The NA62 rare Kaon decay experiment Photon Veto System

The NA62 experiment at CERN SPS is aimed at measuring the rare decay K+→π+νν-bar. This poses very stringent requirements on the particle identification capabilities of the apparatus in order to reject the overwhelming K+→μ+ν and K+→π+π0 background. In particular, a π0 rejection at level of 10-8 is needed to complement the kinematical rejection of π+π0 events. In order to have a full acceptance from 0 to 50 mrad, partly covered by NA48 liquid Kripton calorimeter, a set of veto anti-counters should be placed along the vacuum decay tank, to catch large angle photons with a detection efficiency better than 10-4 in a wide energy range: from few hundreds MeV to 35 GeV. Intense R and D programs have been carried out in order to study different technological solutions: a lead-scintillating fibers calorimeter, lead-scintillator sandwich calorimeter and finally an original re-use of the existing barrel of the OPAL lead-glass electromagnetic calorimeter. We present the results on detector performances and compare the three solutions.