Low- Z shore of the "island of inversion" and the reduced neutron magicity toward O 28

Abstract

The two odd-even fluorine isotopes F27,29 were studied via in-beam γ-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. A secondary beam of Ne30 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 F27 and at 1080(18) keV for F29. Both were assigned a 1/21+ spin and parity. The low transition energy for F29 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 F29, 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 1/21+ level energy in F29 and suggest a persistent reduced neutron gap for O28.