Narrow Results

In a detector exposed to high flux of particles (100 Mrad and more) and whose fibres are the active elements one has to know the level of radiation accumulated to calibrate the fibre read-out. This is the case for the forward calorimeter of the CMS experiment at LHC (HF) made of iron as absorber and quartz fibres generating and transmitting Cerenkov light to PMTs. The light spectrum detected is centered near 450 nm (blue). The important transmission variations at these wavelength (few tens %) due to radiation damage and recovery led us to monitor on line the blue light transmission with special fibres called Raddam fibres. Injecting blue light in a 2.5m long fibre, we used the ratio of reflected signal at the two ends to determine the absorption of the fibre. This paper will describe the principle, the construction and tests of this device.