The odd-even staggering of nuclear masses was recognized in the early days of nuclear physics. Recently, a similar effect was discovered in other finite fermion systems, such as ultrasmall metallic grains and metal clusters. It is believed that the staggering in nuclei and grains is primarily due to pairing correlations (superconductivity), while in clusters it is caused by the Jahn-Teller effect. We find that, for light and medium-mass nuclei, the staggering has two components. The first one originates from pairing while the second, comparable in magnitude, has its roots in the deformed mean field

A new strategy of fitting the coupling constants of the nuclear energy density functional is proposed, which shifts attention from ground-state bulk to single-particle properties. The latter are analyzed in terms of the bare single-particle energies and mass, shape, and spin core-polarization effects. Fit of the isoscalar spin-orbit and both isoscalar and isovector tensor coupling constants directly to the f 5/2 − f 7/2 spin-orbit splittings in 40 Ca, 56 Ni, and 48 Ca is proposed as a practical realization of this new programme. It is shown that this fit requires drastic changes in the isoscalar spin-orbit strength and the tensor coupling constants as compared to the commonly accepted values but it considerably and systematically improves basic single-particle properties including spin-orbit splittings and magic-gap energies. Impact of these changes on nuclear binding energies is also discussed.

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Surfaces of experimental masses of even-even and odd-odd nuclei exhibit a sharp slope discontinuity at N =Z. This cusp (Wigner energy), reflecting an additional binding in nuclei with neutrons and protons occupying the same shell model orbitals, is usually attributed to neutron-proton pairing correlations. A method is developed to extract the Wigner term from experimental data. Both empirical arguments and shell-model calculations suggest that the Wigner term can be traced back to the isospin T =0 part of nuclear interaction. Our calculations reveal the rather complex mechanism responsible for the nuclear binding around the N =Z line. In particular, we find that the Wigner term cannot be solely explained in terms of correlations between the neutron-proton J=1, T =0 (deuteron-like) pairs.PACS numbers: 21.10. Dr, 21.10.Hw, 21.60.Cs Typeset using REVT E X * Science Alliance summer student from University of Missouri -Rolla. 1One of the most interesting avenues in nuclear structure research is the study of nuclear properties near the particle drip lines. Theoretically, due to their unusual shell structure and weak binding, nuclei with highest and lowest N/Z ratios represent a unique challenge for the many-body problem.This Letter concerns the proton-rich border of nuclear binding (N/Z≈1). The proton drip line is relatively well known experimentally, and the discovery of 100 Sn indicates the potential that radioactive nuclear beams will access all bound N=Z nuclei and their bound and unbound neighbors. This opens up the possibility of nuclear structure studies of mediummass and heavy systems with N≈Z.There are many aspects of nuclear structure which make physics near the N=Z line very interesting: proton and diproton emission, isospin mixing, superallowed beta decay, and nuclear constraints for the astrophysical rp-process -all these are timely experimental and theoretical themes. A unique aspect of nuclei with N=Z is that neutrons and protons occupy the same shell-model orbitals. Consequently, the large spatial overlaps between neutron and proton single-particle wave functions are expected to enhance neutron-proton (np) correlations, especially the np pairing.At present, it is not clear what the specific experimental fingerprints of the np pairing are, whether the np correlations are strong enough to form a static condensate, and what their main building blocks are. Most of our knowledge about nuclear pairing comes from nuclei with a sizable neutron excess where the isospin T =1 neutron-neutron (nn) and protonproton (pp) pairing dominate. Now, for the first time, there is an experimental opportunity to explore nuclear systems in the vicinity of the N=Z line which have many valence np pairs; that is, to probe the interplay between the like-particle and neutron-proton (T =0,1, T z =0) pairing channels.This novel situation calls for the generalization of established theoretical models of nuclear pairing. In spite of several early attempts to extend the independent quasi-particle formalism to incorporate the ...

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Odd-even staggering of binding energies is studied in finite fermion systems with pairing correlations. We discuss contributions of the pairing and mean-field to the staggering, and we construct the binding-energy filters which measure the magnitude of pairing correlations and the effective singleparticle spacings in a given system The analysis is based on studying several exactly-solvable manybody Hamiltonians as well as on the analytical formulas that can be applied in the weak and strong pairing limits. PACS number(s): 21.10. Dr, 21.10.Pc, 21.60.Jz, 71.15.Mb

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