Low-Z shore of the 'island of inversion' and the reduced neutron magicity toward ~(28)O

摘要

The two odd-even fluorine isotopes ~(27,29)F were studied via in-beam γ -ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. A secondary beam of ~(30)Ne was used to induce one-proton and one-proton– two-neutron removal reactions on carbon and polyethylene targets at midtarget energies of 228 MeV/u. Excited states were observed at 915(12) keV for ~(27)F and at 1080(18) keV for ~(29)F. Both were assigned a 1/2_1~+ spin and parity. The low transition energy for ~(29)F largely disagrees with shell model predictions restricted to the sd model space. Calculations using effective interactions that include the neutron pf shell indicate that the N = 20 gap is quenched for ~(29)F, thus extending the “island of inversion” to isotopes with proton number Z = 9. Variations of the N = 20 gap further reveal a strong correlation to the 1/2_1~+ level energy in 29F and suggest a persistent reduced neutron gap for ~(28)O.