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The threshold energy for the reaction Br⁸¹ (p,n)Kr⁸¹ was measured and found to be 1.133±.020 Mev. The threshold for Cd¹¹⁶(p,n)ln¹¹⁶ was searched for up to 1.8 Mev but was not found. The density of cosmic neutrons was measured at the earth's surface. It was found to be 4.2±.7x10⁻⁹ n/cm³ over land with 66% in thermal equilibrium. Over the sea, this density was 3.72±.98x10⁻⁹ n/cm³ with 64% in the thermal peak. The earth's albedo or reflectivity to neutrons impinging on it from the atmosphere was estimated to be .22. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Two boron trifluoride proportional counters, one containing normal isotopic boron and the other boron enriched to 96% B¹⁰, have been irradiated with 4.87-Mev neutrons from the D +.D reaction. In addition to the reactions B¹⁰(n,α)Li⁷and B¹⁰(n,α)Li⁷* with Q-values of 2.79 Mev and 2.31 Mev respectively, two other reactions have been observed, (i) F¹⁹(n,α)N¹⁶and F¹⁹ (n, α)N¹⁶* with Q-values of -1.43±0.15 Mev and -1.77±0.15 Mev and (ii) either B¹⁰ (n,p)Be¹⁰ or B¹⁰ (n,t)Be⁸ with a Q-value of +0.35±0.20 Mev. Due to the presence of these two reactions, the analysis of complex fast neutron spectra by the use of such counters is not feasible. The Q-values for reaction (i) yield a value of the N¹⁶ mass of 16.01110±.00020 MU. Neutrons from a pulsed deuterium beam impinging on a tritium target were used to bombard a boron trifluoride proportional counter containing the normal ratio of B¹¹ to B¹⁰. The half-life of the activity and the energy of the particles emitted indicated that they were due to the immediate breakup of Be⁸ into two alphas after the 0.89 sec. beta decay of the Li⁸ formed by the B¹¹ (n,α)Li⁸ reaction. The process was found to have a cross section of the order of 10 millibarns for 14-Mev neutrons. The angular distribution of the non-resonant gamma radiation from the proton bombardment of C¹² has been measured and found to obey the relation:- I(ɵ)α0.02±.02 + sin²ɵ, for a proton energy of 1,580 kev. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Detailed calculations have been carried out which indicate that the small-angle scattering of fast neutrons by lead depends on the polarization, or spin orientation, of the neutrons. When the scattering of neutrons whose spin vectors point upward is observed in the horizontal plane, more neutrons should be found scattered to the right than to the left. For completely polarized 3.1 Mev neutrons, the theory predicts a maximum "right to left" intensity ratio of 14.5:1 at a scattering angle of 0.5°, the ratio decreasing to 1.6:1 at 5°, and approaching unity rapidly as the scattering angle increases. An attempt to detect this effect with neutrons from the reaction D(d,n)He³ failed because the degree of neutron collimation attainable, while satisfactory for most scattering experiments, was insufficient to permit investigation of neutron scattering at very small angles. A three crystal pair spectrometer has been used to investigate the complex gamma ray spectrum arising from bombardment of B¹⁰with deuterons of several energies between 0.8 and 2.2 Mev. Gamma rays of energy 4.46 ≠.04, 4.75 ≠ .03, 5.03 ≠ .09, 5.35 ≠ .05, 6.52 ≠ .05, 6.78 ≠ .07, 7.29 ≠ .04, 8.27 ≠ .09, and 8.87 ≠ .02 Mev have been assigned to transitions in B¹¹ and C¹¹, with excellent agreement in almost all cases with the energy level schemes proposed from other experiments. The excitation curves of three of the gamma rays have been found to rise smoothly between bombarding energies of 0.8 and 2.2 Mev, indicating that the reactions B¹⁰(d,p [symbol omitted])B¹¹l and B¹⁰(d, n [symbol omitted])C¹¹ are primarily of non-resonant character, at least in this energy region. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

An ion accelerator has been constructed which, is capable of accelerating very intense monoenergetic beams of protons or deuterons up to 50 KEV energy. Total ion currents of 500 µ amps containing 30 to 40% atomic ions have been obtained. The intensity and high definition of the beam makes it possible to collimate after magnetic analysis, 50 µ amps of protons or deuterons at 30 KEV energy into a 1/16 inch diameter spot on a target. This deuteron beam incident on a heavy ice target is capable of producing a neutron flux of 3 x 10⁸ neutrons/sec from the "D on D" reaction at 50 KEV bombarding energy or the equivalent of the neutrons from 20 curies of radium used in a Ra-Be source. The intrinsic interest of the D-D reactions to nuclear physics had prompted the construction of a scattering chamber to be used in conjunction with the accelerator. An experimental arrangement has been designed and constructed for the measurement of the energy dependent characteristics of the companion reactions, D(d,n)He³ and D(d,p)H³, which has a potential accuracy considerably greater than previously reported. As well, the extension of studies to much lower energies has been made possible by the large ion currents available. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

