In the neutron-rich sd shell near the N = 20 'island of inversion', the evolution of the N = 20 shell gap is indicated by the energies of negative-parity states which primarily arise due to single neutron excitation to higher lying orbitals across the shell gap. These negative-parity states often have high spin (due to the participation of the 0f7/2 orbital) and are therefore preferentially populated using fusion-evaporation reactions. We have studied the intermediate energy levels of 32Si and 29Al using 12C(22Ne,2p) and 12C(22Ne,alpha p) reactions, identifying several negative-parity states in both nuclides.

We have studied 32Si and 29Al using 12C(22Ne, 2p) and 12C(22Ne, alpha p) fusion-evaporation reactions. In both cases, we observed significant population of high-spin structures distinct from the ground-state yrast bands. In 32Si, most of the high-energy states feed into a J pi = 5-nanosecond isomer. In 29Al, we identified a rotor-like negative-parity band with a J pi = 7/2-band-head. Doppler shift lifetime measurements were performed for all observed states. These results were compared to shell model calculations and interpreted in terms of proton and neutron cross-shell excitation.

We study the recently measured β decay of 70Kr into 70Br within the framework of the large-scale shell model. The enhancement in the Gamow-Teller (GT) transition strength in Br70 compared to the β decay of the lighter Ge62 was suggested as an indication for increased neutron-proton (np) pairing correlation. To explore the np correlations in nuclei, we systematically examined the β-decay properties of the even-even nuclei A=58,62,66, and 70 into N=Z odd-odd nuclei. By employing an interaction involving solely J=1,T=0 and J=0,T=1 pairing matrix elements, we observe that the pairing does not necessarily lead to an enhancement in the GT strength for the same coupling strength. But with the inclusion of the g9/2 orbital, the GT strength can be increased with increasing np pairing in connection with the enhanced contribution from the g9/2 orbital. We further compare those results with realistic calculations in the fp and f5/2pg9/2 model space to gauge the contribution from f7/2 and g9/2 orbitals in the GT strengths. With the JUN45 interaction, there is an increment for the yrast 1+ state for the decay of Kr70 as compared to the decay of Ge62 due to increased g9/2 contribution. Additionally, we probe the effect of np pairing on BGT by modifying the single-particle energies and the T=0 matrix elements of the interaction responsible for the decay transition strength. In calculations with realistic interaction, we find that the accumulated transition strength can increase with enhanced np pairing.

This paper addresses the challenges of solving the quantum many-body problem, particularly within nuclear physics, through the configuration interaction (CI) method. Large-scale shell model calculations often become computationally infeasible for systems with a large number of valence particles, requiring truncation techniques. We propose truncation methods for the nuclear shell model, in which angular momentum is conserved and rotational symmetry is restored. We introduce the monopole-interaction-based truncation and seniority truncation strategies, designed to reduce the dimension of the calculations. These truncations can be established by considering certain partitions based on their importance and selecting physically meaningful states. We examine these truncations for Sn, Xe, and Pb isotopes, demonstrating their effectiveness in overcoming computational limits. These truncations work well for systems with either a single type of valence nucleon or with both types. With these truncations, we are able to achieve good convergence for the energy at a very small portion of the total dimension.