Nuclear fragmentation in particle therapy and space radiation protection: from the standard approach to the FOOT experiment

Today, the application of particle beams in cancer therapy is a well-established strategy and its combination with surgery and chemotherapy is becoming an increasingly reliable approach for some several clinical cases (e.g. skull base tumors). Currently, protons and 12C ions are used for patients’ treatment, due to their characteristic depth-dose deposition profile featuring a pronounced peak (the Bragg Peak) at the end of range. Clinical energies typically span between 60 and 250 MeV for protons and up to 400 MeV/u for 12C ions, in order to deliver treatments to various disease sites. Interactions between the primary beam and the patient’s body always occur during treatment, changing the primary radiation composition, energy and direction and thus affecting its depth dose and lateral profile. In carbon therapy, both projectile and target fragments can be generated during a treatment: the former are characterized by a kinetic energy spectrum peaked at the same energy of the primary beam and are mostly emitted in the forward direction; the latter are emitted with a much lower energy because they are produced from the target, which is at rest before the collision, and they are generated isotropically in the target frame. Moreover, the interaction of carbon ions with the patient's body is currently modeled in the treatment planning on the basis of experimental data measured in water. For all other biological materials, the contribution of nuclear interactions is taken into account by rescaling the values measured in water with a density factor. This approximation neglects the influence of the elemental composition, which might become relevant in cases where the material encountered by the beam significantly differs from water (e.g. bone or lung tissues) and result in a non-uniform and incorrect dose profile. Thus, experimental data with target different from water are clearly needed in order to correctly evaluate the contribution of all biological elements inside the human body. Treatments with protons can only generate target fragments, leading to the production of low-energy and therefore short-range fragments. Heavy secondary fragments will have a higher biological effectiveness than to protons, thus affecting the proton Relative Biological Effectiveness (RBE, i.e. the ratio of photon to charged particles dose necessary to achieve the same biological effect), nowadays assumed as a constant value (RBE=1.1) in clinical practice. Another aspect related to nuclear interactions is the overlap between radiotherapy and space radiation protection. The group of particle species either currently available in radiotherapy or considered promising alternative candidates (i.e. Helium, Lithium and Oxygen) are among the most abundant in the space radiation environment. Moreover, the proton energy range used in radiotherapy is similar to that of Solar Particle Events (SPEs) and Van Allen trapped protons. The radiation environment in space can lead to serious health risks for astronauts, especially in long duration and far from Earth space missions (like human explorations to Mars). Protection against space radiation are of paramount importance for preserving the astronauts’ life. Today, the only possible countermeasure is passive shielding. Nuclear fragmentation processes can occur inside the spaceship hull, causing the production of lighter and highly penetrating radiation that must be considered when a shielding is designed. Therefore, experimental data for beam and targets combinations relevant in space radiation applications must be collected for characterizing the interaction of mixed generated radiation field and assess the radiation-induced health risk. Despite the many fundamental open issues in particle therapy and space radiation protection fields, such the ones mentioned above, the current lack of experimental fragmentation cross section data in their energy range of interest is undeniable. Thus, accurate measurements for different ions species with energies up to 1000 MeV/u would be of great importance in order to further optimize particles treatments and improve the shielding design of spaceship. Moreover, additional experimental data would be of great importance for benchmarking Monte Carlo codes, which are extensively used by the scientific communities in both research fields. In fact, the available transport codes suffer from many uncertainties and they need to be verified with reliable experimental data. Due to high energy and long range of projectile fragments, the standard approach for their identification is collect data from several detector types, usually two plastic scintillators coupled with a Barium Fluoride or LYSO crystal, placed both upstream or downstream the target, providing information about the charge, energy loss, the residual kinetic energy and the time of flight of the emitted fragments. This experimental setup allows the identification of particle species in terms of charge, isotope, emission angle and kinetic energy and it has been widely exploited to perform several fragmentation measurements, both in particle therapy and space application fields. An example is the ROSSINI (RadiatiOn Shielding by ISRU and/or INnovative materIals for EVA, Vehicle and Habitat) project financed by the European Space Agency (ESA) to select innovative shielding materials and provide recommendations on space radioprotection for different mission scenarios. However, such standard approach is not useful for the characterization of target fragments. In fact, because of their low energy and short range, a much more complex setup and finer experimental strategies are required for their detection. The FOOT (FragmentatiOn Of Target) experiment has been designed to measure fragment production cross sections with ~5% uncertainty. Target fragmentation induced by 50-250 MeV proton beams will be studied taking advantage of an inverse kinematic approach. Specifically, O, C and He beams impinging on different targets (e.g., C, C2H4) will be employed, thus boosting the fragments energy and making their detection possible. Fragmentation cross section of hydrogen will be then obtained by subtraction. The same configuration provides also a measurement of projectile fragments with the direct kinematics approach. FOOT experimental setup consists of two different apparatus: a dedicated “table-top” electronic setup, based on a magnetic spectrometer, were conceived for the detection of heavier fragments (Z≥3). Alternatively, an emulsion spectrometer was designed in order to measure the production of low Z fragments (Z≤3) that would not cross the whole magnetic spectrometer. The purpose of the work presented in this doctoral thesis is the experimental characterization of particles originated in nuclear fragmentation processes for targets and beams of interest for particle therapy and space radiation protection, providing inputs to improve the accuracy of Monte Carlo transport codes presently used. Data collected in experimental campaigns using the standard setup to study the interaction of 400 MeV/u 12C ions beam with bone-like materials and 1000 MeV/u 58Ni ions beam with targets relevant for space applications have been analyzed. The presented fragments characterization comprehends the fraction of primary particles surviving the target and the yield and kinetic energy spectra of charged particles emitted at several angles with respect to the primary beam direction. The )*Ni beam data were collected in the frame of the ROSSINI experiment and focused on characterizing secondary neutrons production. Moreover, the analysis of the performances and fragments reconstruction capabilities of the FOOT electronic setup has been accomplished with Monte Carlo simulations. A dedicated analysis software has been developed in order to reconstruct fragments charge and mass, energy yields and production cross sections. A preliminary analysis of experimental data collected by a partial FOOT electronic setup is presented as well.