Abstract
Strained-layer GaAs and strained-superlattice GaAs photocathodes are used at Jefferson Laboratory to create high average current beams of highly spin-polarized electrons. High electron yield, or quantum efficiency (QE), is obtained only when the photocathode surface is atomically clean. For years, exposure to atomic hydrogen or deuterium has been the photocathode cleaning technique employed at Jefferson Laboratory. This work demonstrates that atomic hydrogen cleaning is not necessary when precautions are taken to ensure that clean photocathode material from the vendor is not inadvertently dirtied while samples are prepared for installation inside photoemission guns. Moreover, this work demonstrates that QE and beam polarization can be significantly reduced when clean high-polarization photocathode material is exposed to atomic hydrogen from an rf dissociator-style atomic hydrogen source. Surface analysis provides some insight into the mechanisms that degrade QE and polarization due to atomic hydrogen cleaning.
Highlights
The Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Laboratory is a nuclear physics research facility where high average current beams of highly spin-polarized electrons can be delivered to three experimental end stations simultaneously
The photocathode is formed when cesium (Cs) and nitrogen trifluoride (NF3) gas are applied to the surface to create a negative electron affinity condition (NEA) [1]
Wetchemical etching techniques were replaced with atomic hydrogen cleaning to obtain high quantum efficiency (QE) from bulk GaAs [2]
Summary
The Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Laboratory is a nuclear physics research facility where high average current beams of highly spin-polarized electrons can be delivered to three experimental end stations simultaneously (combined beam current up to 200 A with polarization >70%). Quantum efficiency (QE), is obtained only when the photocathode surface is atomically clean. Wetchemical etching techniques were replaced with atomic hydrogen cleaning to obtain high QE from bulk GaAs [2]. An rf-inductive discharge dissociator apparatus was chosen as the atomic hydrogen source. The merits of atomic hydrogen cleaning are examined. Results indicate that vendors provide sufficiently clean photocathode material to warrant atomic hydrogen cleaning superfluous. Very high QE can be obtained without implementing any form of cleaning, atomic hydrogen cleaning or other techniques, if precautions are taken to ensure clean material is not inadvertently dirtied when photocathode samples are prepared for installation within photoemission guns.
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