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A cryogenically pumped, windowless gas target has been used to study the radiative alpha-capture reaction on ¹⁶O and ¹²C targets, giving information on the electromagnetic decay properties of various unbound states in ²⁰Ne and ¹⁶O. Several states in ²⁰Ne have been observed in this reaction for the first time. The first five unit-isospin, natural parity states in ²⁰Ne have been observed. The isospin assignment of the 11.27 MeV (1^-)) state has been made definite by the observation of a strong Ml decay to the 8.85 MeV (1^-)state, and the analogue of the 1.97 MeV state in ²⁰F has been identified with the ²⁰Ne state at 12.25 MeV. Angular distribution measurements have shown the spin of this state to be 3^-), and the total width has been shown to be less than 1 keV, contradicting previous reports. The 12.22 MeV (2⁺)state has been resolved clearly as an individual resonance for the first time. The electromagnetic transition rates for the lower T=l states in ²⁰Ne are compared with shell model calculations. Five zero-isospin capture resonances in ²⁰Ne have been studied, the gamma-decay of the 8.70 MeV (1^-), 9.11 MeV (3^-) and 11.92 MeV (4⁺) states being observed for the first time. A remeasurement has been made of the enhanced E2 decay strength of the 8⁺ member of the ground state rotational band, giving ^ω=l31 ^±18 meV, in agreement with a previous less accurate value. This result is compared with shell model calculations. A study of the capture reaction on ¹²C has been performed in the beam energy range 5.22 MeV < Eα < 9.53 MeV. No new capture resonances have been found but the known inelastic resonances have been seen. The 11.10 MeV (4⁺) state has been studied and the strength for the decay to the 6.13 MeV (3^-) state has been shown to be only half of a previous result. The 12.05 MeV (0⁺) state has not been observed, but a limit has been placed on its strength.

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Neste trabalho nós propomos uma extensão da teoria Hadrodinâmica Quântica (QHD) para o estudo do emparelhamento nucleônico na matéria nuclear. O trabalho segue a formulação usual da QHD, descrevendo a interação-NN em termos de campos médios, tanto para a auto-energia usual, E, como para a energia de emparelhamento, A. As equações dos campos são obtidas em termos dos propagadores nucleônicos exatos, os quais são expandidos até a primeira ordem na interação-NN, definindo a aproxirnação auto-consistente Hartree-Fock-Bogoliubov (HFB) para E e A. Esta abordagem representa um avanço sobre as tradicionais formulações não-relativísticas, no sentido de que ela permite uma descrição simultânea de muitas propriedades da matéria nuclear como o ponto de saturação, a massa efetiva do nucleon, o gap de energia no espectro de partícula única, etc., de uma maneira consistente considerando apenas os graus de liberdade hadrônicos, ou seja, os nucleons interagindo na matéria nuclear através da troca de mésons. Os campos médios resultantes E e A, tem grandes componentes no espaço de Lorentz, que adequadamente se cancelam nas expressões tanto da energia de ligação por nucleon como no parâmetro de gap\", de forma que os valores resultantes dessas quantidades concordam com os resultados experimentais. Embora o parâmetro de gap possa ser adequadamente estimado com modelos não-relativísticos, em termos de interações de dois nucleons, o ponto de saturação da matéria nuclear não pode. O aspecto atraente do presente modelo é sua formulação fundamental bastante simples em termos de graus de liberdade hadrônicos e da aproximação HFB usual à teoria de muitos corpos, o que aponta para avanços futuros como poderiam ser obtidos com a aproximação Brueckner-HF auto-consistente. / In this work we propose an extension of the relativistic Quantum Hadrodynamic theory (QHD) to the study of nucleonic pairing in nuclear matter. The work follows the usual formulation of QHD by describing the NN-interaction in terms of mean fields either for the usual self-energy, E, or the pairing energy, A. The equations of the fields are obtained in terms of the exact nucleon propagators which are expanded up to the first order in the NN-interaction, defining the self-consistent Hartree-Fock-Bogoliubov (HPB) approximation E for A and .This approach represents an improvement over the old non-relativistic formulations in the sense that it allows for a simultaneous description of many nuclear matter properties as the saturation point, the effective mass or the nucleon, the energy gap in the sp-spectrum, etc., in a consistent fashion, by considering only the hadronic degrees of freedom, that is, the nucleons interacting in nuclear matter through the exchange of mesons. The resulting mean field energies, E and A, have large components in the Lorentz space, which adequately cancel each other in the expressions of either the energy per nucleon or the gap parameter\", so that the resulting values of these quantities agree with the experimental data. Although the gap parameter can be adequately estimated with non-relativistic models, in terms of two nucleon interactions, the saturation point of nuclear matter cannot be. The appealing feature of the present model is its rather simple fundamental formulation in terms of hadronic degrees of freedom and the usual HFB approximation to the many-body field theory, which points toward future improvements as would be obtained with a self-consistent Brueckner-HF approximation.

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No description available.

No description available.