Excitations of the unstable nuclei ^{48}Ni and ^{49}Ni

The isoscalar E1 and E2 resonances in the proton-rich nuclei ^{48,49}Ni and the {f_{7/2}3^-} multiplet in ^{49}Ni have been calculated taking into account the single-particle continuum exactly. The analogous calculations for the mirror nuclei ^{48}Ca and ^{49}Sc are presented. The models used are the continuum RPA for ^{48}Ni, ^{48}Ca and the Odd RPA for ^{49}Ni, ^{49}Sc, the latter has been developed recently and describes both single-particle and collective excitations of an odd nucleus on a common basis. In all four nuclei we obtained a distinct splitting of the isoscalar E1 resonance into 1 h-bar omega and 3 h-bar omega peaks at about 11 MeV and 30 MeV, respectively. The main part of the isoscalar E1 EWSR is exhausted by the 3 h-bar omega resonances. The 1 h-bar omega resonances exhaust about 35% of this EWSR in ^{48,49}Ni and about 22% in ^{48}Ca and ^{49}Sc. All seven {f_{7/2}3^-} multiplet members in ^{49}Ni are calculated to be in the (6-8) MeV energy region and have noticeable escape widths.Comment: 11 pages, 3 Postscript figure

On Cooper Pairing in Finite Fermi Systems

In order to analyse the role of the quasiparticle-phonon interaction in the origin of nuclear gap, we applied an approach which is similar to the Eliashberg theory for usual superconductors. We obtained that the averaged contribution of the quasiparticle-phonon mechanism to the observed value of the pairing gap for $^{120}$Sn is 26% and the BCS-type mechanism gives 74% . Thus, pairing is of a mixed nature at least in semi-magic nuclei -- it is due to the quasiparticle-phonon and BCS mechanisms, the first one being mainly a surface mechanism and the second one mainly a volume mechanism. The calculations of the strength distribution for the odd-mass nuclei $^{119}Sn$ and $^{121}Sn$ have shown that the quasiparticle-phonon mechanism mainly improves the description of the observed spectroscopic factors in these nuclei. For the case of nuclei with pairing in both proton and neutron systems it is necessary to go beyond the Eliashberg-Migdal approximations and include the vertex correction graphs in addition to the rainbow ones. The estimations for spectroscopic factors performed within a three-level model have shown that the contribution of the vertex correction graphs was rather noticeable.Comment: The 7-th International Spring Seminar on Nuclear Physics, "Challenges of Nuclear Structure",Maiori, May 27-31, 200

Covariant theory of particle-vibrational coupling and its effect on the single-particle spectrum

The Relativistic Mean Field (RMF) approach describing the motion of independent particles in effective meson fields is extended by a microscopic theory of particle vibrational coupling. It leads to an energy dependence of the relativistic mass operator in the Dyson equation for the single-particle propagator. This equation is solved in the shell-model of Dirac states. As a result of the dynamics of particle-vibrational coupling we observe a noticeable increase of the level density near the Fermi surface. The shifts of the single-particle levels in the odd nuclei surrounding 208-Pb and the corresponding distributions of the single-particle strength are discussed and compared with experimental data.Comment: 27 pages, 8 figure

Self-consistent calculations of the strength function and radiative neutron capture cross section for stable and unstable tin isotopes

The E1 strength function for 15 stable and unstable Sn even-even isotopes from A=100 till A=176 are calculated using the self-consistent microscopic theory which, in addition to the standard (Q)RPA approach, takes into account the single-particle continuum and the phonon coupling. Our analysis shows two distinct regions for which the integral characteristics of both the giant and pygmy resonances behave rather differently. For neutron-rich nuclei, starting from $^{132}$Sn, we obtain a giant E1 resonance which significantly deviates from the widely-used systematics extrapolated from experimental data in the $\beta$-stability valley. We show that the inclusion of the phonon coupling is necessary for a proper description of the low-energy pygmy resonances and the corresponding transition densities for $A132$ region the influence of phonon coupling is significantly smaller. The radiative neutron capture cross sections leading to the stable $^{124}$Sn and unstable $^{132}$Sn and $^{150}$Sn nuclei are calculated with both the (Q)RPA and the beyond-(Q)RPA strength functions and shown to be sensitive to both the predicted low-lying strength and the phonon coupling contribution. The comparison with the widely-used phenomenological Generalized Lorentzian approach shows considerable differences both for the strength function and the radiative neutron capture cross section. In particular, for the neutron-rich $^{150}$Sn, the reaction cross section is found to be increased by a factor greater than 20. We conclude that the present approach may provide a complete and coherent description of the $\gamma$-ray strength function for astrophysics applications. In particular, such calculations are highly recommended for a reliable estimate of the electromagnetic properties of exotic nuclei

