Δοσιμετρία νετρονίων σε περιβάλλον πηγών θρυμματισμού

Abstract

Spallation is an efficient endothermal process for releasing neutrons from nuclei. In order to sustain a spallation reaction, an energetic beam of particles, most commonly protons, must be supplied onto a heavy target. Spallation is a multifragmentation reaction which leads to only one heavy fragment whose mass is close to the target mass AT and a lot of light particles, such as neutrons, protons, deuterons etc. However spallation can play an important role as a source of neutrons whose flux can be easily controlled via the driving beam. A spallation neutron spectrum is determined by the type, the flux and the energy of the beam particles, as well as from the mass and the target geometry. The neutron spectrum of a spallation source in general is a continuous spectrum from intermediate up to fast neutrons. Therefore a spallation source can be used as an example in the material science, or the transmutation and incineration of nuclear wastes. Two different spallation sources were designed and their neutron production was investigated during this study. The first spallation neutron source “Gamma-2” consists of cylindrical Pb target with 8 cm in diameter and length which was varied from 20 cm up to 40 cm. The Pb target was covered with a paraffin moderator. The second spallation source, “Energy plus transmutation” consist of a lead target, with 48 cm length and 8 cm diameter, surrounded by natural Uranium blanket. Four sections of natural Uranium blankets constitute the spallation source. Each section consists of 30 U-rods with 10.4 cm in length and 3.6 cm in diameter. The whole system was surrounded by a polyethylene moderator covered with 1mm thickness Cd. The neutron spectrum produced by the irradiation of the “Energy plus Transmutation” is harder than the neutron spectrum from “Gamma-2” set-up. In the current work the spallation neutron sources were used for transmutation and incineration purposes. In order to transmute an isotope via (n,γ) or (n,f) reaction with thermal neutron a soft neutron spectrum is needed, while a harder neutron spectrum is necessary in order to transmute an isotope via (n,xn) or (n,f) with fast neutrons. For radiation protection purposes spallation neutron sources must be surrounded by an appropriate shielding. In this work measurements of the neutron flux produced by the spallation neutron sources were performed using Solid State Nuclear Track Detectors (SSNTDs). The number of neutrons escaping the shielding materials was also measured in the current work using SSNTDs. In addition phenomenological calculations of the neutron flux behind different shielding materials, as paraffin, polyethylene and concrete, based on empirical relations coming from high energy Physics, and measurements were made. The experimental results were compared with the phenomenological and with Monte Carlo calculations. According to this comparison the phenomenological calculations can be used in order to estimate the dose behind various shielding materials. Moreover the phenomenological method has the advantage that their execution time of the calculation is independent of the depth or thickness of shielding for which the computation is done. Consequently phenomenological method compares favourably with the Monte Carlo calculations and is able to obtain results where the Monte Carlo statistics are poor. According to the phenomenological calculations the most appropriate shielding to reach radiation protection recommendation (1μSv/h ) is proposed to be 140 cm of Concrete and 30 cm of Iron. The experiments were performed in Nuclotron accelerator at the Laboratory of High Energies at Joint Institute for Nuclear Research (JINR), in Dubna Russia.