A low chamber (2-4Χ10-7 Torr) pressure with fully loaded
(10-20 mTorr) target gas cell is crucial for obtaining high
quality, low background electron spectra, particularly with
position sensitive detection since it also reduces the dead
time of the entire data acquisition This necessitates the
use of a doubly differentially pumped gas cell with its
own independent pumping system a concept first used with
great success in Kansas.
5.1 Design of doubly differentially
pumped gas cell: calls for a voltage floatable target gas cell
enclosed in an outer cell and directly pumped by an independent
turbo pump with μm XYZ alignment capabilities.
Supporting the gas cell at the correct focusing distance
is very important and will be studied in simulation using
SIMION CPO software in
conjunction with the input lens settings of the hemispherical
analyzer. A 50-100 lt/s turbo pump with back up pump will
have to be ordered and installed.
5.2 Gas cell construction
and placement in collision chamber:
Gas cell manifold, aperture assembly, pumping system and
support will be built according to the design in the shop
and positioned.
5.3 Connection to gas
feed through and pressure stabilization unit: The gas pressure will be stabilized and monitored
by a feedback system, typically a capacitive manometer,
which will also have to be bought.
5.4 Gas cell alignment and support: involves the crucial optical alignment of the gas
cell with the optical axis and is first performed in air
with a telescope. Final micro adjustment to minimize electron
background from slit edge scattering is performed by taking
electron spectra with ion beam on target (WP8.1).
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