Caratterizzazione dosimetrica della facility a neutroni termici del reattore TRIGA di Pavia: studio della dose da fotoni mediante rivelatori ESR di alanina

The optimization of the procedures of Neutron Capture Therapy (NCT) for cancer treatments involves research for beam characterization. One major issue for this therapy is the reliable dosimetric determination of the various (neutronic and photonic) components of the employed beam. In particular, the precise and accurate measurements of the gamma photon component is fundamental for evaluating the risks to healthy tissues hit by the mixed field. Among solid state dosimeters the alanine detectors read by Electron Paramagnetic Resonance (EPR) technique present several advantages such as: tissue equivalence for photon and electron beams, linearity of its dose-response over a wide range, high stability of radiation induced free radicals, no destructive read-out procedure, no sample treatment before EPR signal measurement and low cost of the dosimeters. These features associated with the possibility of recognizing the various components of a mixed radiation fields makes alanine a good candidate for dosimetry in neutron-gamma fields. In this work we determine the gamma component of the mixed radiation field in thermal column of the Triga Reactor of University of Pavia (which is used for experimental activities on NCT) by means of alanine EPR dosimeters. Commercial alanine dosimeters produced by Synergy Health (Germany) were exposed in three positions in the thermal column; the irradiations were performed inside graphite holders to avoid use of hydrogenous phantoms for minimizing the gamma contribution due to the plastic holders. EPR measurements were carried out through Bruker ECS106 spectrometer equipped with a TE102 rectangular cavity. In order to isolate the gamma components of the mixed field two kinds of irradiations were carried out inside a lithium carbonate box (wherein the thermal neutron component is heavily reduced) and outside of it. MCNP Monte Carlo simulations of the irradiation set-up were carried out, calculating the contributions of the various components present in the mixed field (thermal and fast neutron and gamma). The experimental values are compared with the computations of the Monte Carlo simulations and the results are discussed on the basis of the mixed field features and on the response of alanine dosimeters to high and low LET radiations.