The N = 16 spherical shell closure in 24O

The unbound excited states of the neutron drip-line isotope 24O have been investigated via the 24O(p,p')23O+n reaction in inverse kinematics at a beam energy of 62 MeV/nucleon. The decay energy spectrum of 24O* was reconstructed from the momenta of 23O and the neutron. The spin-parity of the first excited state, observed at Ex = 4.65 +/- 0.14 MeV, was determined to be Jpi = 2+ from the angular distribution of the cross section. Higher lying states were also observed. The quadrupole transition parameter beta2 of the 2+ state was deduced, for the first time, to be 0.15 +/- 0.04. The relatively high excitation energy and small beta2 value are indicative of the N = 16 shell closure in 24O.Comment: to be submitted to Physical Review Letter

Measurement of Unbound Excited States of (24)O

The unbound excited states of the most neutron-rich oxygen isotope, (24)O (Z = 8, N = 16), have been investigated using the (24)O(p,p')(24)O*-->(23)O+n reaction at RIKEN, where a 63 MeV/neuclon secondary beam of (24)O was produced in a Be production target by the projectile-like fragmentation of a 95 MeV/u (40)Ar primary beam. The first 2(+) excited state was observed in the invariant mass spectrum. This experiment; will give us a deeper understanding of the new magic number at N = 16 in the neutron-rich oxygen isotopes

Neutron occupancy of 0d5/2 orbital in 24O

The partial one-neutron removal cross section of 24O leading to the first excited state in 23O has been measured to be 65.7 ± 5.3 mb with a proton target in inverse kinematics at the beam energy of 62 MeV/nucleon. The decay energy spectrum of the neutron unbound state of 23O was reconstructed from the measured four momenta of 22O and the emitted neutron. A sharp peak was observed at Edecay = 48 ± 3 keV, confirming the previous measurements. The measured partial longitudinal momentum distribution of 23O* showed the d-wave knockout character, providing a support for the Jπ assignment of 5/2+ for this state. The spectroscopic factor for the neutrons in 0d5/2 orbital was deduced, for the first time, to be C2S(0d5/2) = 3.7 ± 0.3 (preliminary) by comparing with the result of Glauber model. The relatively large spectroscopic factor supports the N = 16 shell closure in the neutron drip-line nucleus 24O