Berlin, Freie Universiẗat, Diss., 2000. / Dateiformat: zip, Dateien im PDF-Format.

Neste trabalho, foram estudadas as reações envolvendo núcleos pesados e prótons no regime de energias intermediárias e altas através de simulações computacionais com o modelo Monte Carlo CRISP (Colaboração Rio - Ilhéus - São Paulo). Os principais processos nucleares investigados foram a cascata intranuclear e a competição evaporação-fissão. Em estudos preliminares, determinou-se que o CRISP reproduz satisfatoriamente a multiplicidade de nêutrons de evaporação (E < 20 MeV) da reação p (1200 MeV) + Pb-208 e a produção de resíduos de spallation para p (1000 MeV) + Pb-208. Já o estudo da relação entre multiplicidade de nêutrons e fissão para a reação de prótons com energia até 85 MeV com Th-232 indicou que o CRISP superestimou as emissões, enquanto subestimou a seção de choque de fissão dessa reação - reflexo das limitações do modelo de cascata nuclear para baixas energias (da ordem de 50 MeV). A reação p (1200 MeV) + Pb-208 foi escolhida para o estudo de uma fonte de nêutrons de spallation. A cascata intranuclear foi responsável pela emissão dos nêutrons energéticos da reação (E > 20 MeV), enquanto que a evaporação foi responsável pelo maior número de nêutrons emitidos. A seção de choque de fissão encontrada foi de 209 mb, enquanto que a de spallation foi de 1788 mb - ambas comparáveis aos valores experimentais. A distribuição de massa dos fragmentos indicou fissão simétrica. Finalmente, foi utilizado outro código Monte Carlo, o MCNP, para o transporte de radiação, a fim de compreender o papel da fonte de nêutrons de spallation em um reator nuclear ADS (Accelerator Driven System). Simulou-se um reator PWR, inicialmente, para estudar a produção de nuclídeos no processo de queima do combustível nuclear. Em seguida, simulou-se uma primeira tentativa de adaptação de uma fonte de spallation a um reator térmico de dimensões industriais. Constatou-se que não houve redução da concentração de elementos transurânicos com o modelo de reator adotado e alterações foram propostas. / In the present work, intermediate- and high-energy nuclear reactions involving heavy nuclei and protons were studied with the Monte Carlo CRISP (Rio - Ilhéus - São Paulo Collaboration) model. The most relevant nuclear processes studied were intranuclear cascade and fission-evaporation competition. Preliminary studies showed fair agreement between CRISP model calculation and experimental data of multiplicity of evaporated neutrons (E < 20 MeV) from the p (1200 MeV) + Pb-208 reaction and of spallation residues from the p (1000 MeV) + Pb-208 reaction. The investigation of neutron multiplicity from proton-induced fission of Th-232 up to 85 MeV showed that it was being overestimated by CRISP model; on the other hand, fission cross section were being underestimated. This behavior is due to limitations of the intranuclear cascade model for low-energies (around 50 MeV). The p (1200 MeV) + Pb-208 reaction was selected for the study of a spallation neutron source. High-energy neutrons (E > 20 MeV) were emitted mostly in the intranuclear cascade stage, while evaporation presented larger neutron multiplicity. Fission cross section of 209 mb and spallation cross section of 1788 mb were calculated - both in agreement with experimental data. The fission process resulted in a symmetric mass distribution. Another Monte Carlo code, MCNP, was used for radiation transport in order to understand the role of a spallation neutron source in a ADS (Accelerator Driven System) nuclear reactor. Initially, a PWR reactor was simulated to study the isotopic compositions in spent nuclear fuel. As a first attempt, a spallation neutron source was adapted to an industrial size nuclear reactor. The results showed no evidence of incineration of transuranic elements and modifications were suggested.

