Convergence properties of the effective interaction

The convergence properties of two perturbative schemes to sum the so-called folded diagrams are critically reviewed, with an emphasis on the intruder state problem. The methods we study are the approaches of Kuo and co-workers and Lee and Suzuki. The suitability of the two schemes for shell-model calculations are discussed.Comment: 10 pages in revtex ver. 3.0. 3 figs can be obtained upon request. Univerisity of Oslo report UiO/PHYS/93-2

Suppression of core polarization in halo nuclei

We present a microscopic study of halo nuclei, starting from the Paris and Bonn potentials and employing a two-frequency shell model approach. It is found that the core-polarization effect is dramatically suppressed in such nuclei. Consequently the effective interaction for halo nucleons is almost entirely given by the bare G-matrix alone, which presently can be evaluated with a high degree of accuracy. The experimental pairing energies between the two halo neutrons in $^6$He and $^{11}$Li nuclei are satisfactorily reproduced by our calculation. It is suggested that the fundamental nucleon-nucleon interaction can be probed in a clearer and more direct way in halo nuclei than in ordinary nuclei.Comment: 11 pages, RevTex, 2 postscript figures; major revisions, matches version to appear in Phys. Rev. Letter

Low momentum nucleon-nucleon potential and shell model effective interactions

A low momentum nucleon-nucleon (NN) potential V-low-k is derived from meson exhange potentials by integrating out the model dependent high momentum modes of V_NN. The smooth and approximately unique V-low-k is used as input for shell model calculations instead of the usual Brueckner G matrix. Such an approach eliminates the nuclear mass dependence of the input interaction one finds in the G matrix approach, allowing the same input interaction to be used in different nuclear regions. Shell model calculations of 18O, 134Te and 135I using the same input V-low-k have been performed. For cut-off momentum Lambda in the vicinity of 2 fm-1, our calculated low-lying spectra for these nuclei are in good agreement with experiments, and are weakly dependent on Lambda.Comment: 5 pages, 5 figure

Realistic Shell-Model Calculations for Proton-Rich N=50 Isotones

The structure of the N=50 isotones 98Cd, 97Ag, and 96Pd is studied in terms of shell model employing a realistic effective interaction derived from the Bonn-A nucleon-nucleon potential. The single-hole energies are fixed by resorting to an analysis of the low-energy spectra of the isotones with A>= 91. Comparison shows that our results are in very satisfactory agreement with the available experimental data. This supports confidence in the predictions of our calculationsComment: 8 pages, 3 figures, to be published on Journal of Physics

Novel Methods for Determining Effective Interactions for the Nuclear Shell Model

The Contractor Renormalization (CORE) method is applied in combination with modern effective-theory techniques to the nuclear many-body problem. A one-dimensional--yet ``realistic''--nucleon-nucleon potential is introduced to test these novel ideas. It is found that the magnitude of ``model-space'' (CORE) corrections diminishes considerably when an effective potential that eliminates the hard-momentum components of the potential is first introduced. As a result, accurate predictions for the ground-state energy of the there-body system are made with relatively little computational effort when both techniques are used in a complementary fashion.Comment: 14 pages, 5 figures and 2 tabl

Renormalization of the weak hadronic current in the nuclear medium

The renormalization of the weak charge-changing hadronic current as a function of the reaction energy release is studied at the nucleonic level. We have calculated the average quenching factors for each type of current (vector, axial vector and induced pseudoscalar). The obtained quenching in the axial vector part is, at zero momentum transfer, 19% for the sd shell and 23% in the fp shell. We have extended the calculations also to heavier systems such as $^{56}$Ni and $^{100}$Sn, where we obtain stronger quenchings, 44% and 59%, respectively. Gamow--Teller type transitions are discussed, along with the higher order matrix elements. The quenching factors are constant up to roughly 60 MeV momentum transfer. Therefore the use of energy-independent quenching factors in beta decay is justified. We also found that going beyond the zeroth and first order operators (in inverse nucleon mass) does not give any substantial contribution. The extracted renormalization to the ratio $C_P/C_A$ at q=100 MeV is -3.5%, -7.1$%, -28.6%, and +8.7% for mass 16, 40, 56, and 100, respectively.Comment: 28 pages, 6 figure

Bonn Potential and Shell-Model Calculations for 206,205,204Pb

The structure of the nuclei 206,205,204Pb is studied interms of shell model employing a realistic effective interaction derived from the Bonn A nucleon-nucleon potential. The energy spectra, binding energies and electromagnetic properties are calculated and compared with experiment. A very good overall agreement is obtained. This evidences the reliability of our realistic effective interaction and encourages use of modern realistic potentials in shell-model calculations for heavy-mass nuclei.Comment: 4 pages, 4 figures, submitted to Physical Review