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Transpector XPR 3+ Operating Manual
There are four classes of interactions between the sensor and the immediate
vacuum environment which can have a significant effect on the detected gas
composition.
First, the analyzer itself is a source of gas molecules because of outgassing from
its surfaces. Usually, the outgassing levels can be reduced by baking the analyzer
in vacuum. When operating in the ultrahigh vacuum (UHV) region, it is best to bake
the sensor overnight at the maximum permissible temperature with the electronics
removed. See the bakeout temperature specifications for the Transpector XPR 3+
sensor. A second overnight bakeout should be performed at the maximum sensor
operating temperature. (It can take more than three hours for all parts of the sensor
to reach maximum temperature during a bakeout, and more than six hours to cool
back down.)
Make sure that the Electron Multiplier high voltage is
turned off for this (second) bakeout temperature,
otherwise, permanent damage to the EM may result.
Second, it is possible that the opposite of outgassing can occur; that is, gas
molecules can be captured by the surfaces of the sensor. This effect is called
“pumping.” In such cases, the magnitude of the signals of the gases pumped will
be lower than is properly representative of the composition of the gas in the
vacuum chamber.
Third, reactions involving gas molecules on surfaces of the analyzer can result in
a change of composition. Gases can either be consumed by the surfaces, or
produced by the surfaces. One example of gas consumption is the reaction of
oxygen with a hot filament, particularly when tungsten filaments are used. The
typical result is an anomalously low concentration of oxygen detected. See
O’Hanlon’s book (Chapter 8, Section 2) for more information on filament materials
and their interactions with the gas being analyzed. An example of gases being
produced from surfaces is the liberation of carbon monoxide molecules from a
thorium oxide coated iridium filament by a sputtering mechanism in the presence
of significant quantities of argon. This latter mechanism makes the combination of
a pressure reduction system and an RGA Sensor unsuitable for measuring
nitrogen contamination in argon at the low parts-per-million (PPM) level from a
sputter deposition process. A special type of inlet system and ion source (often
referred to as a Closed Ion Source [CIS]) should be used for this type of application.
Fourth, there are cases where at least some of the ions detected are emitted from
surfaces in the ion source under electron bombardment, and are not generated in
the gas phase from neutral molecules. This process is known as electron
stimulated desorption (ESD), or sometimes as electron induced desorption (EID).
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