Halo nuclei are among the strangest nuclear structures. They are viewed as a core containing most of the nucleons surrounded by one or two loosely bound nucleons. These have a high probability of presence at a large distance from the core. Therefore, they constitute a sort of halo surrounding the other nucleons. The core, remaining almost unperturbed by the presence of the halo is seen as a usual nucleus. <P> The Coulomb breakup reaction is one of the most useful tools to study these nuclei. It corresponds to the dissociation of the halo from the core during a collision with a heavy (high <I>Z</I>) target. In order to correctly extract information about the structure of these nuclei from experimental cross sections, an accurate theoretical description of this mechanism is necessary. <P> In this work, we present a theoretical method for studying the Coulomb breakup of one-nucleon halo nuclei. This method is based on a semiclassical approximation in which the projectile is assumed to follow a classical trajectory. In this approximation, the projectile is seen as evolving in a time-varying potential simulating its interaction with the target. This leads to the resolution of a time-dependent Schrödinger equation for the projectile wave function. <P> In our method, the halo nucleus is described with a two-body structure: a pointlike nucleon linked to a pointlike core. In the present state of our model, the interaction between the two clusters is modelled by a local potential. <P> The main idea of our method is to expand the projectile wave function on a three-dimensional spherical mesh. With this mesh, the representation of the time-dependent potential is fully diagonal. Furthermore, it leads to a simple representation of the Hamiltonian modelling the halo nucleus. This expansion is used to derive an accurate evolution algorithm. <P> With this method, we study the Coulomb breakup of three nuclei: ¹¹Be, ¹⁵C and ⁸B. ¹¹Be is the best known one-neutron halo nucleus. Its Coulomb breakup has been extensively studied both experimentally and theoretically. Nevertheless, some uncertainty remains about its structure. The good agreement between our calculations and recent experimental data suggests that it can be seen as a <I>s1/2</I> neutron loosely bound to a ¹⁰Be core in its 0⁺ ground state. However, the extraction of the corresponding spectroscopic factor have to wait for the publication of these data. <P> ¹⁵C is a candidate one-neutron halo nucleus whose Coulomb breakup has just been studied experimentally. The results of our model are in good agreement with the preliminary experimental data. It seems therefore that ¹⁵C can be seen as a ¹⁴C core in its 0⁺ ground state surrounded by a <I>s1/2</I> neutron. Our analysis suggests that the spectroscopic factor corresponding to this configuration should be slightly lower than unity. <P> We have also used our method to study the Coulomb breakup of the candidate one-proton halo nucleus ⁸B. Unfortunately, no quantitative agreement could be obtained between our results and the experimental data. This is mainly due to an inaccuracy in the treatment of the results of our calculations. Accordingly, no conclusion can be drawn about the pertinence of the two-body model of ⁸B before an accurate reanalysis of these results. <P> In the future, we plan to improve our method in two ways. The first concerns the modelling of the halo nuclei. It would be indeed of particular interest to test other models of halo nuclei than the simple two-body structure used up to now. The second is the extension of this semiclassical model to two-neutron halo nuclei. However, this cannot be achieved without improving significantly the time-evolution algorithm so as to reach affordable computational times.

A physico-chemical approach (theory of dissociation and reaction) is developed, which can be used in binary diffusion couple to determine diffusion parameters of the product phases with wide homogeneity range, as well as phases with narrow homogeneity range. It is demonstrated that this approach is basically equivalent to the diffusion based treatment. However, physico-chemical approach pedagogically sheds light on the chemical reactions occurring during interdiffusion at the interphase interfaces and morphology develops in the interdiffusion zone. This theory can be used in any binary systems for any end-member condition to explain single phase or multiphase diffusion controlled growth. Ni-Al and Ag-Zn systems are considered here to calculate diffusion parameters following physico-chemical approach. It is evident from our theoretical analysis and experimental evidence that in the presence of a stable Kirkendall marker plane one should expect duplex grain morphology in a particular phase layer. On the other hand, there is another model which is used rather frequently, is the theory of partitioning of flux. Although, the theory of partitioning of flux is used several times, we found that this theory does not count the mobility of both the species and therefore is not suitable to use in most of the interdiffusion systems. We have first modified this theory to take into account the mobility of both the species and then further extended to develop the relations for the integrated diffusion coefficient and the ratio of diffusivities of the species. The versatility of these two different models (that is the theory dissociation and reaction and the partitioning of flux) is examined in the Co-Si system with respect to different end-member compositions. From our analysis, we found that the applicability of the theory of partitioning of flux is rather limited but the theory of dissociation and reaction can be used in any binary systems. The theory of dissociation and reaction is then used to elucidate this behaviour in a single phase of β-NiAl and to calculate the diffusion parameter at the Kirkendall marker planes in the interdiffusion zone. To apply the physico-chemical approach, Ni-and Al-rich part of the phase is treated as two different phases and the plane corresponding to eqiatomic composition is considered as virtual interface between them. Possible dissociation and reaction equations are considered to combine with the flux equations to derive the relation for diffusion coefficient. Further experiments are conducted in the Cu-Sn, Au-Sn and Ni-Sn systems, which are important for flip chip bonding related to micro electronics industry. Different diffusion parameters, such as integrated diffusion coefficient, tracer diffusion coefficient of elements and the ratio of diffusivities are determined, which shed lights on the atomic mechanism of diffusion. Subsequently, the theory of dissociation and reaction is used when possible to explain the growth of the phases in the interdiffusion zone.

