The CHArged ANTI Counter for the NA62 experiment at CERN SPS

The NA62 experiment at CERN is intended to measure, with 10% relative error, the Branching Ratio of the K decay K+ -> pi(+) nu nu which is expected to occur only in about 8 out 10 11 kaon decays. The measurement strategy is to detect the decaying kaon and its product pi(+) and to reject all the background channels, which are up to ten orders of magnitude more abundant than the signal, using both kinematical constraints and particle identification systems as veto. The exceptionally low signal yield makes it necessary to prevent also background which is not due to kaon decays, but is rather connected to the interaction of the intense hadron beam with the residual material enconuntered along the decay volume including the beam spectrometer itself: in order to reduce to an acceptable level this last background we designed the Charged Anti Counter (CHANTI). CHANTI is designed to be a compact, efficient and fast response detector to be operated in vacuum and is made up assembling FNAL-NICADD scintillator bars and fast wavelength shifter fibers. It is read out via silicon photomultipliers coupled to fast electronics. It is composed by 6 stations with x-y views and tracking capability. We already constructed and characterized the bars for two stations and we fully assembled the first station. We also developed, in the NA62 framework (based on the GEANT4 package), a Monte Carlo simulation of the CHANTI and we got an estimation of its efficiency.