Letteratura scientifica selezionata sul tema "Compound Nucleus Formation probability"

The potential energy landscape for the [Formula: see text] synthesis is evaluated in the framework of the macroscopic–microscopic model. The fragmentation potential calculated in a configuration space characterized by five degrees of freedom associated to elongation, mass asymmetry, necking, and left/right deformations reveals the existence of several minima and some pronounced valleys. The valleys correspond mainly to the [Formula: see text] and [Formula: see text] quasifission channels, but also for some paths in the formation of the compound nuclear system. In this context, two different paths are obtained for the [Formula: see text] and [Formula: see text] entrance channels. The [Formula: see text] path leads to the formation of an isomeric minimum that decays by fission. The [Formula: see text] path evidences a larger probability for the formation of the compound nucleus. Therefore, the two distributions obtained for the associated quasifission processes are very different.

We present calculations on the formation of 258Rf in the reaction 50Ti+208Pb within the Langevin framework. The initital stage after contact of the colliding nuclei is dominated by a drift in the center-of-mass direction, while the subsequent process of forming a compound nucleus is diffusion dominated. The probability for the composite system to diffuse over the inner saddle is obtained by performing Metropolis random walks in a five-dimensional potential-energy landscape.

(1) Purpose: Conditions of formation of compound nuclear systems needed for synthesis of heavy nuclei in pycnonuclear reactions in compact stars are studied on a quantum mechanical basis. (2) Methods: The method of multiple internal reflections is applied for pycnonuclear reactions in compact stars with new calculations of quasibound spectra and spectra of zero-point vibrations. (3) Results: Peculiarities of the method are analyzed for reaction with isotopes of Carbon. The developed method takes into account continuity and conservation of quantum flux (describing pycnonuclear reaction) inside the full spacial region of reaction, including the nuclear region. This gives the appearance of new states (called quasibound states) in which compound nuclear systems of Magnesium are formed with the largest probability. These states have not been studied yet in synthesis of elements in stars. Energy spectra of zero-point vibrations and spectra of quasibound states are estimated with high precision for reactions with isotopes of Carbon. For the first time, the influence of plasma screening on quasibound states and states of zero-point vibrations in pycnonuclear reactions has been studied. (4) Conclusions: The probability of formation of a compound nucleus in quasibound states in pycnonuclear reaction is essentially larger than the probability of formation of this system in states of zero-point vibrations studied by Zel’dovich and followers. Therefore, synthesis of Magnesium from isotopes of Carbon is more probable through the quasibound states than through the states of zero-point vibrations in compact stars. Energy spectra of zero-point vibrations are changed essentially after taking plasma screening into account. Analysis shows that from all studied isotopes of Magnesium, only 24Mg is stable after synthesis at an energy of relative motion of 4.881 MeV of the incident nuclei 12C.

The mixing of the quasifission component to the fissionlike cross section causes ambiguity in the quantitative estimation of the complete fusion cross section from the observed angular and mass distributions of the binary products. We show that the partial cross section of quasifission component of binary fragments covers the whole range of the angular momentum values leading to capture. The calculated angular momentum distributions for the compound nucleus and dinuclear system going to quasifission may overlap: competition between complete fusion and quasifission takes place at all values of initial orbital angular momentum. Quasifission components formed at large angular momentum of the dinuclear system can show isotropic angular distribution and their mass distribution can be in mass symmetric region similar to the characteristics of fusion-fission components. As result the unintentional inclusion of the quasifission contribution into the fusion-fission fragment yields can lead to overestimation of the probability of the compound nucleus formation.

The amorphous formation mechanism was studied from the viewpoint of the crystallography. The experimental results showed that the heterogeneity atoms with chemical bonds had a higher capability of forming into compounds than that of congener atoms. It was proposed that the banded structure was induced by the variation of active atoms with a proper thickness, alternated depletion and enrichment of anions in the diffusion layer due to generation and evolution of hydrogen gas. The chemical force plays an important part in the formation. The factors working on chemical reaction affect the formation in the same. A number of steady motes are in a high speed movement, and collide with each other in all probability. The formation of amorphous alloys is the result of massive nucleus appearance, which was proposed from the crystallography in this paper. The cause of the chaotic distribution is of the suppression of each nucleus growth. The short-range order atomic structure is of the atomic nucleus growth orientation. And the long-range disorder structure is of the massive nucleus suppression growth.

The amorphous formation mechanism was studied from the viewpoint of the crystallography. The experimental results showed that the heterogeneity atoms with chemical bonds had a higher capability of forming into compounds than that of congener atoms. It was proposed that the banded structure was induced by the variation of active atoms with a proper thickness, alternated depletion and enrichment of anions in the diffusion layer due to generation and evolution of hydrogen gas. The chemical force plays an important part in the formation. The factors working on chemical reaction affect the formation in the same. A number of steady motes are in a high speed movement, and collide with each other in all probability. The formation of amorphous alloys is the result of massive nucleus appearance, which was proposed from the crystallography in this paper. The cause of the chaotic distribution is of the suppression of each nucleus growth. The short-range order atomic structure is of the atomic nucleus growth orientation. And the long-range disorder structure is of the massive nucleus suppression growth.

In our new approach, evaporation residue cross-sections for new superheavy nuclei with atomic numbers [Formula: see text] are estimated by calculation of vital characteristics of superheavy nuclei synthesis such as the fission barrier height, the compound nucleus formation probability and the survival probability of the residue nuclei. Our presented estimation is in good agreement with available experimental data. In addition, this new approach allowed us to predict the evaporation residue cross-sections for superheavy nuclei with [Formula: see text] and 120 via introducing synthesis box and compare our results with other models. It is shown that the fission barrier heights of two nuclei with [Formula: see text] and 120 are comparable with their corresponding neutron separation energies. It is suggested that for the synthesis of new superheavy nuclei, it is proper to use nearly double magic nuclei such as [Formula: see text] as our projectile, so that the fission barrier heights remain high.

In this work we suggest some methods based on the statistical and knock-on models, for evaluation of the α-clustering factor or α-clustering probability in (n, α) reactions induced by slow and fast neutrons. The main purpose of this study is to compare the values of the α-clustering factors obtained by the compound and direct mechanisms for the same nuclear reactions. Also, our results are compared with values estimated by other authors.

The crystal structures of 2-methyl-4-phenyl-1H-imidazole, C10H10N2, (3a), 4-(4-chlorophenyl)-2-methyl-1H-imidazole hemihydrate, C10H9ClN2·0.5H2O, (3b), and 4-(4-methoxyphenyl)-2-methyl-1H-imidazole, C11H12N2O, (3c), have been analyzed. It was found that the electron-donating/withdrawing tendency of the substituent groups in the aryl ring influence the acid–base properties of the 2-methylimidazole nucleus, changing the strength of the intermolecular N—H...N interactions. This behaviour not only influences the crystal structure but also seems to have an important effect on the antifungal activity. Considering the substituent groups, that is, H in (3a), Cl in (3b) and OMe in (3c), the formation of strong N—H...N connections has the probability (3a) > (3b) > (3c), while compound (3c) proves to be more active than (3a) and (3b) at all concentrations against C. neoformans.

