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Polarized electron source (PES) has been
researched and developed for a future linear collider project. The
polarized electron beam is produced by using photoemission from
GaAs-type semiconductors. This type of polarized electron source is
based on a combination of two fundamental technologies; optical pumping
by circularly polarized light and emission of conduction electrons into
vacuum using a negative electron affinity (NEA) surface. Here, the
present problems of the PES are improvements of beam emittance and
lifetime of NEA surface state. In order to overcome these problems, we
started to study a new type of spin polarized electron source using
field emission mechanism. First, we tried to use a GaAs crystal that
has a pyramidal shape (tip-GaAs). Using the tip-GaAs, electron can be
emitted from a small area at the top of pyramid, and the beam emittance
is expected to decrease. The emission mechanism also enables the
photocathode to extract electrons from the positive electron affinity
(PEA) surface, and alleviates the NEA lifetime problem.
The measured electrical characteristics of tip-GaAs and its
polarization exhibited distinctive field emission behavior. I-V
characteristic of tip-GaAs shows that the electron beam was extracted
by field emission mechanism, because a linear dependence was obtained
in Fowler-Nordheim (F-N) plot. Furthermore the quantum efficiency (QE)
spectrum rose rapidly at 1.6 eV, which did not correspond to the band
gap energy of GaAs. The polarization of tip-GaAs was 20-38 % under
illuminating with circularly polarized light of 700-860 nm, and the
peak polarization was 37.4±1.4 % at a wavelength of 731 nm.
These
experimental results indicated that spin polarized electrons can be
extracted from the conduction band into vacuum by a field emission
mechanism. We regard tip-GaAs with field emission as a spin polarized
electron photocathode that can produce a low emittance and achieve long
lifetime because it avoids the use of NEA surface. It is hoped that
this photocathode will be widely applicable to high energy accelerators
and analytical instruments such as the electron microscope.
Talk Slides: (Slides)
For more information, please
contact Dr.
Alex Bogacz, Chair of CASA Seminar Committee
contact casaweb@jlab.org