Because gas cells operate more predictably and avoid maintenance problems, they have become more common. Gas cells consist of concentric tubes, perhaps 0.3 m long. The ends are closed except for small holes through which the ion beam passes. The outer tube is connected to an extra vacuum pump and gas is leaked from an external source into the inner tube to produce a pressure of 1 to 10 millitorr. This differential pumping in the outer tube reduces the amount of gas leaking into the beam line.
Lower pressures are possible with a stripper foil.
However, the modest collisional cross section of high energy ions with gas molecules diminishes the requirement for high vacuum in RBS experiments.