• time of flight
• hadrontherapy
• Silicon Photomultipliers

Hadrontherapy, also called Particle Therapy, is a consolidated clinical treatment for
solid tumors based on the application of charged particles beams. The main advantage of
this technique with respect to conventional radiotherapy, based on photons and electrons
beams, resides in the different energy loss mechanism that characterizes the interaction of
protons and heavier ions with matter. Charge particles release most of their energy at the
end of their path inside the medium, while the energy deposition in the entrance channel is
low. The effective result is the delivery of a high dose to the anatomic district of interest,
sparing the healthy tissues. Another advantage of adopting heavy charged particles in
treating tumors consists in their enhanced biological effectiveness with respect to photons
and electrons and, as a consequence, their capability to produce a direct damage to the
cancerous cells is higher.
Nuclear fragmentation processes induced by the interaction of protons and heavier nuclei
with matter are one of the most discussed topics in hadrontherapy. In heavy ion treatments,
the main effect of nuclear inelastic interactions consists in the fragmentation of the
projectile. The reaction products have generally the same velocity and direction of the
projectile, but lower mass. Therefore they have a longer range than the primary particle
and they will deliver a undesirable dose beyond the tumor volume. Differently, in proton
therapy the production of short range target fragments can lead to an enhanced local
dose deposition in the entrance region. The lack of experimental data about the reaction
cross sections in the energy range of hadrontherapy make the evaluation of the fragments
contribution to the dose difficult.
The main goal of the FOOT (FragmentatiOn Of Target) experiment is to measure the
target and projectile double differential cross section of nuclear fragmentation reactions
relevant for hadrontherapy. To reach this goal, the experimental apparatus is especially
designed to adopt inverse and direct kinematic approaches to accurately characterize both
the cases. To identify the nuclear fragments produced by different particles beams, FOOT
performs measurements of mass, charge and velocity. Charge identification is performed
with a combined measure of Time Of Flight (TOF) and energy deposited in a detector
named TOF-Wall (TW).
The TW is composed by two orthogonal layers of 20 plastic scintillator bars. The readout
of the TW signals is performed by Silicon PhotoMultipliers (SiPMs) optically coupled at
the ends of each bar. The requirements of FOOT experiment are to reconstruct the Z
of the fragments and their mass with an accuracy respectively of 2-6 % and 3-6%. To
reach this goal, an energy resolution of  few % and a TOF resolution of about 100 ps
are required.
This thesis work is focused on the optimization of the TW operation conditions to meet
the FOOT requirements. SiPMs were firstly characterized to measure their main parameters
(gain, dark count rate, crosstalk and afterpulse) as a function of the applied
overvoltage. The obtained results were then included in a Monte Carlo simulation to
model the SiPMs response when they are irradiated by the scintillation photons produced
by the particles impinging on the bars. Simulation outcomes were studied to determine
the energy resolution for different particles and by varying the operation parameters of the
SiPMs to understand the effective impact of the photodetector in energy measurements.
At a fixed SiPMs overvoltage, a resolution of about 6.1% was obtained for protons, while
for carbons it varied from a minimum of 2.7% (115 MeV/u Carbon ions) to a maximum
of 5.6% (400 MeV/u Carbon ions). Based on these results, the main contribution to the
energy resolution is due to the plastic scintillaor. In fact, from an additional comparison
with an ideal case (i.e. SiPMs PDE at 100% and without dead regions), SiPMs noises and
finite PDE have no significant impact on the energy resolution. Finally, the simulation
performed for a fixed particle (i.e. protons) by varying the SiPMs overvoltage highlights
that the impact of the PDE is relevant only for low overvoltage.