Monte Carlo neutronic calculations for the design of VESPA shielding at the European Spallation Source

Abstract

The European Spallation Source (ESS) is the next world’s most powerful pulsed neutron source, under construction in Lund, in the south of Sweden. Many scientific and industrial fields will benefit from ESS, like pharmaceutical drugs, manufacturing, biotechnology, information technology, chemistry, and so on. The facility will produce neutrons by spallation reactions, induced by high energetic protons, accelerated up to the energy of 2 GeV and eventually conveyed to impinge on a rotating tungsten target. Neutrons will then pass through a complex moderator system and will be delivered to the experimental stations of a suite of instruments through dedicated beamlines. Each instrument has a unique experimental station, neutron guide and design that are optimized and conceived for a specific scientific research field. ESS will be capable of producing neutrons with energies up to that of the incident proton beam. Such high energetic neutrons constitute a matter of particular care for radiological protection purposes. In fact, neutrons represent a higher concern respect to charged particles due to the fact that they carry no electric charge. Therefore, they can’t interact with matter by mean of the coulomb force, which dominates the energy loss mechanism for charged particles. In particular slow neutrons, of energy up to a few eV, are likely to be absorbed by the atomic nuclei by radiative capture reactions, that lead to the emission of gamma particles. Similarly to neutrons, gammas carry no electric charge, but their interaction mechanisms in matter differ from those of neutrons, and depend on the atomic number Z of the nuclei [1]. Both neutron and gamma radiations are an issue for radiological protection in neutron facilities, due to their higher penetrability respect to charged particles. In particular, being able to attenuate and shield that radiation down to acceptable limits is a crucial aspect within the ESS project. The radiological shielding of each component of the facility, from the accelerator down to the beam-lines, is an essential matter of studies and investigations at ESS. Shielding studies can be thought as a type of optimization studies. In fact, the typical purpose is to minimize a radiological quantity, usually the dose, in some particular area of interest, by choosing appropriate materials. The choice is bound to the attenuating capability of the selected materials for the given radiation, to the cost and to the available space that set geometrical constraints. The studies described in this thesis are focused on the design of an appropriate shielding for the beam-line of VESPA, one of the instruments under construction at ESS. VESPA is a joint venture between Consiglio Nazionale delle Ricerche (CNR, Italy) and Science and Technology Facilities Council (STFC, United Kingdom). The acronym VESPA stands for Vibrational Excitation Spectrometer using Pyrolytic-graphite Analyser. As the name suggest, it is a high-resolution broadband chemical spectrometer, enhanced with diffraction capabilities, fully dedicated for in-situ research. It will be capable of providing simultaneous dynamic and structural data on chemical bondings, intra-molecular and inter-molecular interactions and on the vibrational dynamics. VESPA is a 60m long straight instrument, in line of sight with ESS moderator. Most of the instrument, about 45 m, will be built in an area that will be frequently accessed by workers, scientists, ESS personnel and so on. Therefore, the radiation coming from the instrument has to be strongly attenuate by an adequate shielding structure, so to not constitute a radiological hazard. The studies for the shielding of VESPA presented here, were performed by mean of the Monte Carlo transport code MCNP and auxiliary codes like CombLayer and ADVANTG. The investigation benefited from the Common Shielding Project at ESS, that aims to standardize the shielding structures for all the participating instruments, as well as to provide a common teamwork for discussing and validating the investigations. Part of this thesis aims to describe the Monte Carlo method, with a particular care for the variance reduction techniques, especially those that were used in the MCNP calculations. A large part of the thesis is related to the characterization of the sources used in the simulations. The shielding for VESPA is investigated through studying the neutron and photon dose rate maps. The proposed design, in compliance with the Common Shielding requirements and the dose requirements, is given toward the end of the thesis. The last chapter of the thesis is an addendum about the early simulations aimed to design an essential component of the instrument.