β decay of Si 38,40 (Tz=+5, +6) to low-lying core excited states in odd-odd P 38,40 isotopes

Low-lying excited states in P-38,P-40 have been identified in the beta decay of T-z = +5,+6, Si-38,Si-40. Based on the allowed nature of the Gamow-Teller (GT) decay observed, these states are assigned spin and parity of 1(+) and are core-excited 1p1h intruder stateswith a parity opposite to the ground state. The occurrence of intruder states at low energies highlights the importance of pairing and quadrupole correlation energies in lowering the intruder states despite the N = 20 shell gap. Configuration interaction shell model calculations with the state-of-art SDPF-MU effective interaction were performed to understand the structure of these 1p1h states in the even-A phosphorus isotopes. States in P-40 with N = 25 were found to have very complex configurations involving all the fp orbitals leading to deformed states as seen in neutron-rich nuclei with N approximate to 28. The calculated GT matrix elements for the beta decay highlight the dominance of the decay of the core neutrons rather than the valence neutrons.