Neste trabalho, foram estudadas as reações envolvendo núcleos pesados e prótons no regime de energias intermediárias e altas através de simulações computacionais com o modelo Monte Carlo CRISP (Colaboração Rio - Ilhéus - São Paulo). Os principais processos nucleares investigados foram a cascata intranuclear e a competição evaporação-fissão. Em estudos preliminares, determinou-se que o CRISP reproduz satisfatoriamente a multiplicidade de nêutrons de evaporação (E < 20 MeV) da reação p (1200 MeV) + Pb-208 e a produção de resíduos de spallation para p (1000 MeV) + Pb-208. Já o estudo da relação entre multiplicidade de nêutrons e fissão para a reação de prótons com energia até 85 MeV com Th-232 indicou que o CRISP superestimou as emissões, enquanto subestimou a seção de choque de fissão dessa reação - reflexo das limitações do modelo de cascata nuclear para baixas energias (da ordem de 50 MeV). A reação p (1200 MeV) + Pb-208 foi escolhida para o estudo de uma fonte de nêutrons de spallation. A cascata intranuclear foi responsável pela emissão dos nêutrons energéticos da reação (E > 20 MeV), enquanto que a evaporação foi responsável pelo maior número de nêutrons emitidos. A seção de choque de fissão encontrada foi de 209 mb, enquanto que a de spallation foi de 1788 mb - ambas comparáveis aos valores experimentais. A distribuição de massa dos fragmentos indicou fissão simétrica. Finalmente, foi utilizado outro código Monte Carlo, o MCNP, para o transporte de radiação, a fim de compreender o papel da fonte de nêutrons de spallation em um reator nuclear ADS (Accelerator Driven System). Simulou-se um reator PWR, inicialmente, para estudar a produção de nuclídeos no processo de queima do combustível nuclear. Em seguida, simulou-se uma primeira tentativa de adaptação de uma fonte de spallation a um reator térmico de dimensões industriais. Constatou-se que não houve redução da concentração de elementos transurânicos com o modelo de reator adotado e alterações foram propostas. / In the present work, intermediate- and high-energy nuclear reactions involving heavy nuclei and protons were studied with the Monte Carlo CRISP (Rio - Ilhéus - São Paulo Collaboration) model. The most relevant nuclear processes studied were intranuclear cascade and fission-evaporation competition. Preliminary studies showed fair agreement between CRISP model calculation and experimental data of multiplicity of evaporated neutrons (E < 20 MeV) from the p (1200 MeV) + Pb-208 reaction and of spallation residues from the p (1000 MeV) + Pb-208 reaction. The investigation of neutron multiplicity from proton-induced fission of Th-232 up to 85 MeV showed that it was being overestimated by CRISP model; on the other hand, fission cross section were being underestimated. This behavior is due to limitations of the intranuclear cascade model for low-energies (around 50 MeV). The p (1200 MeV) + Pb-208 reaction was selected for the study of a spallation neutron source. High-energy neutrons (E > 20 MeV) were emitted mostly in the intranuclear cascade stage, while evaporation presented larger neutron multiplicity. Fission cross section of 209 mb and spallation cross section of 1788 mb were calculated - both in agreement with experimental data. The fission process resulted in a symmetric mass distribution. Another Monte Carlo code, MCNP, was used for radiation transport in order to understand the role of a spallation neutron source in a ADS (Accelerator Driven System) nuclear reactor. Initially, a PWR reactor was simulated to study the isotopic compositions in spent nuclear fuel. As a first attempt, a spallation neutron source was adapted to an industrial size nuclear reactor. The results showed no evidence of incineration of transuranic elements and modifications were suggested.

A benchmark experiment was designed and conducted that irradiated two small volume mercury targets at the Weapon Neutron Research facility at the Los Alamos Neutron Science Center with 800 MeV protons. Following irradiation the production cross sections of 53 medium and longer-lived spallation residuals using gamma spectroscopy at various decay times up to a year were determined. The measured cross sections were then compared with predicted cross sections from the MCNPX code. After acquisition of the gamma spectroscopy data the targets were drained and disassembled to study the distribution and the deposition of the spallation residuals.

No description available.

In this thesis, the design of a high intensity accumulator ring for the European Spallation Source Neutrino Super Beam (ESSnuSB) is considered. The European Spallation Source (ESS) linear accelerator (Linac), presently being constructed in Lund, Sweden, presents an interesting opportunity to also host an experiment to detect neutrino CP violation. 0.7 ms long H- pulses would be accelerated to 2 GeV and collide with a target, producing pions which then decay into neutrinos. To focus the pions a toroidal magnet (''neutrino horn'') is pulsed with a 350 kA current. The peak current is about 5 μs long, which requires the H- pulses to be shortened to about the same length using an accumulator ring that is located between the linac and the target. The H- would be stripped of their electrons using either a thin carbon foil or a laser beam during injection into the ring. Foil stripping is limited by the lifetime of the foil, which depends on the temperature to which it is heated by the beam. The temperature is simulated in a computer model and the results indicate that it does not rise above the critical temperature (2500 K). The high number of protons (1015) circulating in the ring could cause instabilities due to the collective charge of the particles, known as the space charge effect. The space charge tune shift is calculated for the ESSnuSB and different solutions are discussed. The result of a design accumulator lattice for the ESSnuSB, based on the Spallation Neutron Source, at Oak Ridge National Laboratory, Tennessee, U.S., and made using the computer program Methodical Accelerator Design (MAD), is presented.

