The whole interferometer has a length (tip to detector) of 565 mm. It is constructed rigidly [20] to avoid mechanical vibrations and shielded against electromagnetic noise [21] by a copper and mu-metal tube. The inlet of different background gases is performed by an UHV gas nozzle. The interferometer is placed in a chamber where a minimum pressure of 1 1010 mbar is achieved by an ion getter pump in combination with a cryopump.