The charged anticounter for the NA62 experiment at CERN

The NA62 experiment at CERN aims at measuring the Branching-Ratio (BR) of the ultra-rare kaon decay K+ → π+ ν with 10% relative uncertainty, using an unseparated positive hadron beam of 75GeV/c, in order to test the Standard Model (SM) measuring the |Vtd| element of the Cabibbo-Kobayashi-Maskawa (CKM) flavour mixing matrix and to possibly search for physics Beyond SM. Backgrounds, which are up to 1010 times higher than the signal, will be suppressed by an accurate measurement of the momentum and direction of the K+ and the π+ and by a complex system of veto detectors. A critical background can be induced by inelastic interactions of the hadron beam with the silicon beam tracker. Pions produced in these interactions, emitted at low angle, can mimic a K decay in the fiducial region if no other track is detected. In order to suppress this background a CHarged track ANTIcounter (CHANTI), has been designed and is in construction. The detector consists of a series of six guard counters surrounding the beam and placed immediately after the Silicon tracker. Each ring is made of scintillator bars; light produced in a bar is collected by a wavelength-shifting (WLS) fiber running inside the bar. The fiber is read from one side by Hamamatsu Silicon Photo-Multiplier (SiPM); the other side of the fiber is mirrored. We describe the design and of the CHANTI and report on results obtained testing a prototype.