Collimation: a silicon solution
Silicon crystals could be used very efficiently to deflect high-energy beams. Testing at CERN has produced conclusive results, which could pave the way for a new generation of collimators.
1) The beam passes without "seeing" the crystals and is not deflected
2) The beam is deflected by channelling (with an angle of around 100 μrad)
3) The beam is reflected (with an angle of around 50 μrad).
The intensity of the deflected beam is illustrated by the intensity of the spot. The spot of the reflected beam is clearly more intense than that one of the channelled beam, demonstrating the efficiency of the reflecting method compared to the channelling technique.
Beam cleaning, consisting in the elimination of particles deflected from the normal orbit of the beam, is a crucial problem for the LHC as these wayward particles can cause magnet quenches and damage machine components. The accelerator will therefore be equipped with a sophisticated system consisting of a hundred or so collimators, fibre-reinforced graphite, copper, tungsten and carbon blocks placed at intervals around the ring.
However, a team of researchers from CERN, the Italian Institute for Nuclear Physics (INFN), the St. Petersburg Nuclear Physics Institute (PNPI), the Protvino Institute for High Energy Physics (IHEP) and the Dubna Joint Institute for Nuclear Research (JINR) have carried out important work on silicon crystals, which could prove extremely useful for the collimation of the LHC and other high-energy accelerators.
The team, led by Walter Scandale of CERN, used the reflective properties of the crystals to deflect a 400 GeV proton beam from the Super Proton Synchrotron. Five crystals were aligned to deflect the beam. The results were excellent, with a 98% reflection efficiency per crystal over the volume of the beam. With the five crystals aligned, over 90% of the particles were thereby deflected. This solution also allows a wider range of incidence angles compared to the channelling technique, in which the ordered structure of the crystals is used to guide the charged particles into the potential trough between two crystal planes. In order to be channelled, the particles must only have a limited transverse energy, which makes the method less effective. As an illustration, the channelling technique was able to deflect only 55% of the particles over the same beam.
The team carried out two series of tests, one in 2006 and the other this spring. "We are very pleased to have obtained good results in a very short space of time", notes Walter Scandale with satisfaction. The team is now working on a system that will allow remote alignment of the crystals.
See also the article in the CERN Courier of June 2007 (p. 8).