Thesis (M.A.)-University of California, Berkeley, Feb. 1958. / Bibliography references : p. 31-32.

Este trabalho é dedicado ao estudo de reações de fissão de núcleos pesados induzidas por fótons e prótons a energias intermediárias e altas, fazendo uso, para tanto, do código de Monte Carlo CRISP, sendo um trabalho inteiramente computacional. Estudou-se um novo conjunto de parâmetros para a fórmula de massa semi-empírica proposta por Pearson em seu trabalho de 2001, a partir da qual se passou à simulação de reações de fissão induzidas por fótons dos núcleos 208Pb, 232Th, 237Np e 238U, e fissão induzida por prótons nos núcleos 197Au, 208Pb, 237Np e 241Am. Os modelos de Bohr e Wheeler para fissão, de 1939, e o modelo de Weisskopf para emissão de partículas de 1937 foram empregados na simulação. Para os parâmetros de densidade de níveis, um conjunto de parâmetros foi obtido para as fórmulas empíricas de Dostrovsky de 1958 e para a razão de fissão. Foram calculadas seções de choque de fotofissão para os núcleos 208Pb, 232Th, 237Np e 238U, produtos de reações de spallation induzidas por prótons nos núcleos 208Pb e 197Au, e distribuições de massa dos fragmentos de fissão em reações de Bremsstrahlung com o 238U e em reações induzidas por prótons com os núcleos 197Au, 208Pb, 237Np e 241Am. São apresentadas comparações a dados experimentais de todos os cálculos realizados. Também se obteve um modelo para aproximar os valores de barreira de fissão obtidos pelo método ETFSI (Extended Thomas-Fermi plus Strutinsky Integral). / This work is dedicated to the study of heavy nuclei fission induced by intermediate- and high-energy photons and protons, by using to this end the Monte Carlo code CRISP, therefore being an entirely computacional work. A new set of parameters for the 2001 semi-empirical mass formula proposed by Pearson is studied. Then we simulate fission reactions induced by photons and protons on 208Pb, 232Th, 237Np and 238U, and fission induced by protons on 197Au, 208Pb, 237Np e 241Am. The Bohr-Wheeler model for describing the fission process and the Weisskopf model for the emission probability of particles were used to perform the calculation. Regarding the level density parameters, a set of parameters is presented for the empirical formulas of Dostrovsky and for the fission ratio. This work also presents calculated fotofission cross section for induced reactions on 208Pb, 232Th, 237Np and 238U nuclei, spallation products of proton induced reactions on 208Pb and 197Au, and fragment mass distributions of Bremsstrahlung reactions with 238U and of proton induced reactions on 197Au, 208Pb, 237Np and 241Am. Comparisons to experimental data are shown for all calculations. A model to aproximate the fission barrier values obtaind by the ETFSI - Extended Thomas-Fermi plus Strutinsky Integral method is also presented.

