Gamma-ray spectroscopy of 23F with proton transfer reaction

Neutron-rich fluorine isotopes are interesting in their features. A recent discovery revealed that the stability of neutron-rich isotopes remarkably changes between oxygen and fluorine isotopes. The fluorine isotopes are extended up to 31F with N = 22 beyond the magic number of N 20 while the heaviest oxygen isotope is 24O with N = 16. This feature may indicate that a strong attraction works between a valence proton on the d5/2 shell and valence neutrons in fluorine isotopes. To ensure this feature, it is important to understand the proton shell structures in fluorine isotopes as a function of neutron number. One-proton transfer reactions such as (d,n) and (α,t) are effective tools for investigating proton shell structure. In these reactions, the (α,t) reaction in inverse kinematics is considered to be one of best ones for unstable nuclei, because its total cross section is largest at a beam energy of E = 20-40A MeV, which corresponds to an energy range of unstable beams produced by an intermediate energy fragmentation reaction. The present study aims at searching single proton particle states in 23F. Properties of excited states in 17F, 19F and 21F have been well known. However in the nucleus 23F, excitation energies of six excited states were only reported. Spin and parities of these states are not obvious experimentally. We have, hence, search the excited states in neutron-rich nucleus 23F via the proton transfer reaction onto an unstable nucleus, 4He(22O,23F*) at 35 MeV/nucleon. Identification of excited states was made by de-excitation γ-ray energies from 23F*. From the angular distributions, we deduce the angular momenta of excited states in 23F. Additionally we have measured inelastic scattering of 23F, 4He(23F,23F*), and neutron knockout reaction of 24F, 4He(24F,23F*), in order to identify clearly single particle states by comparison with yields of each states in three different reactions. (author)