CNGS Reflector installed

A major component that will help target the CNGS neutrino beam for its 732km journey through the earth's crust, from CERN to the Gran Sasso laboratory in Italy, has been installed in its final position.

The transport of the huge magnetic horn reflector through the CNGS access gallery.

A team from CNGS and TS/IC, and the contractors DBS, transported the magnetic horn reflector on 5th December, in a carefully conducted operation that took just under two hours. The reflector is 7m long, 1.6m in diameter and 1.6 tonnes in weight. With only a matter of centimetres to spare on either side, the reflector was transported through the CNGS access gallery, before being installed in the experiment's target chamber.

The larger of two magnetic horns, the reflector will help refocus sprays of high energy pions and kaons emitted after a 0.5MW stream of protons from the Super Proton Synchrotron (SPS) strikes nucleons in a graphite target.

The horns are toroidal magnetic lenses and work with high pulsed currents: 150 kA for the first horn, and 180 kA for the reflector. These currents flow for around 7 milliseconds at the instant when a proton beam pulse arrives at the target. The heat deposited by this current requires the thin inner conductors of the toroids to have a complex water cooling system. Further cooling is needed due to the energy deposited by stray particles passing through the inner and outer conductors of the horns.

After passing through the horns, the focussed beam of pions and kaons enters a one-kilometre evacuated tunnel, inside which they decay in-flight into neutrinos and their sister muon particles. At the end of this tunnel a barrier, consisting of 3 metres of graphite followed by 15m of iron, absorbs any 'rogue' protons and blocks remaining un-decayed pions and kaons. This process leaves behind a beam of muons (which are quickly absorbed in rock) and muonic neutrinos, which will traverse a section of the Earth's crust on their journey to Gran Sasso.

Before this milestone, the CNGS team had already completed a test installation of the first magnetic horn in November 2004. The final installation of that horn is scheduled for 24 January 2006.