Status of UA9, the Crystal Collimation Experiment in the SPS

UA9 was operated at the CERN-SPS for more than two years to investigate the feasibility of halo collimation with bent crystals. Silicon crystals 2 mm long with bending angles of about 170 μrad were used as primary collimators. The crystal collimation process was steadily achieved through channeling, with high efficiency. The crystal orientation was easily set and optimized with an installed goniometer that has an angular accuracy of about ± 10 μrad. In channeling orientation, the loss rate of the halo particles interacting with the crystal is reduced by half an order of magnitude, whilst the residual off momentum halo escaping from the crystal-collimator area is reduced by a factor two to five. The crystal channeling efficiency of about 75% is reasonably consistent with simulations and with single pass data collected in the extracted proton beam of the SPS North Experimental Area. The accumulated observations, shown in this paper, support our expectation that the coherent deflection of the beam halo by a bent crystal should help considerably in enhancing the collimation efficiency in LHC.