CHANTI: a fast and efficient charged particle veto detector for the NA62 experiment at cern

This work has been performed into the frame of the NA62 experiment at CERN that aims at measuring the Branching-Ratio of the ultra-rare kaon decay K+→π+ nu nubar with 10% uncertainty - using an unseparated kaon beam of 75GeV/c - in order to test the Standard Model (SM), to look for physics beyond SM and to measure the |Vtd| element of the Cabibbo-Kobayashi-Maskawa (CKM) flavor mixing matrix. Backgrounds, which are up to 10^10 times higher than the signal, will be suppressed by an accurate measurement of the momentum of the K+ (with a silicon beam tracker named GigaTracker) and the π+ (with a straw tracker) and by a complex system of particle identification and veto detectors. A critical background can be induced by inelastic interactions of the hadron beam with the GigaTracker. Pions produced in these interactions, emitted at low angle, can reach the straw tracker and mimic a kaon 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 built in Naples. The detector consists of a series of six guard counters surrounding the beam and placed immediately after the silicon tracker. Each guard counter is made up of two layers, X and Y. Y (X) layer is composed of 24 scintillator bars arranged parallel to X (Y) direction. Each bar is triangularly shaped with a 1.7 mm diameter hole. In order to collect the light of the scintillator, a wavelength shifting (WLS) fiber is inserted into the hole of each bar. The fiber is mirrored at one side and is read by a Hamamatsu silicon photomultiplier (SiPM) at the other side. In this thesis the design philosophy, the construction procedure and the quality tests, adopted during the assembly of the detector, are reported. A careful calibration procedure of the frontend electronics has been setup. It is made up of two different stage. The first one is used to set bias voltage of the SiPMs and to read their current; it also gives a fast amplification of the signal before passing them to the second stage in which a comparator gives an LVDS output of duration equal to the time a signal is above a given threshold. A simulation of the scintillation bar, that includes all the optical processes inside this single channel of the CHANTI, and the complete digitization of the signals have been developed. The performance of the detector have been evaluated both in the laboratory facilities in Naples, with cosmic rays, and in the experimental area of the NA62 experiment with real beam condition. In particular the efficiency, time and space resolutions and accidental veto rate of the detector have been determined analysing the data collected during the first NA62 physics run in 2015.