Radiological protection and nuclear engineering studies in multi-MW target systems

Tese de doutoramento, Física, Universidade de Lisboa, Faculdade de Ciências, 2014

Investigation of molecular polarizabilities and derivatives in halomethanes

The research undertaken was to obtain absolute Raman intensities for the symmetric stretching vibrations of the methyl halides, CH3X with (X=F, CI, Br), by experiment and theory. The intensities were experimentally measured using the Ar+ ion gas laser as excitation source, a Spex 14018 double monochromator and a RCA C-31034 photomultiplier tube as detector. These intensities arise from changes in the derivative of the polarizability (8 a'), with respect to vibration along a normal coordinate (8qi). It was intended that these derivatives obtained with respect to normal coordinates would be converted to derivatives with respect to internal coordinates, for a quantitative comparison with theory. Theoretical numerical polarizability derivatives for the stretching vibrations are obtained using the following procedure. A vibration was simulated in the molecule by increasi.ng and decreasing the respective bond by the amount ±o.oosA for the C-H bonds and ±o.oIA for the C-X (X=F, CI, Br) bond. The derivative was obtained by taking the difference in the polarizability for the equilibrium geometry and the geometry when a particular bond is changed. This difference, when divided by the amount of change in each bond and the number of bonds present results in the derivative of the polarizability with respect to internal coordinate i.e., !1u/!1r. These derivatives were obtained by two methods: I} ab initio molecular orbital calculation and 2} theory of atoms in molecules (AIM) analysis. Due to errors in the experimental setup only a qualitative analysis of the results was undertaken relative to the theory. Theoretically it is predicted that the symmetric carbonhalogen stretch vibrations are more intense than the respective carbon-hydrogen stretch, but only for the methyl chloride and bromide. The carbon fluorine stretch is less intense than the carbon-hydrogen stretch, a fact which is attributed to the small size and high electronegativity of the fluorine atom. The experimental observations are seen to agree qualitatively with the theory results. It is hoped that when the experiment is repeated, a quantitative comparison can be made. The analysis by the theory of atoms in molecules, along with providing polarizabilities and polarizability derivatives, gives additional information outlined below. The theory provides a pictorial description of the main factors contributing to the molecular polarizability and polarizability derivative. These contributions are from the charge transfer and atomic dipole terms i.e., transfer of charge from one atom to another and the reorganization of atomic electronic charge distribution due to presence of an electric field. The linear relationship between polarizability and molecular volume was also observed.

Dénombrement dans les empilements apolloniens généralisés et distribution angulaire dans les extensions quadratiques imaginaires.

Cette thèse traite de deux thèmes principaux. Le premier concerne l'étude des empilements apolloniens généralisés de cercles et de sphères. Généralisations des classiques empilements apolloniens, dont l'étude remonte à la Grèce antique, ces objets s'imposent comme particulièrement attractifs en théorie des nombres. Dans cette thèse sera étudié l'ensemble des courbures (les inverses des rayons) des cercles ou sphères de tels empilements. Sous de bonnes conditions, ces courbures s'avèrent être toutes entières. Nous montrerons qu'elles vérifient un principe local-global partiel, nous compterons le nombre de cercles de courbures plus petites qu'une quantité donnée et nous nous intéresserons également à l'étude des courbures premières. Le second thème a trait à la distribution angulaire des idéaux (ou plutôt ici des nombres idéaux) des corps de nombres quadratiques imaginaires (que l'on peut voir comme la distribution des points à coordonnées entières sur des ellipses). Nous montrerons que la discrépance de l'ensemble des angles des nombres idéaux entiers de norme donnée est faible et nous nous intéresserons également au problème des écarts bornés entre les premiers d'extensions quadratiques imaginaires dans des secteurs.

Sum rules and short-range correlations in nuclear matter at finite temperature

The nucleon spectral function in nuclear matter fulfills an energy weighted sum rule. Comparing two different realistic potentials, these sum rules are studied for Greens functions that are derived self-consistently within the T matrix approximation at finite temperature.

