Breakup reactions of the one-neutron halo nucleus 11 Be on lead and carbon targets at about 70 MeV/nucleon have been investigated at RIKEN by measuring the momentum vectors of the incident 11 Be, outgoing 10 Be, and neutron in coincidence. The relative energy spectra as well as the angular distributions of the 10 Be+n center of mass system (inelastic angular distributions) have been extracted both for Pb and C targets. For the breakup of 11 Be on Pb, the selection of forward scattering angles, corresponding to large impact parameters, is found to be effective to extract almost purely the first-order E1 Coulomb breakup component, and to exclude the nuclear contribution and higher-order Coulomb breakup components. This angle-selected energy spectrum is thus used to deduce the spectroscopic factor for the 10 Be(0 + ) ⊗ ν2s 1/2 configuration in 11 Be which is found to be 0.72 ± 0.04 with a B(E1) strength up to Ex = 4 MeV of 1.05 ± 0.06 e 2 fm 2 . The energy weighted E1 strength up to Ex = 4 MeV explains 70 ± 10 % of the cluster sum rule, consistent with the obtained spectroscopic factor. The non-energy weighted sum rule within the same energy range is used to extract the root mean square distance of the halo neutron to be 5.77( 16) fm, consistent with previously known values. In the breakup with the carbon target, we have observed the excitations to the known unbound states in 11 Be at Ex = 1.78 MeV and Ex = 3.41 MeV. Angular distributions for these states show the diffraction pattern characteristic of L=2 transitions, resulting in J π =(3/2,5/2) + assignment for these states. We finally find that even for the C target the E1 Coulomb direct breakup mechanism becomes dominant at very forward angles.

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An exclusive measurement has been made of the Coulomb dissociation of the two-neutron halo nucleus 11Li at 70 MeV/nucleon at RIKEN. Strong low-energy (soft) E1 excitation is observed, peaked at about Ex = 0.6 MeV with B(E1) = 1.42(18) e2fm2 for Erel < or = 3 MeV, which was largely missed in previous measurements. This excitation represents the strongest E1 transition ever observed at such low excitation energies. The spectrum is reproduced well by a three-body model with a strong two-neutron correlation, which is further supported by the E1 non-energy-weighted cluster sum rule.

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The cross sections for single-neutron removal from the very neutron-rich nucleus 31Ne on Pb and C targets have been measured at 230 MeV/nucleon using the RIBF facility at RIKEN. The deduced large Coulomb breakup cross section of 540(70) mb is indicative of a soft E1 excitation. Comparison with direct-breakup model calculations suggests that the valence neutron of 31Ne occupies a low-l orbital (most probably 2p(3/2)) with a small separation energy (S(n) approximately < 0.8 MeV), instead of being predominantly in the 1f(7/2) orbital as expected from the conventional shell ordering. These findings suggest that 31Ne is the heaviest halo system known.

Reaction cross sections (sigma(R)) for 19C, 20C and the drip-line nucleus 22C on a liquid hydrogen target have been measured at around 40A MeV by a transmission method. A large enhancement of sigma(R) for 22C compared to those for neighboring C isotopes was observed. Using a finite-range Glauber calculation under an optical-limit approximation the rms matter radius of 22C was deduced to be 5.4+/-0.9 fm. It does not follow the systematic behavior of radii in carbon isotopes with N < or = 14, suggesting a neutron halo. It was found by an analysis based on a few-body Glauber calculation that the two-valence neutrons in 22C preferentially occupy the 1s(1/2) orbital.

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