Cette thèse étudie le mécanisme de synthèse des éléments super-lourds (SHE) par des réactions de fusion-évaporation. Il s'agit de noyaux de numéro atomique \(Z \ge 104\) qui n'existent pas dans la nature en raison de leurs barrières de fission macroscopiques qui disparaissent. Ils sont stabilisés par une correction quantique du modèle en couches. La recherche de nouveaux SHE repousse les limites de la physique nucléaire et nous permet de mieux comprendre leur formation, leur stabilité et leur structure. Cependant, la synthèse des SHE est un défi en raison de la diminution des sections efficaces de production à mesure que la charge atomique augmente, ce qui nécessite des simulations théoriques pour guider les expériences et identifier les conditions de réaction optimales.Ce travail se concentre sur l'amélioration du pouvoir prédictif du modèle Kewpie2, conçu pour la simulation de la réaction de fusion-évaporation. Cette réaction est modélisée comme un processus en trois étapes : capture, formation et survie. Alors que Kewpie2 simule de manière indépendante la section efficace de capture et la probabilité de survie, il nécessitait des calculs externes pour la probabilité de formation. Cette thèse met en œuvre l'étape de formation dans le code Kewpie2 pour la première fois en utilisant à la fois les formalismes de sur-critiques et de Langevin complet. La distance du point d'injection (décrivant la configuration initiale de la phase de formation pour le système projectile-cible) est optimisée pour les réactions de fusion froide et chaudes séparément. Une paramétrisation améliorée de la distance du point d'injection, compatible avec le formalisme de Langevin, reproduit les sections efficaces de résidus d'évaporation mesurées pour les réactions de fusion chaude, généralement avec un ratio inférieur à un ordre de grandeur. Pour les réactions de fusion froide, les canaux d'émission de plusieurs neutrons sont expliqués par l'introduction d'un terme structurel supplémentaire, ce qui permet d'obtenir un bon accord avec les données expérimentales. Dans ce cas, les données du canal 1n sont décrites comme ayant une déviation d'un facteur par rapport aux données expérimentales, alors que pour les canaux 2n et 3n, les ratios sont à l'intérieur d'un ordre de grandeur. La thèse étudie également la modélisation de la probabilité de survie en utilisant les données les plus récentes pour SHE. Les étapes de formation et de survie sont testées de manière approfondie et comparées au modèle de fusion par diffusion (FbD) pour 27 réactions de fusion froide et 24 réactions de fusion chaude.L'analyse des coefficients de frottement réduits dans le cadre de l'approche de Langevin suramortie suggère que la dynamique n'est pas sur-critique. Par conséquent, un formalisme de Langevin unidimensionnel complet est étudié et implémenté dans Kewpie2. Le formalisme est appliqué aux données de réaction de fusion chaude. Les paramètres du modèle sont optimisés à l'aide d'une technique d'ajustement systématique, et les résultats confirment que la dynamique n'est pas sur-critique. Dans cette approche, les prédictions du modèle se situent dans un ordre de grandeur d'écart par rapport aux données expérimentales. Les prédictions pour la synthèse d'éléments de numéros atomiques ZCN = 119 et 120 s'accordent avec les résultats d'autres codes. En outre, une méthode d'étude des rapports de probabilités de formation est proposée et discutée pour la synthèse des isotopes 258No et 259Db.En conclusion, ce travail améliore considérablement Kewpie2, ce qui en fait un outil autonome pour l'étude de la synthèse SHE et l'orientation des futurs efforts expérimentaux
This thesis investigates the mechanism of synthesising super-heavy elements (SHE) via fusion evaporation reactions. These are nuclei with atomic numbers \(Z \ge 104\) and do not exist in nature due to their vanishing macroscopic fission barriers. They are stabilised by quantum shell correction. The search for new SHE pushes the boundaries of nuclear physics, furthering our understanding of their formation, stability, and structure. However, synthesising SHE is challenging due to decreasing production cross sections as the atomic charge increases, necessitating theoretical simulations to guide experiments and identify optimal reaction conditions.This work focuses on improving the predictive power of the Kewpie2 model, designed for fusion evaporation simulation. Fusion evaporation is modelled as a three-stage process: capture, formation, and survival. While Kewpie2 independently simulates the capture cross section and survival probability, it has relied on external calculations for formation probability. This thesis implements the formation step in the Kewpie2 code for the first time using both the overdamped and full Langevin formalisms. The injection point distance (describing projectile-target nuclei starting configuration) is optimised for cold and hot fusion reaction datasets in both cases. An improved injection point distance parametrisation, consistent with the Langevin formalism, reproduces measured evaporation residue cross sections for hot fusion reactions, typically with accuracy better than an order of magnitude. For cold fusion reactions, multiple neutron emission channels are explained by introducing an additional structural term, achieving good agreement with experimental data. In this case, the 1n channel data are described as having a factor deviation from the experimental data, while the 2n and 3n channels are within an order of magnitude. The thesis also investigates survival probability modelling using the latest data for SHE. Both the formation and survival steps are extensively tested and compared with the Fusion-by-Diffusion (FbD) model for sets of 27 cold and 24 hot fusion reactions.Analysis of the reduced friction coefficients within the overdamped Langevin approach suggests that the dynamic is not fully damped. Therefore, a full one-dimensional Langevin formalism is investigated and implemented in Kewpie2. The formalism is applied to hot fusion reaction data. The fitting coefficients of the model are optimised using a so-called systematic fitting technique, and the results confirm that the dynamic is not fully damped. In this approach, the model predictions are within an order of magnitude deviations from the experimental data. Predictions for the synthesis of elements with atomic numbers ZCN = 119 et 120 align with results from other codes. Additionally, a method for studying ratios of formation probabilities is proposed and discussed for the synthesis of 258No et 259Db..In conclusion, this work significantly enhances Kewpie2, making it a self-contained tool for studying SHE synthesis and guiding future experimental efforts

In this chapter developments in theories and research on human understandings and judgements of probability are examined. The concept of probability as a degree of belief and the systematic study of human probability judgements have emerged only recently, but have stimulated numerous fruitful debates about the nature of rationality, belief formation, decision-making, and uncertainty itself. The chapter begins with a review of how the connection between probability and degrees of belief was developed and elaborated to form a prescriptive framework, followed by a brief summary of debates concerning rationality and uncertainty. It then surveys models of human probability judgements based on probability weighting functions, ways in which these judgements depend on how relevant information is presented, mental shortcuts (or “heuristics”) underpinning such judgements, the extent to which probability judgements are miscalibrated, fallacies in judgements of probabilities of compound and conditional events, and debates concerning the effective communication of probabilistic information.