Sum rules and correlations in asymmetric nuclear matter

The neutron and proton single-particle spectral functions in asymmetric nuclear matter fulfill energy-weighted sum rules. The validity of these sum rules within the self-consistent Green's function approach is investigated. The various contributions to these sum rules and their convergence as a function of energy provide information about correlations induced by the realistic interaction between the nucleons. The study of the sum rules in asymmetric nuclear matter exhibits the isospin dependence of the nucleon-nucleon correlations.

Two-photon ionization of atomic inner-shells

Die Summation ueber des vollstaendige Spektrum des Atoms, die in der Stoehrungstheorie zweiter Ordnung vorkommt, wurde mit Hilfe der Greenschen Funktion Methode berechnet. Die Methode der Greenschen Funktion verlangt die Berechnung der unterschiedlichen Greenschen Funktionen: eine Coulomb-Greensche-Funktion im Fall von wasserstoffaehnlichen Ionen und eine Zentral-feld-Greensche-Funktion im Fall des Vielelektronen-Atoms. Die entwickelte Greensche Funktion erlaubte uns die folgenden atomaren Systeme in die Zweiphotonenionisierung der folgenden atomaren Systeme zu untersuchen: - wasserstoffaehnliche Ionen, um relativistische und Multipol-Effekte aufzudecken, - die aeussere Schale des Lithium; Helium und Helium-aehnliches Neon im Grundzustand, um taugliche Modelle des atomaren Feldes zu erhalten, - K- und L-Schalen des Argon, um die Vielelektronen-Effekte abzuschaetzen. Zusammenfassend, die relativistische Effekte ergeben sich in einer allgemeinen Reduzierung der Zweiphotonen Wirkungsquerschnitte. Zum Beispiel, betraegt das Verhaeltnis zwischen den nichtrelativistischen und relativistischen Wirkungsquerschnitten einen Faktor zwei fuer wasserstoffaehnliches Uran. Ausser dieser relativistischen Kontraktion, ist auch die relativistische Aufspaltung der Zwischenzustaende fuer mittelschwere Ionen sichtbar. Im Gegensatz zu den relativistischen Effekten, beeinflussen die Multipol-Effekte die totalen Wirkungsquerschnitte sehr wenig, so dass die Langwellennaeherung mit der exakten Naeherung fuer schwere Ionen sogar innerhalb von 5 Prozent uebereinstimmt. Die winkelaufgeloesten Wirkungsquerschnitte werden durch die relativistischen Effekte auf eine beeindruckende Weise beeinflusst: die Form der differentiellen Wirkungsquerschnitte aendert sich (qualitativ) abhaengig von der Photonenenergie. Ausserdem kann die Beruecksichtigung der hoeheren Multipole die elektronische Ausbeute um einen Faktor drei aendern. Die Vielelektronen-Effekte in der Zweiphotonenionisierung wurden am Beispiel der K- und L-Schalen des Argon analysiert. Hiermit wurden die totalen Wirkungsquerschnitte in einer Ein-aktives-Elektron-Naeherung (single-active-electron approximation) berechnet. Es hat sich herausgestellt, dass die Elektron--Elektron-Wechselwirkung sehr wichtig fuer die L-Schale und vernachlaessigbar fuer die K-Schale ist. Das bedeutet, dass man die totalen Wirkungsquerschnitte mit wasserstoffaehnlichen Modellen im Fall der K-Schale beschreiben kann, aber fuer die L-Schale fortgeschrittene Modelle erforderlich sind. Die Ergebnisse fuer Vielelektronen-Atome wurden mittels einer Dirac-Zentral-feld-Greenschen Funktion erlangt. Ein numerischer Algorithmus wurde urspruenglich von McGuire (1981) fuer der Schroedinger-Zentral-feld-Greensche Funktion eingefuehrt. Der Algorithmus wurde in dieser Arbeit zum ersten Mal fuer die Dirac-Gleichung angewandt. Unser Algorithmus benutzt die Kummer- und Tricomi-Funktionen, die mit Hilfe eines zuverlaessigen, aber noch immer langsamen Programmes berechnet wurden. Die Langsamkeit des Programms begrenzt den Bereich der Aufgaben, die effizient geloest werden koennen. Die Zentral-feld-Greensche Funktion konnte bei den folgenden Problemen benutzt werden: - Berechnung der Zweiphotonen-Zerfallsraten, - Berechnung der Zweiphotonenanregung und -ionisierungs-Wirkungsquerschnitte, - Berechnung die Multiphotonenanregung und -ionisierungs-Wirkungsquerschnitte, - Berechnung einer atomaren Vielelektronen-Green-Funktion. Von diesen Aufgaben koennen nur die ersten beiden in angemessener Zeit geloest werden. Fuer die letzten beiden Aufgaben ist unsere Implementierung zu langsam und muss weiter verbessert werden.

