The Large Angle Photon Veto System For The Na62 Experiment At Cern

The NA62 experiment at CERN SPS aims at measuring similar to 100 events of the very rare decay K+ -> pi(+)nu(nu) over bar (BR similar to 8.5x10(-10)). It poses stringent requirements on PID capabilities to reject the overwhelming pi(+)pi(0) (63%) and K-mu 2(21%) backgrounds. The photon veto system must provide a rejection factor of 10(-8) on pi(0) decays. As a main gamma veto detector, the NA48 liquid Kripton calorimeter will be used. To have full geometrical acceptance up to 50 mr, a set of 12 veto stations should be placed along the vacuum decay tank, with an inefficiency <10(-4) in a wide energy range (200 MeV-35 GeV). Good energy resolution (similar to 10% at 1 GeV) for threshold definition, good time resolution (similar to 1 ns) to be used at the trigger level, sensitivity to MIP for calibration with muons of the beam halo are needed. A moderate segmentation in the azimuthal angle is desirable, for reducing the counting rate and providing information on the. direction. We performed an intense R&D program on three solutions: spaghetti calorimeter, lead/scintillator sandwich calorimeter, and original re-use of the existing barrel of the OPAL lead-glass e.m. calorimeter. Studies have been performed at the Frascati BTF beam and all three meet the efficiency requirements. The final choice uses a peculiar radial arrangement of lead-glasses in rings. Front-end electronics has been designed to cover the tree orders of magnitude of the signal, contributing to the trigger, and integrated in the general TDAQ, while keeping low cost and simplicity. The first five full veto stations have been constructed. Two tests have been done and problems found fixed. We will discuss about R&D for the technology choice, LAV construction, test beams results and simulation performance. (C) 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of the organizing committee for TIPP 11.