AB INITIO NO CORE SHELL MODEL STUDY FOR LIGHTER NUCLEI

Abstract

The aim of the present thesis is to perform ab initio no core shell model (NCSM) calculations for lighter nuclei and to perform shell model calculations using valence-space Hamiltonians derived with ab initio approaches like in-medium similarity renormalization group (IM-SRG) and coupled-cluster effective interaction (CCEI) for heavier sd shell nuclei. A systematic study of low-lying energy spectrum for 18−23O and 18−24F isotopes using NCSM is presented. We have used INOY potential, which is a two body interaction but also has the effect of three body forces by short range and nonlocal character. We have also performed calculations with N3LO, and N2LOopt interactions and corresponding results are compared with the experimental data and phenomenological interaction USDB. The largest model space we have reached for 18−21O and 18−19F is Nmax=6 and for other oxygen and fluorine isotopes is Nmax=4. We have also discussed the binding energy for O and F chain. The over binding in ground state (g.s.) energy in neutron-rich oxygen isotopes is observed in our largest model space calculations. We have calculated the energy spectra for 18−22N isotopes using NCSM. To calculate the energy spectrum we have used three different NN potentials: INOY, N3LO, and CDB2K. The calculations have been done at ~Ω=20 MeV, 14 MeV and 12 MeV i ii using INOY, N3LO and CDB2K potentials, respectively. The results of INOY interaction are in reasonable agreement with the available experimental data. We present ab initio shell model calculations for electric quadrupole moments and magnetic dipole moments of sd shell nuclei with interactions derived from ab initio approaches: IM-SRG and CCEI. The results are in a reasonable agreement with the available experimental data as well as with the results from the phenomenological USDB effective interaction. We have also calculated B(E2; 2+ 1 → 0+ 1 ) for Ne, Mg and Si isotopes using ab initio interactions. The Gamow-Teller transition strength B(GT) distributions of sd shell nuclei for thirteen transitions using ab initio effective interactions: IM-SRG and CCEI are reported. The aim of the present work is to test the predictive power of ab initio effective interactions for available experimental data of B(GT) distributions of sd shell nuclei. The ab initio results of the Gamow-Teller (GT+/GT−) strength distributions reproduce the experimental data with reasonable agreement. We also calculate the electron capture reaction rates for 23Na(e−, ν)23Ne and 25Mg(e−, ν)25Na using ab initio and USDB interactions. We have performed shell model calculations to describe the structure of 35,37,39S isotopes using SDPF-U and SDPFMW interactions. Protons and neutrons are restricted to the sd-shell for N < 20 and neutrons start to fill the pf-shell for N > 20. The natural parity states are described by only in-shell mixing and unnatural parity states with 1p-1h inter-shell neutron excitations. The calculated energy levels, electromagnetic properties, and spectroscopic factors are in good agreement with the recently available experimental data. Finally, summary and future prospects are reported.