Physical Principles of Electron Microscopy: An Introduction to TEM ...

The Transmission Electron Microscope 89
Figure 3-16. Schematic diagram of a rotary vacuum pump, illustrating one complete cycle of
the pumping sequence. Spring-loaded vanes A and B rotate clockwise, each drawing in gas
molecules and then compressing them toward the outlet.
During the remainder of the rotation cycle, the air is compressed (Fig. 3-16c)
and driven out of the outlet tube (Fig. 3-16d). Meanwhile, air in the opposite
half of the cylinder has already expanded and is about to be compressed to
the outlet, giving a continuous pumping action. The rotary pump generates a
“rough” vacuum with a pressure down to 1 Pa, which is a factor � 10 5 less
than atmospheric pressure but still too high for a tungsten-filament source
and totally inadequate for other types of electron source.
To produce a “high” vacuum (� 10 -3 Pa), a diffusion pump (DP) can be
added. It consists of a vertical metal cylinder containing a small amount of a
special fluid, a carbon- or silicon-based oil having very low vapor pressure
and a boiling point of several hundred degrees Celsius. At the base of the
pump (Fig. 3-17), an electrical heater causes the fluid to boil and turn into a
vapor that rises and is then deflected downwards through jets by an internal
baffle assembly. During their descent, the oil molecules collide with air