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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Atomic nuclei are finite quantum systems composed of two distinct types of fermion--protons and neutrons. In a manner similar to that of electrons orbiting in an atom, protons and neutrons in a nucleus form shell structures. In the case of stable, naturally occurring nuclei, large energy gaps exist between shells that fill completely when the proton or neutron number is equal to 2, 8, 20, 28, 50, 82 or 126 (ref. 1). Away from stability, however, these so-called 'magic numbers' are known to evolve in systems with a large imbalance of protons and neutrons. Although some of the standard shell closures can disappear, new ones are known to appear. Studies aiming to identify and understand such behaviour are of major importance in the field of experimental and theoretical nuclear physics. Here we report a spectroscopic study of the neutron-rich nucleus (54)Ca (a bound system composed of 20 protons and 34 neutrons) using proton knockout reactions involving fast radioactive projectiles. The results highlight the doubly magic nature of (54)Ca and provide direct experimental evidence for the onset of a sizable subshell closure at neutron number 34 in isotopes far from stability.

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.