Schwalm1 — 1Max-Planck-Institut für Kernphysik, Heidelberg, Germany — 2Institute of Semiconductor Physics, Novosibirsk, Russia
Semiconductor photocathodes with negative electron affinity are promising sources of electron beams with low longitudinal and transversal energy spreads. To investigate the energy distribution of electrons photoemitted from GaAs(Cs,O) we developed a method which proceeds by “marking” electrons with fixed longitudinal energy and a subsequent measurement of the associated differential transverse energy distribution, applying an adiabatic magnetic compression technique. We found that scattering near the interface strongly influences the electron escape process, so that photoemitted electrons spread out over a wide range (up to the effective electron affinity) of transverse and longitudinal energies. Nevertheless, a sufficient part of electrons exhibits low transverse energies of about kT at the bulk material, once electrons with energies near the conduction band minimum are selected. This opens up the possibility to produce ultra cold electron beams by using cryogenic temperatures. Indeed, measurements at 90 K showed that an electron beam with primary transverse and longitudinal energy spreads of ∼7 and ∼10 meV, respectively, is available with an effective quantum efficiency of several per cent. Photocathode operation at current density of 15 mA/cm2 and an extrapolated 1/e lifetime of about 10 hours was demonstrated.