• hadronterapy

Nowadays, one of the most advanced methods for solid tumor treatment is represented by hadrontherapy, a high precision external radiotherapy technique which applies collimated beams of protons or heavier ions to defeat radioresistant tumors and which has given a boost to the use of radiation in the fight against cancer.
Compared to the standard X-ray based treatments, due to the profile of their released dose in tissue, charged hadron beams can be very effective in destroying the tumor and, at the same time, avoiding the adjacent healthy tissue. On the other hand, the use of accelerated particles requires appropriate methods to accurately evaluate the dose distribution inside and outside the planned target volume during the irradiation treatment. The most important difference between protons and heavier ions is the increased biological effectiveness of the latter, i.e., with heavy ions a lower physical dose is needed to obtain a given biological effect. Carbon ions are particularly attractive for hadrontherapy, due to the fact that their energy deposition is highly localized and their relative biological effectiveness (RBE) is high towards the end of the range, thus offering an additional advantage for slowly growing radioresistant tumors. However, when the carbon beam penetrates matter, the primary ions can be fragmented as a result of the collisions with the tissue atomic nuclei. The collisions along the carbon path lead to the attenuation of the primary beam intensity and the production of secondary fragments, such as neutrons and ions lighter than carbon (hydrogen, helium, lithium, beryllium and boron isotopes). These lighter fragments have longer ranges and wider energy distributions than the primary particles and give rise to a characteristic dose tail behind the Bragg peak (BP). The biological effect of a certain type of ion radiation is dependent also on these secondary products, therefore, a detailed knowledge of the fragmentation process is essential to guarantee the appropriate treatment accuracy.

The determination of the RBE of the fragments by means of radiobiological experiments is difficult and, in the energy range of biomedical interest, limited cross section data are available for the production of heavy recoils after proton irradiation. The new experiment FOOT (FragmentatiOn Of Target) has therefore been proposed to measure the fragmentation cross sections of the target in proton therapy and of the projectile in ion therapy. To overcome the difficulties related to the short fragment range, the FOOT experiment adopts an inverse kinematic approach. The final FOOT apparatus will consist of a high precision tracking system in magnetic field (composed by silicon pixel trackers and silicon strip detectors), a time-of-flight dE/dx measurement system (hereafter called TOF dE/dx detector) and a calorimeter. The beam monitoring system of the FOOT experiment will include a plastic scintillator detector to measure the incoming rate of the ion beam and a drift chamber to measure the incident beam direction and position.

The final TOF dE/dx detector will be composed by two orthogonal layers of 22 plastic scintillator bars each, and each bar will be readout at both ends by silicon photomultipliers (SiPM). This detector will be used to measure the Time Of Flight of the fragments and the energy released in the scintillator (for the calculation of the mass number A). The requirements of the FOOT experiment are: enrgy resolution 2% and TOF resolution 100 ps.
In the development of the final TOF dE/dx detector, some critical parameters need to be taken into account.
These are, for example, the thickness of the plastic scintillator bar, the efficiency of the light collection (i.e. the number of SiPMs at the ends of the bars and the type of connection among them) and the data acquisition parameters which are used to digitize the signal.
Before developing the final detector, these parameters need to be studied and tuned.
The aim of the present thesis is therefore the development and the characterization of a TOF dE/dx protoype that consists of a plastic scintillator bar coupled on both sides to SiPMs and to study how the time and energy resolution depend on factors such as the beam energy, the beam interaction position inside the scintillator bar, the SiPM applied voltage and the electronics sampling rate.

In Chapter 1, the state of the art of hadrontherapy is introduced, followed by a description of the relevant radiobiological parameters adopted in Particle Terapy. Chapter 2 shows the measurements strategy and the experimental setup adopted in the FOOT experiment. In Chapter 3, the structure of the TOF dE/dx prototype detector and the calculation of the time and energy information is explained. The preliminary results obtained with a TOF dE/dx detector prototype at the Proton Therapy Centre (PTC) of the Trento Hospital with a 70-230 MeV proton beam are presented and discussed in Chapter 4. In the final Chapter, the conclusions of the work are drawn and the future perspectives are outlined.

The results of the characterization of the detector prototype developed in this thesis have shown that a detector with time resolution lower than 100 ps and energy resolution lower than 2%, can be developed for particle beams with energy and atomic number in the range of interest for the FOOT experiment. In addition, these results will allow to develop a final detector with time and energy performance compliant with the requirements of the FOOT experiment.