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Mesure de la masse atomique du noyau N=Z, 74Rb, avec le spectromètre MISTRAL

Abstract : Measuring the atomic mass of nuclei permits to obtain the binding energy which reflects all the interactions prevailing within the nucleus. For N=Z nuclei, the binding energy presents a singularity which can be reproduced by adding a Wigner term in the mass formulas. This effect is studied via the neutron-proton interaction which exhibits an excess for N=Z nuclei which tends to disappear for the even-even nuclei when the mass number A increases. However, for odd-odd N=Z nuclei, the neutron-proton interaction tends to be stabilized beyond A=40 with an excess of approximately 2 MeV. The atomic mass of the N=Z=37 74Rb was measured with the radiofrequency spectrometer MISTRAL which is installed at CERN. The cyclotron frequency of the 74Rb inside a magnetic field is measured by synchronizing it with the frequency of an electric field, which modulates the kinetic energy of the ions. This cyclotron frequency is compared with that of a reference nucleus, of well-known mass, inside the same magnetic field and the ratio of the two frequencies gives the ratio of the two atomic masses. This measurement whose the obtained precision is 116 keV, confirmed the stabilization of the neutron-proton interaction for odd-odd N=Z nuclei. Various mass formulas were studied and the neutron-proton interaction was calculated starting from the masses predicted by these formulas, and compared with the experimental values. In particular, the IBM-4 model, based on SU(4) symmetry, was used to calculate the masses of N=Z nuclei and even-even nuclei, so that the neutron-proton interaction for even-even N=Z nuclei could be deduced from it. Another interesting point of the 74Rb is its super-allowed transition to the 74Kr whose energy and half-life permit to calculate the coupling vector constant of the electroweak theory, which is supposed to be independent of the nucleus according to the CVC assumption.
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Submitted on : Monday, March 24, 2003 - 3:27:24 PM
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  • HAL Id : tel-00002578, version 1

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Nelson Vieira. Mesure de la masse atomique du noyau N=Z, 74Rb, avec le spectromètre MISTRAL. Physique Nucléaire Théorique [nucl-th]. Université Pierre et Marie Curie - Paris VI, 2002. Français. ⟨tel-00002578⟩

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