Gamma-ray induced optical absorption in Ge and P-doped fibres at the LHC

From 2007 the Large Hadron Collider (LHC) at the European Organisation for Nuclear Research (CERN) will provide 14 TeV proton interactions by colliding two counter-rotating 7 TeV proton beams at four points on the 27 km circumference accelerator. Experiments situated at the interaction points will use the decay products to test the standard model of fundamental forces and search for new physics. Many LHC electronics will be installed in the tunnel close to beam in order to optimise performance, increase the S/N ration and to reduce cabling costs. The 3 major distributed control systems in the tunnel that use WorldFIP protocol over fibre optical links are the Quench Protection System, Power Converters and the Cryogenics system. Another big consumer of fibres in the tunnel is the BLM/BPM system that transmits raw data from the low level BPMs/BLMs crates directly over a fibre optical links to the surface at a rate of 40 MHz. Fibres in the LHC tunnel will suffer from radiation-induced attenuation caused by the ionizing component of the mixed LHC radiation field. This may eventually halt communication of data after a few years of nominal operation of the LHC. Radiation-induced attenuation depends mainly on the type and the concentration of dopants (such as Phosphor (P), Fluorine (F) and Germanium (Ge)) in the amorphous silicon dioxide (a-SiO:) core. At present Ge-P-doped and Ge-doped fibres are being installed in the LHC tunnel. Ge-P-doped fibres have been excluded from use in IR7 in favour of the more radiation tolerant Ge-doped fibres. In addition the routing of fibres has been optimized to minimize exposure in the hottest radiation areas and minimize attenuation. Fraunhofer INT has been commissioned to investigate, for different fibre types, the dose rate dependence of the attenuation increase and assess the impact of annealing at the time-scales of the LHC duty cycle.