Simulation, realization and test of veto systems for the NA62 experiment
Palladino, Vito (2010) Simulation, realization and test of veto systems for the NA62 experiment. [Tesi di dottorato] (Inedito)
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During my PhD course I have been involved in research activities into the frame of the NA62 experiment, which main goal is the measurement of the branching fraction B(K+ -> pi+ nu anti-nu). NA62 is a very challenging experiment due to ultra rare nature of this decay (BR is about 8e(-11)). The study of the decay K+ -> pi+ nu anti-nu is very important because it allows the first direct measurement of CKM element Vtd. Moreover it could provide signals of physics beyond Standard Model (SM) as that decay is highly sensitive to new physics. The experiment will be located at CERN experimental SPS North Area (hence the acronym NA). It is a fixed target experiment done using a unseparated 75 GeV/c beam of positive hadrons, produced by a 400 GeV/c proton beam. Positive Kaons, although being only 6% of the beam, are produced in a very abundant fashion, and will allow us to collect enough statistics to reach a about 10% relative uncertainty after a 2 years long data taking. To reach such level of uncertainty and to keep signal/background ratio below 10, the apparatus is designed in order to provide both particle identification and kinematical rejection. My work was focused on two veto subsystems with different scope, different architecture and different dimensions: the Large Angle photon Veto (or LAV) and the CHarged ANTIcounter (or CHANTI). The LAV is made of 12 different stations all along the decay region. Each station is ring shaped. The main task of LAV is to veto photons from neutral pions decay with an inefficiency lower than 10e(-4), to reduce background from K+ -> pi+ pi0. In order to choose the best technology to implement the detector we have had an intense R&D program. Three different prototypes were tested and finally a solution that uses lead glass blocks from former OPAL electromagnetic calorimeter was chosen. The prototype used to measure inefficiency was made in Naples and tested at Laboratori Nazionali di Frascati beam test facility with electrons of energy between 300 and 500 MeV/c. Unfortunately the area at CERN where blocks were stored was flooded and all blocks were involved. This major event forced a massive recovering campaign, of which I was responsible. During recovering operations many problems were found and (most of them) solved. However not all flooded blocks may be used for LAV. Part of them were damaged (9%) and an other part showed an abnormal behaviour. During the recovering operation the design of the LAV was refined and construction started. In order to reduce costs we opted for a read-out electronics based on Time over Threshold (ToT) technique. The first station out of 12 was realized and tested at CERN in June-September 2009. It was an important mile stone. The test was intended, basically, as confirmation of ToT usefulness and as check of off-line equalization procedure that each block needes to pass before being mounted. To test ToT, signals were splitted and read on one side by custom ToT electronics and from the other side by a commercial CAEN QDC, in order to produce a ToT versus charge curve. Surprisingly not a unique curve was found. After an accurate data analysis the problem was isolated and a solution proposed, consisting in changing the HV dividers of all blocks. A second module was built with new dividers and tested again at CERN. Preliminary data analysis (still going) is showing that the problem is solved and ToT can be used as read-out solution. The other item I was involved on is the CHANTI project. It is a small detector that will be placed just after last station of incoming Kaon tracking system (called Giga TracKer or GTK). GTK is made of three silicon station hit by the 800 MHz incoming beam of hadrons. About 0.3% of particles crossing the GTK undergo in inelastic collision in which the incoming hadron strongly interacts with a nuclei of GTK station producing many particles, among them pions. If inelastic events involves the third station a signal event could be mimicked if only a pion is detected. CHANTI aims to veto these events detecting the cloud of particles produced together with pion. It is made by a series of guard ring made by X and Y scintillating bars planes. Bars have a triangular shape, thus are naturally staggered. Each bar is read through a Wave Length Shifter fiber coupled to a Silicon PhotoMultiplier (SiPM). A detailed Monte Carlo was adopted to improve geometry design and to estimate the efficiency of CHANTI; moreover neutron fluence, crucial if SiPM are to be used, was estimated. Finally we designed and constructed a first full size prototype. Preliminary results about response and time resolution have been done using cosmic rays.
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