Innovative silicon detectors for measuring the energy of clinical proton beams.

During my master thesis I investigated the use of innovative silicon detectors optimized for time resolution (Ultra Fast Silicon Detectors, UFSDs) to assess the beam energy of clinical proton beams. The research activity I performed is involved in the MoVeIt project of the INFN (Istituto Nazionale di Fisica Nucleare), aiming at developing new beam monitors for future treatments in charge particle therapy. The capability to detect single protons and the outstanding time resolution provided by the UFSD technology are exploited to measure protons’ time-of-flight (TOF), obtaining the energy and, consequently, the depth of penetration (i.e. the range) of the beams in the tissues. In the first part of the thesis, the work is focused on the analysis of the signals acquired by two different UFSD detector prototypes during the beam tests in two Italian facilities: CNAO (Centro Nazionale di Adroterapia Oncologica) and Trento Protontherapy Center. The detector description and the experimental setup are described in chapter 2. The TOF has been calculated as the difference of protons’ time-of-arrival in two UFSD sensors in a telescope configuration, using the constant fraction algorithm. I developed and implemented two different methods to analyze the coincident signals in the two detectors, as described in chapter 2. The third chapter describes the detector calibration, needed to estimate the time offset (due to the cables and the dead time of the acquisition system) and the distance between the sensors in the telescope with an uncertainty of few hundreds of μm. I developed two calibration approaches, as reported in chapter 3. The first one (named absolute approach) is relying on the nominal energies provided by the clinical facilities, while the second one (relative approach) is independent from the nominal energies. The beam energies are then obtained from the TOF values, the distances and the time offset through an analytical approximation validated with Geant4 simulations, taking into account the energy lost in the air between sensors. The preliminary results, obtained with two UFSDs at relative distances ranging between 7 and 97 cm on clinical proton beams with energies between 62 and 228 MeV, showed an error smaller than 1 MeV (at 228 MeV, 97 cm) in the energy estimation. In the fourth chapter of my thesis, I developed two MATLAB applications for the analysis and the computational simulation of a UFSD sensor specifically designed for the project. The latter, segmented in 8 strips, allows the simultaneous acquisition of 16 signals (8 from each sensor of the telescope). The two methods implemented to analyze the signal of one channel, as described in the second chapter, are extended and applied to measure the TOF, identifying the coincident signals. The relative approach of the calibration process, described in the third chapter, is validated against the simulation results. The preliminary experimental results and the simulations of the sensor segmented in 8 strips verified the feasibility of a UFSD prototype for the beam energy measurement. The perspectives and the open-points, like the experimental validation of a final prototype reading out all the 8 strips of the UFSD sensors and the improvement of the data acquisition and processing (to allow the energy estimation in few milliseconds) will be described in the final chapter.