An accurately colliraated monenergetic fast neutron beam of small angular width was produced by bombarding a deuterium target with 2 MeV deuterons. Background reduction was achieved by using the associated particle technique and extensive shielding. Dual parameter pulse height analysis was used to reduce the coincidence time resolution between the detected neutrons and the associated He³ recoil particle to 4.6 nanoseconds by taking into account the variations in flight times of the He³ particles. A rotating thin film deuterated polyethylene target was developed which enabled a substantial increase in the neutron flux to be made. The beam of 70 neutrons/second/millisteradian produced, was of sufficient intensity to perform a small angle scattering experiment. The technique was tested by a measurement of the angular distribution from 40 degrees to 10 degrees of neutrons elastically scattered from lead. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Dans ce travail, nous avons amélioré un modèle de diffusion des défauts ponctuels (lacunes et interstitiels) en introduisant les hétéro-interstitiels. Ce modèle permet ainsi de simuler par Monte Carlo cinétique (MCC) la formation d’amas riches en solutés observés expérimentalement dans des alliages modèles ferritiques irradiés de type Fe – Cu MnNiSiP – C.Des calculs de structure électronique nous ont permis de caractériser les interactions existant entre les interstitiels et les différents atomes de solutés et aussi le carbone. Le P interagit avec les lacunes et très fortement avec l'interstitiel, le Mn interagit également avec l’interstitiel pour former un dumbbell mixte. Le C, en position octaédrique, interagit fortement avec la lacune et plus faiblement avec l'interstitiel. Les énergies de liaison, de migration ainsi que d’autres propriétés à l’échelle atomique, déterminées par calculs ab initio, nous ont conduits à un jeu de paramètres pour le code de MCC. Dans un premier temps, ces paramètres ont été optimisés sur des expériences, de la littérature, de recuits isochrones d’alliages binaires préalablement irradiés aux électrons. Les simulations de recuit isochrone, en reproduisant les tendances observées expérimentalement, nous ont permis d'associer précisément un mécanisme à chacune des évolutions de la résistivité au cours du recuit. Par ailleurs, les limites de solubilité des différents éléments ont été déterminées par Monte Carlo Metropolis. Dans un second temps, nous avons simulé l’évolution à 300 °C de la microstructure sous irradiation dans des alliages de complexité croissante : fer pur, alliages binaires, ternaires, quaternaires, et enfin alliages complexes de compositions proches de celle de l'acier de cuve. L'ensemble des simulations montrent que le modèle reproduit globalement les tendances des résultats expérimentaux, ce qui a permis de proposer des mécanismes pour expliquer les différents comportements observés. / In this work, we have improved a diffusion model for point defects (vacancies and self-interstitials) by introducing hetero-interstitials. The model has been used to simulate by Kinetic Monte Carlo (KMC) the formation of solute rich clusters that are observed experimentally in irradiated ferritic model alloys of type Fe – CuMnNiSiP – C.Electronic structure calculations have been used to characterize the interactions between self-interstitials and all solute atoms, and also carbon. P interacts with vacancies and strongly with self-interstitials. Mn also interacts with self-interstitials to form mixed dumbbells. C, with occupies octahedral sites, interacts strongly with vacancies and less with self-interstitials. Binding and migration energies, as well as others atomic scale properties, obtained by ab initio calculations, have been used as parameters for the KMC code. Firstly, these parameters have been optimized over isochronal annealing experiments, in the literature, of binary alloys that have been electron-irradiated. Isochronal annealing simulations, by reproducing experimental results, have allowed us to link each mechanism to a single evolution of the resistivity during annealing. Moreover, solubility limits of all the elements have been determined by Metropolis Monte Carlo. Secondly, we have simulated the evolution at 300 °C of the microstructure under irradiation of different alloys of increasing complexity: pure Fe, binary alloys, ternaries, quaternaries, and finally complex alloys which compositions are close to those of pressure vessel steels. The results show that the model globally reproduces all the experimental tendencies, what has led us to propose mechanisms to explain the behaviours observed.

Orientador: Marcello Damy de Souza Santos / Tese (doutorado) - Universidade Estadual de Campinas. Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-07-15T08:52:51Z (GMT). No. of bitstreams: 1 Rodrigues_Claudio_D.pdf: 2532753 bytes, checksum: d92733a3f7ef2e6d0dbe802823326baf (MD5) Previous issue date: 1979 / Resumo: São apresentadas medidas do espalhamento de nêutrons frios pelo álcool metílico na fase líquida, na temperatura ambiente. O espalhamento quase-elástico é interpretado de acordo com o modelo de Larsson-Bergstedt (L-B), que leva em conta movimentos intra-moleculares e difusão molecular. Resulta um valor 2,4 x 10-12s para o tempo de relaxação da rotação restrita do grupo CH3 na molécula. A análise do espalhamento quase-elástico, de acordo com o modelo L-B, explica de maneira consistente os resultados experimentais no intervalo de transferência de quantidade de movimento de 0,80 a 1,55 Å-1. Na região inelástica alguma estrutura é observada para transferências de energia 22,17 e 5 meV. A transferência de energia em 17 meV é associada com a transição 1 ® 0 das oscilações torsionais do grupo metil. A energia de ativação para esse movimento resulta 1,3 kcal/mol, em bom acordo com o valor da altura da barreira para rotação interna do CH3 no metanol, obtida por técnica de microondas / Abstract: Not informed. / Doutorado / Física / Doutor em Ciências