The Vacuum Pump / Gauge

You probably recognized a small hole at the top of the detector schematic. It is used to attach a flexible tube for the pressure gauge and the vacuum pump.
Usually such Geiger counters utilize a noble gas such as helium or argon, but ordinary air works also, albeit at a higher operating voltage.

The best operating area for the Geiger counters is about 10% of air pressure (100mbar or 7cm of mercury) as written in the sciam.com article.

UPDATE:
I've been told that a pressure of 100mbar is probably too high for use with plain air (check the ParticleDetector-Group archive). Much better would be a vacuum of around 10mbar, achieved with an old refrigerator compressors as a vacuum source.

Comments from group-members:
"They [refrigerator compressors] make a really decent vacuum, especially the ones out Frigidaire units."

"The more common refrig pump found at the dump is the piston style which will achieve a modest vacuum of 50 Torr (66.7 mbar) or so. The better style (but a bit harder to locate) are the rotary vane style of pump. These can get to about 1 Torr (1.3 mbar) or a bit better."

Such compressors usually have 3 electrical connections: 2 for AC power and one to turn on the compressor via a thermal switch. "Be sure to recover the switch when you get the pump. Some of the start switches are thermal others magnetic."

Be very careful when handling high voltages!
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