Improvement of photon transport model by including coupled photon-electron transport and kernel refinement

The first part of this work deals with the inverse problem solution in the X-ray spectroscopy field. An original strategy to solve the inverse problem by using the maximum entropy principle is illustrated. It is built the code UMESTRAT, to apply the described strategy in a semiautomatic way. The application of UMESTRAT is shown with a computational example. The second part of this work deals with the improvement of the X-ray Boltzmann model, by studying two radiative interactions neglected in the current photon models. Firstly it is studied the characteristic line emission due to Compton ionization. It is developed a strategy that allows the evaluation of this contribution for the shells K, L and M of all elements with Z from 11 to 92. It is evaluated the single shell Compton/photoelectric ratio as a function of the primary photon energy. It is derived the energy values at which the Compton interaction becomes the prevailing process to produce ionization for the considered shells. Finally it is introduced a new kernel for the XRF from Compton ionization. In a second place it is characterized the bremsstrahlung radiative contribution due the secondary electrons. The bremsstrahlung radiation is characterized in terms of space, angle and energy, for all elements whit Z=1-92 in the energy range 1–150 keV by using the Monte Carlo code PENELOPE. It is demonstrated that bremsstrahlung radiative contribution can be well approximated with an isotropic point photon source. It is created a data library comprising the energetic distributions of bremsstrahlung. It is developed a new bremsstrahlung kernel which allows the introduction of this contribution in the modified Boltzmann equation. An example of application to the simulation of a synchrotron experiment is shown.

Development and performance assessment of a Plasma Focus electron beam generator for Intra-Operative Radiation Therapy

The Plasma Focus is a device designed to generate a plasma sheet between two coaxial electrodes by means of a high voltage difference. The plasma is then driven to collapse into a “pinch”, where thermonuclear conditions prevail. During the “pinch phase” charged particles are emitted, with two main components: an ion beam peaked forward and an electron beam directed backward. The electron beam emitted backward by Plasma Focus devices is being investigated as a radiation source for medical applications, using it to produce x-rays by interaction with appropriate targets (through bremsstrahlung and characteristic emission). A dedicated Plasma Focus device, named PFMA-3 (Plasma Focus for Medical Applications number 3), has been designed, put in operation and tested by the research groups of the Universities of Bologna and Ferrara. The very high dose rate (several gray per discharge, in less than 1 µs) is a peculiarity of this device that has to be investigated, as it might modify the relative biological effectiveness (RBE). Aim of this Ph.D. project was to investigate the main physical properties of the low-energy x-ray beams produced by a Plasma Focus device and their potential medical applications to IORT treatments. It was necessary to develop the optimal geometrical configuration; to evaluate the x-rays produced and their dose deposited; to estimate the energy electron spectrum produced in the “pinch phase”; to study an optimal target for the conversion of the x-rays; to conduct simulations to study the physics involved; and in order to evaluate the radio-biological features of the beam, cell holders had to be developed for both irradiations and cell growth conditions.