Boccia, Vincenzo (2025) Measurement of nuclear fragmentation cross sections for Particle Therapy with nuclear emulsion detectors. [Tesi di dottorato]

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Abstract

According to the World Health Organization, cancer is one of the leading causes of death, accounting for nearly 10 million deaths in 2022. Cancer treatments include surgery, radiotherapy, chemotherapy, or a combination of these. It is estimated that approximately 50% of all European cancer patients have an indication for radiotherapy at least once during the course of their disease. While conventional radiotherapy employs megavoltage photon beams, particle therapy mainly uses protons and carbon ions. Charged particles exhibit a favorable depth–dose profile, characterized by a low entrance dose followed by a sharp maximum known as the Bragg Peak. This shape enables precise targeting of the tumor volume with improved sparing of healthy tissues compared to traditional treatments. The main advantage of proton beams compared to conventional radiotherapy is this “ballistic precision,” while their effectiveness in cell killing is comparable to that of photons. Higher biological effectiveness can be achieved with heavier ions due to the Z² dependence of the stopping power, which is especially useful for treating radioresistant tumors. However, with increasing Z, the peak-to-plateau ratio decreases and the dose delivered to normal tissues becomes significant. Carbon ions are therefore considered a good compromise between biological effectiveness and an acceptable peak-to-plateau ratio. Due to their higher mass compared with protons, carbon ions also experience reduced lateral and longitudinal scattering. To reach deep-seated tumors, heavy ions must be accelerated to energies up to a few hundred MeV per nucleon. This requirement significantly increases the cost of constructing and maintaining a particle therapy facility. Moreover, there remain significant uncertainties associated with the interactions of heavy ions with biological matter. An important source of these uncertainties is nuclear spallation reactions, which lead to the fragmentation of both the projectile and target nuclei. Nuclear fragmentation can severely degrade the ballistic accuracy of carbon ion beams because a significant fraction of projectile fragments deposit dose beyond the Bragg Peak, forming the so-called fragmentation tail. Target fragments, on the other hand, are usually produced almost at rest, significantly damaging cells near the production site. In proton therapy, it has been suggested that neglecting target fragmentation may lead to underestimating the dose delivered to patients, particularly in the entrance channel. Currently, available data on nuclear fragmentation cross sections remain scarce, especially for target fragmentation processes, because the resulting fragments typically have extremely short ranges—down to a few hundred nanometers—making them difficult to detect. The FOOT (Fragmentation Of Target) experiment is the result of an international collaboration designed to overcome these challenges by tracking and identifying every charged fragment produced in nuclear fragmentation events between heavy ions and various targets. The goal of the experiment is to measure both projectile and target fragmentation differential cross sections using direct and inverse kinematics, respectively. The selected ions are helium-4, carbon-12, and oxygen-16—chosen for their abundance in biological tissues—at kinetic energies between 200 and 800 MeV per nucleon. Two complementary setups have been designed: • A magnetic spectrometer with electronic detectors optimized for identifying fragments heavier than helium-4 • An emulsion spectrometer using the Emulsion Cloud Chamber (ECC) design, optimized for detecting and identifying lighter fragments (up to lithium) The present thesis work focuses on data from the emulsion spectrometer. Its main goal has been the measurement of projectile fragmentation of oxygen-16 ions with energies from 80 to 200 MeV per nucleon on carbon and polyethylene targets. By combining the results from these two targets, the fragmentation cross section of oxygen-16 on hydrogen nuclei could also be obtained. This dataset has applications beyond charged particle therapy, extending into nuclear physics. In particular, the high spatial and angular resolution of the emulsion spectrometer enables investigation of the fragmentation mechanisms of oxygen nuclei, with emphasis on the production of intermediate states such as beryllium-8 in the ground state. This study fits within the framework of nuclear structure models based on alpha clustering. The final part of this work involved the first direct measurement of target fragmentation induced by protons using a solid-state detector. This effort forms part of the DAMON experiment (Direct Measurement Of Target Fragmentation), which employs a new type of fine-grained nuclear emulsion known as Nano Imaging Trackers (NIT). NIT technology enables the detection of particle tracks down to scales of a few hundred nanometers. Significant effort was dedicated to optimizing NIT for target-fragmentation studies in proton therapy. The research and development process, along with the first results, is presented. This thesis is divided into five chapters: • Chapter 1 covers the physical and radiobiological foundations of particle therapy and highlights the importance of measuring fragmentation cross sections. • Chapter 2 describes the FOOT experiment and its physics program, presenting recent results from both electronic detectors and the nuclear emulsion spectrometer. It also outlines the structure of the emulsion spectrometer and introduces nuclear emulsions. • Chapter 3 discusses the first fragmentation cross-section measurements performed with the FOOT emulsion spectrometer. • Chapter 4 presents the first application of FOOT data to alpha-clustering nuclear-structure models, focusing on the production of beryllium-8 in the ground state from oxygen-16 fragmentation. • Chapter 5 describes the direct detection of proton-induced target fragmentation with NIT, detailing both the R&D activities and the first experimental results.

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