Bonner Spheres neutron spectrometry for Boron Neutron Capture Therapy purposes

Boron Neutron Capture Therapy (BNCT) consists of an experimental form of localized radiotherapy involving a thermal neutron absorption reaction carried out by 10-B previously accumulated in cancer cells. During the treatment, the patient is irradiated with an epithermal neutron beam, which undergoes thermalization after crossing the skin and prior to its arrival on the target cell. At the physics department of the University of Turin, photons exiting a medical linear accelerator were engaged in a photonuclear reaction to produce a fast neutron beam, which was subsequently slowed down to epithermal energy through a moderator structure. This dissertation outlines the steps that were taken in order to acquire the energy spectrum of the aforementioned moderated beam and assess its epithermal component. The experimental count rates were acquired through a Bonner Spheres Spectrometer (BSS), consisting of a set of polyethylene spheres with different diameters. Each one is equipped with a Thermal Neutron Rate Detector (TNRD), based on a couple of silicon detectors sensitive to thermal neutrons after deposition of a thin layer of 6-LiF. The first step was the construction of the response matrix of the BSS system, namely the set of response functions associated to each sphere, that was extracted by means of transport codes using the MCNP6 software. A series of ROOT macros was developed throughout the study to analyze collected data and simulated outputs. Starting from the experimental readings given by the spectrometer, the neutron fluence rate as a function of energy was obtained. This was achieved through a mathematical method known as unfolding, performed by the FRUIT-SIX code using a simulated epithermal guess spectrum. A fine correction, taking into account the scattered neutron component after each exposition, was also studied and implemented. In addition, a folding process was conducted to verify the quality of the response matrix used in the study. For this purpose, the predicted ideal energy spectrum for BNCT was selected. This estimation was performed in Pavia, at the local section of the National Institute of Nuclear Physics (INFN). The characterization of the neutron beam ultimately led to an energy spectrum compatible with the predicted epithermal one. Moreover, a spectrum consistent with the BNCT ideal one was obtained from the folding process, confirming the accuracy of the BSS response curves.