The gas injection system allows to increase the rest gas pressure in LHC vacuum to about 10^-8 millibars, enough to observe a clean image from electrons.

In SPS there is no need to inject gas.

The system is shown in the bottom of the PVSS control panel. It contains a needle valve for precise control of small gas flux and a buffer volume where the gas pressure is tuned before opening the valve for the machine vacuum.

The gas used is Neon. Heavier noble gas would be favourable (more signal) but they are difficult to pump with ion pumps. On the other hand helium is used in the machine for leak detection. Therefore Neon has been chosen.

The document describing the calibration of the system and operational procedures is here (restricted). Person responsible is Didier Calegari (VAC).