Control of Background Rates in the Underground Pysics Experiments of the CERN SPS Proton Antiproton Collider

The beam halo produces spurious signals (background) in the physics detectors. The background rates are reduced to a nearly constant level of less than 2% of the beam-beam event rate by means of a set. of adjustable collimator blocks inside the SPS vacuum chamber. The principle of the collimation in the SPS environment is explained. The control of the devices and some of their constructional details are discussed.