No abstract

The unbound nucleus ^{26}O has been investigated using invariant-mass spectroscopy following one-proton removal reaction from a ^{27}F beam at 201  MeV/nucleon. The decay products, ^{24}O and two neutrons, were detected in coincidence using the newly commissioned SAMURAI spectrometer at the RIKEN Radioactive Isotope Beam Factory. The ^{26}O ground-state resonance was found to lie only 18±3(stat)±4(syst)  keV above threshold. In addition, a higher lying level, which is most likely the first 2^{+} state, was observed for the first time at 1.28_{-0.08}^{+0.11}  MeV above threshold. Comparison with theoretical predictions suggests that three-nucleon forces, pf-shell intruder configurations, and the continuum are key elements to understanding the structure of the most neutron-rich oxygen isotopes beyond the drip line.

The heaviest bound isotope of boron 19 B has been investigated using exclusive measurements of its Coulomb dissociation, into 17 B and two neutrons, in collisions with Pb at 220 MeV=nucleon. Enhanced electric dipole (E1) strength is observed just above the two-neutron decay threshold with an integrated E1 strength of BðE1Þ ¼ 1.64 AE 0.06ðstatÞ AE 0.12ðsysÞ e 2 fm 2 for relative energies below 6 MeV. This feature, known as a soft E1 excitation, provides the first firm evidence that 19 B has a prominent two-neutron halo. Three-body calculations that reproduce the energy spectrum indicate that the valence neutrons have a significant s-wave configuration and exhibit a dineutronlike correlation.

Very neutron-rich Z ∼ 60 isotopes produced by in-flight fission of 345 MeV/nucleon 238 U beam at the RI Beam Factory, RIKEN Nishina Center have been studied by delayed γ-ray spectroscopy. New isomers are discovered in the neutron-rich N = 100 isotones, 162 Sm, 163 Eu, and 164 Gd. Half-lives, γ-ray energies and relative intensities of these isomers were obtained. Level schemes were proposed for these nuclei and the first 2 + and 4 + states were assigned for the even-even nuclei. configurations with similar excitation energy. The results suggest that neutron-rich N = 100 nuclei are well deformed and the deformation gets larger as Z decreases to 62. The onset of K isomers with the same configuration at almost the same energy in N = 100 isotones indicates that the neutron single-particle structures of neutron-rich isotones down to Z = 62 do not change significantly from those of the Z = 70 stable nuclei. Systematics of the excitation energies of new isomers can be explained without the predicted N = 100 shell gap.

Detailed spectroscopy of the neutron-unbound nucleus 28 F has been performed for the first time following proton/neutron removal from 29 Ne/ 29 F beams at energies around 230 MeV/nucleon. The invariant-mass spectra were reconstructed for both the 27 F ( * ) + n and 26 F ( * ) + 2n coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the 28 F ground state, with Sn( 28 F) = −199(6) keV, while analysis of the 2n decay channel allowed a considerably improved Sn( 27 F) = 1620(60) keV to be deduced. Comparison with shell-model predictions and eikonal-model reaction calculations have allowed spin-parity assignments to be proposed for some of the lower-lying levels of 28 F. Importantly, in the case of the ground state, the reconstructed 27 F+n momentum distribution following neutron removal from 29 F indicates that PACS numbers: