Monte Carlo neutronic calculations for the design of VESPA shielding at the European Spallation Source
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Scionti, Jimmy
Cipparrone, Gabriella
Agostino, Raffaele Giuseppe
Gorini, Giuseppe
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Università della Calabria. Dipartimento di Fisica. Dottorato di ricerca in Scienze e Tecnologie Fisiche, Chimiche e dei Materiali. Ciclo XXXII; 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.Soggetto
Shielding; Monte Carlo; Neutronics; Simulations
Relazione
FIS/01;