Magnetic moments of odd-odd spherical nuclei

Magnetic moments of more than one hundred odd-odd spherical nuclei in ground and excited states are calculated within the self-consistent TFFS based on the EDF method by Fayans {\it et al}. We limit ourselves to nuclei with a neutron and a proton particle (hole) added to the magic or semimagic core. A simple model of no interaction between the odd nucleons is used. In most the cases we analyzed, a good agreement with the experimental data is obtained. Several cases are considered where this simple model does not work and it is necessary to go beyond. The unknown values of magnetic moments of many unstable odd and odd-odd nuclei are predicted including sixty values for excited odd-odd nuclei.Comment: 10 page

M1 Resonances in Unstable Magic Nuclei

Within a microscopic approach which takes into account RPA configurations, the single-particle continuum and more complex $1p1h\otimes phonon$ configurations isoscalar and isovector M1 excitations for the unstable nuclei ${56,78}$Ni and ${100,132}$Sn are calculated. For comparison, the experimentally known M1 excitations in ${40}$Ca and $^{208}$Pb have also been calculated. In the latter nuclei good agreement in the centroid energy, the total transition strength and the resonance width is obtained. With the same parameters we predict the magnetic excitations for the unstable nuclei. The strength is sufficiently concentrated to be measurable in radioactive beam experiments. New features are found for the very neutron rich nucleus ${78}$Ni and the neutron deficient nucleus ${100}$Sn.Comment: 17 pages (LATEX), 12 figures (available from the authors), KFA-IKP(TH)-1993-0

On the mechanisms of superfluidity in atomic nuclei

A system of equations is obtained for the Cooper gap in nuclei. The system takes two mechanisms of superfluidity into account in an approximation quadratic in the phonon- production amplitude : a Bardeen- Cooper- Schrieffer (BCS) type mechanism and a quasiparticle- phonon mechanism. These equations are solved for 120 Sn in a realistic approximation. If the simple procedures proposed are used to determine the new particle- particle interaction and to estimate the average effect, then the contribution of the quasiparticle- phonon mechanism to the observed width of the pairing gap is 26% and the BCS-type contribution is 74%. This means that at least in semimagic nuclei pairing is of a mixed nature - it is due to the two indicated mechanisms, the first being mainly a surface mechanism and the second mainly a volume mechanism.Comment: 6 page

Self-consistent calculations of radiative nuclear reaction characteristics for 56

The photon strength functions (PSF), neutron capture cross sections and average radiative widths of neutron resonances for three double-magic nuclei 56Ni, 132Sn and 208Pb have been calculated within the self-consistent version of the microscopic theory. Our approach includes phonon coupling (PC) effects in addition to the standard QRPA approach. With our microscopic PSFs, calculations of radiative nuclear reaction characteristics have been performed using the EMPIRE 3.1 nuclear reaction code. Three nuclear level density (NLD) models have been used: the phenomenological so-called GSM, phenomenological Enhanced GSM (EGSM) and microscopical combinatorial HFB model. For all the considered characteristics, we found a noticeable contribution of the PC effects and a significant disagreement between the results obtained with the GSM and the other two NLD models. The results confirm the necessity of using consistent microscopic approaches for calculations of radiative nuclear characteristics in double-magic nuclei

Covariant response theory beyond RPA and its application

The covariant particle-vibration coupling model within the time blocking approximation is employed to supplement the Relativistic Random Phase Approximation (RRPA) with coupling to collective vibrations. The Bethe-Salpeter equation in the particle-hole channel with an energy dependent residual particle-hole (p-h) interaction is formulated and solved in the shell-model Dirac basis as well as in the momentum space. The same set of the coupling constants generates the Dirac-Hartree single-particle spectrum, the static part of the residual p-h interaction and the particle-phonon coupling amplitudes. This approach is applied to quantitative description of damping phenomenon in even-even spherical nuclei with closed shells $^{208}$Pb and $^{132}$Sn. Since the phonon coupling enriches the RRPA spectrum with a multitude of ph$\otimes$phonon states a noticeable fragmentation of giant monopole and dipole resonances is obtained in the examined nuclei. The results are compared with experimental data and with results of the non-relativistic approach.Comment: 12 pages, 4 figures, Proceedings of the NSRT06 Conferenc