arXiv subjects

N. Van Giai

Publications and source records attributed to N. Van Giai.

27 records · Page 2Linked to original sources

arXiv2000

Spin-Orbit Splitting in Non-Relativistic and Relativistic Self-Consistent Models

The splitting of single-particle energies between spin-orbit partners in nuclei is examined in the framework of different self-consistent approachs, non-relativistic as well as relativistic. Analytical expressions of spin-orbit potentials are given for various cases. Proton spin-orbit splittings are calculated along some isotopic chains (O, Ca, Sn) and they are compared with existing data. It is found that the isotopic dependence of the relativistic mean field predictions is similar to that of some Skyrme forces while the relativistic Hartree-Fock approach leads to a very different dependence due to the strong non-locality.

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arXiv2000

On dipole compression modes in nuclei

Isoscalar dipole strength distributions in spherical medium- and heavy-mass nuclei are calculated within random phase approximation (RPA) or quasiparticle RPA. Different Skyrme-type interactions corresponding to incompressibilities in the range 200 - 250 MeV are used. The results are discussed in comparison with existing data on isoscalar giant dipole resonances. Two main issues are raised, firstly the calculated giant resonance energies are somewhat higher than the observed ones, and secondly a sizable fraction of strength is predicted below 20 MeV which needs to be experimentally confirmed.

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arXiv2000

Microscopic theories of neutrino-^{12}C reactions

In view of the recent experiments on neutrino oscillations performed by the LSND and KARMEN collaborations as well as of future experiments, we present new theoretical results of the flux averaged $^{12}C(ν_e,e^-)^{12}N$ and $^{12}C(ν_μ,μ^-)^{12}N$ cross sections. The approaches used are charge-exchange RPA, charge-exchange RPA among quasi-particles (QRPA) and the Shell Model. With a large-scale shell model calculation the exclusive cross sections are in nice agreement with the experimental values for both reactions. The inclusive cross section for $ν_μ$ coming from the decay-in-flight of $π^+$ is $15.2 \times 10^{-40} cm^2$ to be compared to the experimental value of $12.4 \pm 0.3 \pm 1.8 \times 10^{-40} cm^2$, while the one due to $ν_{e}$ coming from the decay-at-rest of $μ^+$ is $16.4 \times 10^{-42} cm^2$ which agrees within experimental error bars with the measured values. The shell model prediction for the decay-in-flight neutrino cross section is reduced compared to the RPA one. This is mainly due to the different kind of correlations taken into account in the calculation of the spin modes and partially due to the shell-model configuration basis which is not large enough, as we show using arguments based on sum-rules.

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arXiv2000

Proton decay of high-lying states in odd nuclei

In the framework of the quasiparticle-phonon model, we study the non-statistical proton decay of excited states in odd nuclei towards low-lying collective states. Partial cross sections and branching ratios for the proton decay of the high angular momentum states in $^{41}$Sc, $^{59}$Cu and $^{91}$Nb %{\it excited by means of the ($^{7}$Li,$^{6}$He) reaction} are evaluated. The calculated branching ratios predict strong direct proton decays to the low-lying vibrational states in $^{41}$Sc and $^{91}$Nb. A general agreement with existing experimental data is found.

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arXiv1999

Compression modes in nuclei: microscopic models with Skyrme interactions

The isoscalar giant monopole resonances (ISGMR) and giant dipole resonances (ISGDR) in medium-heavy nuclei are investigated in the framework of HF+RPA and HF-BCS+QRPA with Skyrme effective interactions. It is found that pairing has little effect on these modes. It is also found that the coupling of the RPA states to 2p-2h configurations results in about (or less than) 1 MeV shifts of the resonance energies and at the same time gives the correct total widths. For the ISGMR, comparison with recent data leads to a value of nuclear matter compression modulus close to 215 MeV. However, a discrepancy between calculated and measured energies of the ISGDR in $^{208}$Pb is found and remains an open problem.

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arXiv1999

Cluster ionization via two-plasmon excitation

We calculate the two-photon ionization of clusters for photon energies near the surface plasmon resonance. The results are expressed in terms of the ionization rate of a double plasmon excitation, which is calculated perturbatively. For the conditions of the experiment by Schlipper et al., we find an ionization rate of the order of 0.05-0.10 fs^(-1). This rate is used to determine the ionization probability in an external field in terms of the number of photons absorbed and the duration of the field. The probability also depends on the damping rate of the surface plasmon. Agreement with experiment can only be achieved if the plasmon damping is considerably smaller than its observed width in the room-temperature single-photon absorption spectrum.

physics.atm-clus↗

arXiv1998

Widths of Isobaric Analog Resonances: a microscopic approach

A self-consistent particle-phonon coupling model is used to investigate the properties of the isobaric analog resonance in $^{208}$Bi. It is shown that quantitative agreement with experimental data for the energy and the width can be obtained if the effects of isospin-breaking nuclear forces are included, in addition to the Coulomb force effects. A connection between microscopic model predictions and doorway state approaches which make use of the isovector monopole resonance, is established via a phenomenological ansatz for the optical potential.

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arXiv1997

Neutrino-nucleus reactions on ^{12}C and ^{16}O

Exclusive and inclusive $(ν_μ, μ^-), (ν_e, e^-)$ cross-sections and $μ^-$-capture rates are calculated for ^{12}C and ^{16}O using the consistent random phase approximation (RPA) and pairing model. After a pairing correction is introduced to the RPA results the flux-averaged theoretical $(ν_μ, μ^-), (ν_e, e^-)$ cross-sections and $μ^-$-capture rates in $^{12}$C are in good agreement with experiment. In particular when one takes into account the experimental error bars, the recently measured range of values for the $(ν_μ, μ^-)$ cross-section is in agreement with the present theoretical results. Predictions of $(ν_μ, μ^-)$ and $(ν_e, e^-)$ cross-sections in ^{16}O are also presented.

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arXiv1994

Escape and Spreading Properties of Charge-Exchange Resonances in Bi 208

The properties of charge-exchange excitations of ${}^ {208}$Pb with $ΔL = 0$, i.e., the isobaric analog and Gamow-Teller resonances, are studied within a self-consistent model making use of an effective force of the Skyrme type. The well-known isobaric analog case is used to assess the reliability of the model. The calculated properties of the Gamow-Teller resonance are compared with recent experimental measurements with the aim of better understanding the microscopic structure of this mode.

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