βdecay ofSi38,40(Tz=+5,+6) to low-lying core excited states in odd-oddP38,40isotopes

Abstract

Low-lying excited states in $^{38,40}\mathrm{P}$ have been identified in the $\ensuremath{\beta}$ decay of ${T}_{z}=+5,\phantom{\rule{0.16em}{0ex}}+6, ^{38,40}\mathrm{Si}$. 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 states with 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 $^{40}\mathrm{P}$ 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 $N\ensuremath{\approx}28$. The calculated GT matrix elements for the $\ensuremath{\beta}$ decay highlight the dominance of the decay of the core neutrons rather than the valence neutrons.