Narrow Results

Shell model calculations for Ni, Cu and Zn isotopes by modifying fpg interaction due to Sorlin et al. [Phys. Rev. Lett. 88, 092501 (2002)] have been reported. In the present work, 28 two-body matrix elements of the earlier interaction have been modified. Present interaction is able to explain new experimental results for this region.

The $\gamma$-bands are analyzed through the variation of the energy of the $2_{\gamma }^{+}$ excitation and the energies of excited level sequence of $\gamma$-bands with respect to various parameters. The shape phase transition observed at N = 88–90 is reviewed through its influence on the energies of $\gamma$-band. The correlation of the $2_{\gamma }^{+}$ excitation energies with the collective shape signature observable $R_{4/2}$ indicates a connection with the nuclear equilibrium structure. The study of excited level sequence in the $\gamma$-band with respect to the ground band signifies that the two bands differ in deformation.

Yrast levels of Ni, Cu and Zn isotopes for 40 ≤ N ≤ 50 have been described by state-of-the-art shell model calculations with three recently available interactions using ⁵⁶Ni as a core in the f_5/2pg_9/2 model space. The results are unsatisfactory viz. large E(2⁺) for very neutron-rich nuclei, small B(E2) values in comparison to experimental values. These results indicate an importance of inclusion of πf_7/2 and νd_5/2 orbitals in the model space to reproduce collectivity in this region.

The recently measured experimental data of ^79,81,83Se isotopes have been interpreted in terms of shell model (SM) calculations. The calculations have been performed in f_5/2pg_9/2 space with the recently derived interactions, namely JUN45 and jj44b. To study the importance of the proton excitations across the Z = 28 shell in this region, we have also performed calculation in fpg_9/2 valence space using an fpg effective interaction with ⁴⁸Ca core and imposing a truncation. Excitation energies, B(2) values, quadrupole moments and magnetic moments are compared with the experimental data available. Present study reveals the importance of proton excitations across the Z = 28 shell for predicting quadrupole and magnetic moments.

In this paper, we review recent developments that aim to achieve further understanding of the structure of atomic nuclei, by capitalizing on exact symmetries as well as approximate symmetries found to dominate low-lying nuclear states. The findings confirm the essential role played by the Sp(3, ℝ) symplectic symmetry to inform the interaction and the relevant model spaces in nuclear modeling. The significance of the Sp(3, ℝ) symmetry for a description of a quantum system of strongly interacting particles naturally emerges from the physical relevance of its generators, which directly relate to particle momentum and position coordinates, and represent important observables, such as, the many-particle kinetic energy, the monopole operator, the quadrupole moment and the angular momentum. We show that it is imperative that shell-model spaces be expanded well beyond the current limits to accommodate particle excitations that appear critical to enhanced collectivity in heavier systems and to highly-deformed spatial structures, exemplified by the second 0⁺ state in ¹²C (the challenging Hoyle state) and ⁸Be. While such states are presently inaccessible by large-scale no-core shell models, symmetry-based considerations are found to be essential.

We study the effects produced by interactions among neutrinos upon extra-galactic neutrino-fluxes. We have assumed a separable type of pair interactions and performed a transformation to a quasi-particle mean field followed by a Tamm–Damcoff diagonalization. In doing so, we have adopted techniques originated in the quantum many-body problem, and adapted them to this specific case. The solutions of the associated eigenvalue problem provide us with energies and amplitudes which are then used to construct the neutrino response functions at finite density and temperature. The formalism is applied to the description of neutrinos produced in a SN environment.

Invariant mass spectroscopy is a powerful tool to study low-lying excitation of the halo nuclei. We have applied the invariant mass spectroscopy to the E1 excitation of ¹¹Li and to the 2⁺ transition of ¹⁴Be. The former study used the Coulomb breakup of ¹¹Li on Pb at 70 MeV/nucleon, which revealed a strong E1 transition at very low excitation energies. Such a strong soft E1 excitation indicated dineutron-like spatial correlation in ¹¹Li. The latter study used inelastic scattering of ¹⁴Be on carbon and proton targets at about 70 MeV/nucleon. The first 2⁺ state was observed in energy spectra for both targets. The extracted deformation parameters were smaller than those of the neighboring ¹²Be nucleus, indicating the hindered collectivity at the drip-line nucleus ¹⁴Be.

Results of two experimental campaigns are presented investigating shell structure in neutron-rich nuclei near ⁵⁴Ca and ¹³²Sn, respectively. In the first experiment excited states in ⁵⁵Ti were investigated at the middle focus of the GSI FRS via one-neutron knock-out from ⁵⁶Ti. Longitudinal momentum distributions were measured inclusively and in coincidence with a newly discovered gamma-ray at 955 keV detected by the MINIBALL gamma-ray detector array. From the momentum distributions the νp_1/2 single-particle structure of the ground state was determined for the first time while the excited state at 955 keV is identified as the νp_3/2 single-particle state. Secondly, results from the Coulomb excitation of neutron-rich nuclei ^122,124,126Cd, ^{138,140,142,144}Xe are presented. These experiments were performed at the REX-ISOLDE accelerator at CERN also using the MINIBALL array. The obtained B(E2)-values follow the expected systematic behavior that correlates the energy of the first excited 2⁺ state with the B(E2)-values and also agree well with the results of theoretical predictions.

The question of the persistence of the N = 40 harmonic oscillator closure in neutron-rich nuclei as well as the sudden increase of collectivity when approaching N = 40 has recently interested both experimentalists and theorists. I have studied the development of collectivity through the pf-shell in iron and chromium isotopes within the interacting boson model (IBM-2). Comparison with available experimental data as well as recent theoretical calculations is made and the results are found to be in good agreement. The onset of collective behavior is seen from the systematics of the states as well as B(E2) values. Furthermore, the shape transition from a spherical vibrator to γ-soft rotor with nucleus ⁵⁸Cr standing at the critical point is shown clearly.