Les réactions 136Xe + p et 136Xe + 12C à 1 GeV par nucléon d’énergie cinétique du projectile dans le centre de masse ont été étudiées en cinématique inverse à l’aide du dispositif expérimental SPALADIN, installé auprès de l’accélérateur d’ions lourds de GSI. Ma thèse décrit l’analyse de ces collisions réalisées au printemps 2009. La détection en coïncidence des fragments de l’état final (résidu du projectile, neutrons et fragments légers chargés) avec une grande efficacité géométrique est obtenue par la combinaison de la cinématique inverse, d’un aimant dipolaire de grande ouverture physique et de l’utilisation de grands détecteurs. Ces coïncidences, mesurées événement par événement, nous ont permis de sélectionner, indépendamment du modèle de désexcitation, le pré-fragment, noyau excité issu de la cascade intranucléaire en fonction de son énergie d’excitation. Ainsi, nous avons pu étudier l’évolution du mécanisme de désexcitation (par exemple, l’évaporation de particules légères, la cassure binaire asymétrique ou la fragmentation multiple simultanée) en fonction de l’énergie d’excitation du pré-fragment. Les données de la réaction 136Xe + p sont comparées principalement à trois modèles de désexcitation (SMM, GEMINI++ et ABLA07) couplés au code de cascade intranucléaire INCL4. Si ces modèles semblent décrire globalement les observables de notre expérience, des désaccords significatifs sont apparus notamment en ce qui concerne la production des fragments de masse intermédiaire (IMF). La comparaison des données 136Xe + 12C avec celles de la réaction 136Xe + p fait apparaître une grande similitude dans la désexcitation des pré-fragments qui laisse entrevoir que la cascade intranucléaire mène, dans les deux cibles, aux mêmes types de pré-fragments sur la plage d’énergie d’excitation commune aux deux réactions (0 à 4 MeV par nucléon). Le régime d’excitation au-dessus de 4 MeV par nucléon, accessible uniquement pour la réaction 136Xe + 12C, montre une différence qualitative dans la désexcitation du pré-fragment, avec notamment une production beaucoup plus importante d’IMF, avec des multiplicités élevées par événement, qui croît avec l’énergie d’excitation. / The collision of 136Xe with the proton and with 12C at 1 GeV per nucleon of projectile kinetic energy in the centre of mass has been studied in inverse kinematics using the SPALADIN experimental setup at the GSI facility. This manuscript describes the analysis of these collisions realized in spring 2009. The detection in coincidence of the final state fragments (projectile residues, neutrons and light charged fragments) with a large geometrical efficiency is provided by the inverse kinematics combined with a large aperture dipole magnet and large detectors. Such a coincidence, measured on an event basis, allows selecting, in a model-independent way, the prefragment, the excited nuclear system formed after the intranuclear cascade as a function of its excitation energy. Hence, we were able to study the evolution of the prefragment deexcitation mechanism (evaporation of light particles, asymmetric binary decay, multiple fragmentation…) as a function of its excitation energy. The data of the 136Xe + p reaction have been compared mainly to three deexcitation models (SMM, GEMINI++ and ABLA07) coupled to the intranuclear cascade code INCL4. Despite the relatively good and global agreement between these models and our data, significant discrepancies appeared concerning in particular the production of intermediate mass fragments (IMF). Comparison between the 136Xe + 12C and the 136Xe + p data exhibits an important similarity in the deexcitation of the pre-fragments. This suggests that the nuclear cascade leads, for both targets, to similar prefragment types in the range of excitation energy (0 to 4 MeV per nucleon) common to both reactions. Higher excitation energies, reached only in the 136Xe + 12C reaction, show a qualitative difference in the deexcitation of the pre-fragment, with much higher multiplicities of IMF per event, increasing with the excitation energy.

Lintérêt pour les réactions de spallation comme source intense de neutrons sest renouvelé ces dix dernières années avec les études de transmutation des actinides mineurs dans les systèmes hybrides (ADS). Parmi les modèles de réaction de spallation, le modèle de cascade intranucléaire de Liège (INCL), couplé au modèle dévaporation ABLA, sest révélé être lun des plus prédictifs pour des nucléons incidents de 200 MeV à quelques GeV. Cependant le modèle INCL présente encore quelques lacunes et fait donc encore lobjet de développements. Dans cette thèse nous avons tenté de remédier aux points suivants: - évaluation du pic quasi-élastique en introduisant la dépendance en isospin et en énergie du champ nucléaire moyen des baryons, - les voies faisant intervenir les pions en étendant les sections efficaces pion-nucléon de part et dautre de la résonnance D et en introduisant un potentiel moyen pour les pions déterminé par ajustement sur de nombreuses données expérimentales de production de pions et induites par des pions. En vue de lextension du modèle INCL aux basses énergies nous avons également comparé ce modèle à des modèles de prééquilibre, en théorie mieux adaptés pour des énergies incidentes comprises entre 10 et 200 MeV. Finalement nous avons étudié limpact de nos investigations pour létude de cibles de spallation épaisses et dADS. Le résultat le plus important de nos recherches tant pour des cibles minces que pour la conception de cibles épaisses, est une nette amélioration (réduction) de lestimation des isotopes hautement radiotoxiques du Po à partir de cibles en Bi.