• readout
• neutrons
• radiation
• simulation
• cflv
• performances
• digitizer
• calorimetet
• electronic
• Mu2e

The Mu2e experiment at Fermilab will search for Charged Lepton Flavour Violation (CLFV) looking for the conversion of a negative muon into an electron in the field of a nucleus. This process is heavily suppressed in the Standard Model of particle interactions (SM), with a branching ratio BR<10^(−50) and its detection would be a striking indication of new physics. The experiment sets out to achieve a single event sensitivity of 3×10^(−17) on the ratio between the probability of the conversion of a negative muon into an electron and the one of the muon capture by the nucleus. Even in case of no observation, this sensitivity will allow to improve by 4 orders of magnitude the present limit on the BR of this process, probing the predictions of different extensions of the SM. Mu2e is a complex experimental apparatus whose main subdetectors are a straw tube tracker, an Electromagnetic Calorimeter (ECAL) made of pure CsI crystals and a Cosmic Ray Veto system. The ECAL is particularly important for distinguishing electrons and muons to suppress the cosmic background, that is the main background of the experiment. To achieve this task the ECAL needs an excellent time resolution and a very good energy resolution. A waveform digitizer board (DiRAC) has been specifically designed to sample the calorimeter readout channels. The sampling frequency has been tuned through Monte Carlo simulations to 200 MHz. The work of this thesis has been focused on the first tests of the DiRAC board and in particular to the analysis of the data collected with the calorimeter prototype using cosmic rays. The board has firstly been checked with a custom system, developed during this thesis work, emulating the detector response, to check the correct reading of each pulse. Stress tests have been performed to optimize the board firmware and validate its functionality at high rates. The board has then been connected to a system made of a Silicon photomultiplier and a CsI crystal, emulating the full readout chain of Mu2e. All the components were the final ones that will be used in the experiment so this can be considered the first slice testof the ECAL readout system. Data from cosmic rays collected through this setup were used both the evaluate the integrity of the readout stream and to optimize the gain of the analog electronics using the minimum ionizing particles energy distribution as a reference. The main part of the thesis presents the experimental setup and data analysis of a much more complex Vertical Slice Test, performed at the INFN National Laboratories of Frascati, using a small scale prototype of the ECAL, called Module 0. This is equipped with 51 crystals and 20 of them are read by a single DIRAC board. All the system was kept in vacuum and thermalized to the real Mu2e expected running temperature. A large cosmic data sample has been collected using a continuous data taking, similar to the one that will run in the experiment, or an external trigger provided by two scintillator counters. The analysis of these data has allowed to confirm the full functionality of the board, to characterize the noise levels of the full acquisition chain, to validate the time and energy calibration procedures and the dynamic range of ECAL electronics and finally, for the first time, to evaluate the time and energy resolution of the calorimeter with the final readout chain in the expected running conditions. The time and energy resolutions measured with the Vertical Slice Test are in agreement with the ones obtained at previous test beams, using a commercial readout system. A realistic evaluation of ECAL performances has to take into account also the effects of neutron and ionizing radiation on the calorimeter components. Both crystals and SiPMs response can significantly differ from the ones measured at the test beam after a long exposure to radiation. For this reason it’s important to measure the neutron fluence and the total ionizing dose reaching the calorimeter. A section of this thesis will report on the work done to develop an innovative Neutron Radiation Detector based on the readout of dark current of SiPMs. Neutron radiation would increase the dark current value of SiPMs, and this physical issue can be used to extract the amount of radiation taken by the SiPMs itself. Copies of this detector will be placed in different parts of the calorimeter to continuously monitor the neutron fluence helping to set up the possible countermeasures. Finally, the results of the Vertical Slice Test have been used to tune the ECAL simulation used by the collaboration, including the effect of radiation damage in the degradation of ECAL performances. A multivariate analysis classifier has been trained to identify the conversion electrons against muons. The results confirm that the cosmic backgroud can be suppressed at the level needed to achieve the experiment sensitivity goal.