Profile simulations of gas chopping etching processes : model development and comparison with experiments

The progress in microsystem technology or nano technology places extended requirements to the fabrication processes. The trend is moving towards structuring within the nanometer scale on the one hand, and towards fabrication of structures with high aspect ratio (ratio of vertical vs. lateral dimensions) and large depths in the 100 µm scale on the other hand. Current procedures for the microstructuring of silicon are wet chemical etching and dry or plasma etching. A modern plasma etching technique for the structuring of silicon is the so-called "gas chopping" etching technique (also called "time-multiplexed etching"). In this etching technique, passivation cycles, which prevent lateral underetching of sidewalls, and etching cycles, which etch preferably in the vertical direction because of the sidewall passivation, are constantly alternated during the complete etching process. To do this, a CHF3/CH4 plasma, which generates CF monomeres is employed during the passivation cycle, and a SF6/Ar, which generates fluorine radicals and ions plasma is employed during the etching cycle. Depending on the requirements on the etched profile, the durations of the individual passivation and etching cycles are in the range of a few seconds up to several minutes. The profiles achieved with this etching process crucially depend on the flow of reactants, i.e. CF monomeres during the passivation cycle, and ions and fluorine radicals during the etching cycle, to the bottom of the profile, especially for profiles with high aspect ratio. With regard to the predictability of the etching processes, knowledge of the fundamental effects taking place during a gas chopping etching process, and their impact onto the resulting profile is required. For this purpose in the context of this work, a model for the description of the profile evolution of such etching processes is proposed, which considers the reactions (etching or deposition) at the sample surface on a phenomenological basis. Furthermore, the reactant transport inside the etching trench is modelled, based on angular distribution functions and on absorption probabilities at the sidewalls and bottom of the trench. A comparison of the simulated profiles with corresponding experimental profiles reveals that the proposed model reproduces the experimental profiles, if the angular distribution functions and absorption probabilities employed in the model is in agreement with data found in the literature. Therefor the model developed in the context of this work is an adequate description of the effects taking place during a gas chopping plasma etching process.

Nuclear charge radii from X-ray transitions in muonic atoms of carbon, nitrogen and oxygen

Energies of muonic X-rays of the K-series of carbon, nitrogen and oxygen have been measured with an accuracy of about 15 eV. Root mean square radii of the nuclear charge distributions were deduced. The results 2.49±0.05 fm for carbon, 2.55 ±0.03 fm for nitrogen and 2.71 ±0.02 fm for oxygen are in good agreement at comparable accuracy with recent electron scattering data.

Simple finite field nuclear relaxation method for calculating vibrational contribution to degenerate four-wave mixing

A simple extended finite field nuclear relaxation procedure for calculating vibrational contributions to degenerate four-wave mixing (also known as the intensity-dependent refractive index) is presented. As a by-product one also obtains the static vibrationally averaged linear polarizability, as well as the first and second hyperpolarizability. The methodology is validated by illustrative calculations on the water molecule. Further possible extensions are suggested

Interaction of strangelets with ordinary nuclei

Strangelets (hypothetical stable lumps of strange quarkmatter) of astrophysical origin may be ultimately detected in specific cosmic ray experiments. The initial mass distribution resulting from the possible astrophysical production sites would be subject to reprocessing in the interstellar medium and in the earth`s atmosphere. In order to get a better understanding of the claims for the detection of this still hypothetic state of hadronic matter, we present a study of strangelet-nucleus interactions including several physical processes of interest (abrasion, fusion, fission, excitation and de-excitation of the strangelets), to address the fate of the baryon number along the strangelet path. It is shown that, although fusion may be important for low-energy strangelets in the interstellar medium (thus increasing the initial baryon number A), in the earth`s atmosphere the loss of the baryon number should be the dominant process. The consequences of these findings are briefly addressed.