Abstract The neutron is a very weak probe, and so neutron scattering does not unduly disturb the properties of the sample under investigation. From the point of view of scattering theory this is a significant advantage, as it allows us to use the Born approximation (whereby the scattering process is treated by first-order perturbation theory) in describing the neutron–nucleus interaction. However, the weakness of its interaction with matter also renders neutrons difficult to detect directly. Thermal neutrons produce negligible ionization and they can only be detected by the production of secondary ionization arising after neutron absorption. The absorption process leads to the formation of a compound nucleus, which decays either by the emission of a gamma ray or by a splitting of the compound nucleus into two charged nuclei. In neither case the energy of the incident neutron can be measured directly, as the absorption reactions are strongly exothermic with the release of several MeV of energy.

Abstract In the statistical model of nuclear reactions, discussed in the next chapter, the basic hypothesis is made of the formation of a fully equilibrated compound nucleus whose decay is independent of the mode of formation and entirely governed by the phase space available to the reaction products (Weisskopf and Ewing 1940). A similar hypothesis, the equiprobability of all decay modes of the excited intermediate (projectile + target) composite system, was made by Griffin (1966) in developing the exciton model for pre-equilibrium reactions, and was thereafter adopted in all subsequent theories, including the recent quantum mechanical ones.

The equivalent of a heat-seeking missile in chemistry is a ‘nucleophile’, a lover of nuclei. The ‘missile’ in this case is a special sort of molecule which sniffs out not heat but positive electric charge, the charge of a nucleus glinting through depleted regions of electron cloud in a target molecule. A candidate for such a missile is a negatively charged ion, which can be attracted to the partially exposed positive charge of a nucleus in the target. With nucleus-sniffing in mind, you can perhaps appreciate that if it is possible to blow away some of the electron cloud from the region of a particular C atom in the target molecule, then the nucleophile will home in on that atom and perhaps attach to it. By management of the electron clouds and adroit choice of the incoming missile, therefore, it is possible to build where and what we want. You shouldn’t lose sight of the fact that the ‘missile’ doesn’t just fly through the air and smash into a molecule: it jostles past the solvent molecules, which move aside or sometimes block it and turn it back. When thinking about the reactions in this section, think of the target atom in a molecule as glowing with positive charge and the incoming missile, the reagent, as having an abundance of negative charge spread over at least one of its atoms. The target and missile are drawn together through the solvent, and might snap together, in a manner I shall describe, once they are close to one another. A missile’s successful hunt for a target might then result in reaction and the formation of a new compound. How are the target prepared and the missile launched? One procedure is to go the whole electrical hog and prepare a molecule with a full positive charge on the C atom that has been chosen to be the foundation of the new construction, the proposed site of reaction. The whole hog can be achieved by snipping out an atom that initially is attached to the C atom.

Former studies assumed that, after fission process occurs, the highly ionized new born atoms (20–22 positive charge), ionize the media in which they pass through before becoming stable atoms in a manner similar to 4-MeV-particles. Via ordinary chemical reactions with the surroundings, each stable atom has a probability to form chemical compound. Since there are about 35 different elemental atoms created through fission processes, a large number of chemical species were suggested to be formed. But, these suggested chemical species were not found in the environment after actual releases of FP during accidents like TMI (USA, 1979), and Chernobyl (former USSR, 1986), also the models based on these suggested reactions and species could not interpret the behavior of these actual species. It is assumed here that the ionization states of the new born atoms and the long term high temperature were not dealt with in an appropriate way and they were the reasons of former models failure. Our new approach of DEEP ATOMIC BINDING (DAB) based on the following: 1. The new born atoms which are highly ionized, 10–12 electrons associated with each nucleus, having a large probability to create bonds between them to form molecules. These bonds are at the L, or M shells, and we call it DAB. 2. The molecules stay in the reactor at high temperatures for long periods, so they undergo many stages of composition and decomposition to form giant molecules. By applying DAB approach, field data from Chernobyl, TMI and nuclear detonations could be interpreted with a wide coincidence resulted.