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Tine Curk, 2012, master's thesis

Abstract: A grafted polymer layer can be used to prevent the deposition of colloidal particles on a solid surface. This thesis presents Monte Carlo simulations of hard-sphere colloids pushed to a polymer brush under the influence of external fields (e.g. gravity). For weak fields colloids can not penetrate the brush and the effective potential acting on a single colloid is approximately quadratic. The phase diagram of three-dimensional hard-sphere colloids, that in one dimension are constrained to a plane by a harmonic potential, is presented. Under the influence of sufficiently strong external fields colloids penetrate the brush and form internally ordered, columnar structures that span the polymer layer. The morphology of the patterns that form depends sensitively on the strength of the applied field. We propose a simple phenomenological theory that accounts for the main characteristics of the observed behaviour. The present results suggest a simple experimental method to determine the surface polymer coverage.
Keywords: Soft Matter, Monte Carlo Method, Wang-Landau Algorithm, Free Energy, Polymer Brush, Colloids, Ordering
Published: 04.03.2013; Views: 1621; Downloads: 71
.pdf Full text (6,75 MB)

Alberto Milocco, 2012, dissertation

Abstract: The construction of the nuclear fusion plant 'ITER' has started in 2009 at Cadarache, France. The ITER machine represents a milestone in the civil use of the nuclear fusion energy. The physics of ITER is based on the fusion reaction between deuteron and triton nuclei (d-t). The deuteron-deuteron reaction (d-d) is also interesting and is foreseen for the next generation of fusion reactors. The experimental activities carried out in the context of the ITER neutronics involve intense fields of neutrons produced with a linear accelerator for deuterons, a target containing tritium or deuterium and auxiliary structures, such as the detector system, cooling system, room walls, etc. Experimental data have been obtained from the FNG (Frascati Neutron Generator, Italy), FNS (Fast Neutron Source, Japan), OKTAVIAN (Osaka University, Japan) and IRMM (Institute for Reference Materials and Measurements, EU). An independent method was developed at FNG for the simulation of the d-t neutron spectra at different angles. The FNG source routine models the Monte Carlo deuteron transport in solid tritiated targets as done in the well known SRIM code. The neutrons are generated according to the tabulated probability of the d-t reactions as for the DROSG2000 code. The FNG source routine is implemented into the MCNP distributions. The user is asked to define into the MCNP input file the deuteron energy (up to 10 MeV), the beam width and the target dimensions and composition. This source routine has been chosen as starter for the present thesis. Improvements and extensions were introduced. - The methodology, originally developed for the d-t neutron source, has been extended to d-d neutron sources. - Assuming the the SRIM code constitutes the reference calculation for the deuteron transport in matter, its implementation in the source routine has been cross-checked by extracting from the latter the same quantities as provided by the original code. II - In the present version of the source routines, the cross sections are internally generated from built in table based on modern evaluated nuclear data files instead of tables obtained from the DROSG200 code. - Since the model may be used up to 10 MeV deuteron energy, the relativistic kinematics has been implemented to avoid unnecessary approximations. - Simulations of the bare neutron source spectra and angular yields measurements have been carried out to validate the model. - New editions of the d-t and d-d source routine have been released for the latest versions of the MCNP codes and tested on LINUX and WINDOWS machines. The validation activities with the FNG and IRMM experimental data suggested a possible application of the source routine for the characterisation of neutron spectrometers in the MeV energy region. The source routine has been adopted to simulate integral benchmark experiments at FNG, FNS and OKTAVIAN. Brand new MCNP benchmark models have been developed for inclusion of all the available experimental information. It is shown that the d-t source routine is an accurate tool for the generation of the source eutrons. It also demonstrates to be useful for the evaluation of the neutron source term and associated uncertainties. The accuracy of the analyses is pursued to the point that the quality of the nuclear data employed in the simulation can be assessed. To this extent, the case of a new evaluation of the neutron interaction nuclear data for Manganese-55 is tested. A set of integral benchmark experiments has been used in the validation phase of the nuclear data. The computational models rely on the source routine, the object of the thesis. In conclusion, the source routine claims the inclusion of the major features responsible for the experimental resolution associated with the source term. The doctoral thesis explores its usage in the context of the experimental activities for ITER. The future exploitation of the source routine for the simulation of worldwide experiments might become an occasion to compare it with the source models available in the other laboratories
Keywords: deuteron-triton reactions, low-energy deuterons, neutron source model, Monte Carlo method, solid tritium target, solid deuterium target, fusion neutronics, benchmark experiments, diamond detectors.
Published: 07.03.2012; Views: 3296; Downloads: 100
.pdf Full text (8,50 MB)

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