Looking for intrinsic charm in the forward region at BNL RHIC and CERN LHC

The complete understanding of the basic constituents of hadrons and the hadronic dynamics at high energies are two of the main challenges for the theory of strong interactions. In particular, the existence of intrinsic heavy quark components in the hadron wave function must be confirmed (or disproved). In this paper we propose a new mechanism for the production of D-mesons at forward rapidities based on the Color Glass Condensate (CGC) formalism and demonstrate that the resulting transverse momentum spectra are strongly dependent on the behavior of the charm distribution at large Bjorken x. Our results show clearly that the hypothesis of intrinsic charm can be tested in pp and p(d)A collisions at RHIC and LHC. (C) 2010 Elsevier B.V. All rights reserved.

Fragment mass distributions in the fission of heavy nuclei by intermediate- and high-energy probes

Recent experiments have shown that the multimode approach for describing the fission process is compatible with the observed results. Asystematic analysis of the parameters obtained by fitting the fission-fragment mass distribution to the spontaneous and low-energy data has shown that the values for those parameters present a smooth dependence upon the nuclear mass number. In this work, a new methodology is introduced for studying fragment mass distributions through the multimode approach. It is shown that for fission induced by energetic probes (E > 30 MeV) the mass distribution of the fissioning nuclei produced during the intranuclear cascade and evaporation processes must be considered in order to have a realistic description of the fission process. The method is applied to study (208)Pb, (238)U, (239)Np and (241)Am fission induced by protons or photons.

Sound waves and solitons in hot and dense nuclear matter

Assuming that nuclear matter can be treated as a perfect fluid, we study the propagation of perturbations in the baryon density. The equation of state is derived from a relativistic mean field model, which is a variant of the non-linear Walecka model. The expansion of the Euler and continuity equations of relativistic hydrodynamics around equilibrium configurations leads to differential equations for the density perturbation. We solve them numerically for linear and spherical perturbations and follow the propagation of the initial pulses. For linear perturbations we find single soliton solutions and solutions with one or more solitons followed by ""radiation"". Depending on the equation of state a strong damping may occur. We consider also the evolution of perturbations in a medium without dispersive effects. In this case we observe the formation and breaking of shock waves. We study all these equations also for matter at finite temperature. Our results may be relevant for the analysis of RHIC data. They suggest that the shock waves formed in the quark gluon plasma phase may survive and propagate in the hadronic phase. (C) 2009 Elseiver. B.V. All rights reserved.

Can the pi(+) chi(c1) resonance structures be D*(D)over-bar* and D(1)(D)over-bar molecules?

We use QCD sum rules to study the recently observed resonance-like structures in the pi(+)chi(c1) mass distribution, Z(1)(+) (4050) and Z(2)(+) (4250), considered as D*(+) (D) over bar*(0) and D(1)(+) (D) over bar (0) + D(+) (D) over bar (0)(1) molecules with the quantum number J(P) = 0(+) and J(P) = 1-, respectively. We consider the contributions of condensates up to dimension eight and work at leading order in alpha(s). We obtain m(D*D*) = (4.15 +/- 0.12) GeV, around 100 MeV above the D*D* threshold, and m(D1D) = (4.19 +/- 0.22) GeV, around 100 MeV below the D(1)D threshold. We conclude that the D*(+)(D) over bar*(0) state is probably a virtual state that is not related with the Z(1)(+) (4050) resonance-like structure. In the case of the D(1)D molecular state, considering the errors, its mass is consistent with both Z(1)(+)(4050) and Z(2)(+)(4250) resonance-like structures. Therefore, we conclude that no definite conclusion can be drawn for this state from the present analysis. (C) 2008 Elsevier B.V All rights reserved.

The Trojan horse method in nuclear astrophysics: recent results

Difficulties in cross-section measurements at very low energies, when charged particles are involved, led to the development of some indirect methods. The Trojan horse method (THM) allows us to bypass the Coulomb effects and has been successfully applied to several reactions of astrophysical interest. A brief review of the THM applications is reported together with some of the most recent results.