Thermochemical nonequilibrium widely exists in supersonic combustion, cold plasma and hypersonic flight. The effect can influence heat transfer, surface ablation and aerodynamic loads. One distinct feature of it is the coupling between internal energy excitation and chemical reactions, particularly the vibration-dissociation coupling. The widely used models are empirical and calibrated based on limited experimental data. Advances in theories and computational power have made the first-principle calculation of thermal nonequilibrium reaction rates by methods like quasi-classical trajectory (QCT) almost a routine today. However, the approach is limited by the uncertainties and availability of potential energy surfaces. To the best of our knowledge, there is no study of thermal nonequilibrium transport properties with this approach. Most importantly, non-trivial effort is required to process the QCT data and implement it in flow simulation methods. In this context, the first part of this work establishes the approach to compute transport properties by the QCT method and studies the influence of thermal nonequilibrium on transport properties for N₂-O molecules. The preponderance of the work is the second part, a comprehensive study of the development of a new thermal nonequilibrium reaction model based on reasonable assumptions and approximations. The new model is as convenient as empirical models. By validating against recent QCT data and experimental results, we found the new model can predict nonequilibrium characteristics of dissociation reactions with nearly the same accuracy as QCT calculations do. In general, the results show the potential of the new model to be used as the standard dissociation model for the simulation of thermochemical nonequilibrium flows.

Halo nuclei are among the strangest nuclear structures.<p>They are viewed as a core containing most of the nucleons<p>surrounded by one or two loosely bound nucleons. <p>These have a high probability of presence at a large distance<p>from the core.<p>Therefore, they constitute a sort of halo surrounding the other nucleons.<p>The core, remaining almost unperturbed by the presence<p>of the halo is seen as a usual nucleus.<p><p><P><p><p>The Coulomb breakup reaction is one of the most useful<p>tools to study these nuclei. It corresponds to the<p>dissociation of the halo from the core during a collision<p>with a heavy (high <I>Z</I>) target.<p>In order to correctly extract information about the structure of<p>these nuclei from experimental cross sections, an accurate<p>theoretical description of this mechanism is necessary.<p><p><P><p><p>In this work, we present a theoretical method<p>for studying the Coulomb breakup of one-nucleon halo nuclei.<p>This method is based on a semiclassical approximation<p>in which the projectile is assumed to follow a classical trajectory.<p>In this approximation, the projectile is seen as evolving<p>in a time-varying potential simulating its interaction with the target.<p>This leads to the resolution of a time-dependent Schrödinger<p>equation for the projectile wave function.<p><p><P><p><p>In our method, the halo nucleus is described<p>with a two-body structure: a pointlike nucleon linked to a<p>pointlike core.<p>In the present state of our model, the interaction between<p>the two clusters is modelled by a local potential.<p><p><P><p><p>The main idea of our method is to expand the projectile wave function<p>on a three-dimensional spherical mesh.<p>With this mesh, the representation of the time-dependent potential<p>is fully diagonal.<p>Furthermore, it leads to a simple<p>representation of the Hamiltonian modelling the halo nucleus.<p>This expansion is used to derive an accurate evolution algorithm.<p><p><P><p><p>With this method, we study the Coulomb breakup<p>of three nuclei: ¹¹Be, ¹⁵C and ⁸B.<p>¹¹Be is the best known one-neutron halo nucleus.<p>Its Coulomb breakup has been extensively studied both experimentally<p>and theoretically.<p>Nevertheless, some uncertainty remains about its structure.<p>The good agreement between our calculations and recent<p>experimental data suggests that it can be seen as a<p><I>s1/2</I> neutron loosely bound to a ¹⁰Be core in its<p>0⁺ ground state.<p>However, the extraction of the corresponding spectroscopic factor<p>have to wait for the publication of these data.<p><p><P><p><p>¹⁵C is a candidate one-neutron halo nucleus<p>whose Coulomb breakup has just been studied experimentally.<p>The results of our model are in good agreement with<p>the preliminary experimental data. It seems therefore that<p>¹⁵C can be seen as a ¹⁴C core in its 0⁺<p>ground state surrounded by a <I>s1/2</I> neutron.<p>Our analysis suggests that the spectroscopic factor<p>corresponding to this configuration should be slightly lower<p>than unity.<p><p><P><p><p>We have also used our method to study the Coulomb breakup<p>of the candidate one-proton halo nucleus ⁸B.<p>Unfortunately, no quantitative agreement could be obtained<p>between our results and the experimental data.<p>This is mainly due to an inaccuracy in the treatment<p>of the results of our calculations.<p>Accordingly, no conclusion can be drawn about the pertinence<p>of the two-body model of ⁸B before an accurate reanalysis of these<p>results.<p><p><P><p><p>In the future, we plan to improve our method in two ways.<p>The first concerns the modelling of the halo nuclei.<p>It would be indeed of particular interest to test<p>other models of halo nuclei than the simple two-body structure<p>used up to now.<p>The second is the extension of this semiclassical model to<p>two-neutron halo nuclei.<p>However, this cannot be achieved<p>without improving significantly the time-evolution algorithm so as to<p>reach affordable computational times. / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished