By using the 1H(6Li,6Be)n charge-exchange reaction, continuum states in 6Be were populated up to E_t=16 MeV, E_t being the 6Be energy above its three-body decay threshold. In kinematically complete measurements performed by detecting alpha+p+p coincidences, an E_t spectrum of high statistics was obtained, containing approximately ~5x10^6 events. The spectrum provides detailed correlation information about the well-known 0^+ ground state of 6Be at E_t=1.37 MeV and its 2^+ state at E_t=3.05 MeV. Moreover, a broad structure extending from 4 to 16 MeV was observed. It contains negative parity states populated by Delta L=1 angular momentum transfer without other significant contributions. This structure can be interpreted as a novel phenomenon, i.e. the isovector soft dipole mode associated with the 6Li ground state. The population of this mode in the charge-exchange reaction is a dominant phenomenon for this reaction, being responsible for about 60% of the cross section obtained in the measured energy range.Comment: 8 pages, 7 figure

Expérience GANIL/SPIRAL/MUST2/E525SThe low-lying spectroscopy of 6He was investigated via the 2-neutron transfer reaction p(8He, t) with the 8He beam delivered by the SPIRAL facility at 15.4 A MeV. The light charged particles produced by the direct reactions were measured using the MUST2 Si-strip telescope array. Above the known 2+ state, two new resonances were observed: at E∗ = 2.6±0.3 MeV (width Γ = 1.6±0.4 MeV) and at 5.3±0.3 MeV with Γ = 2 ± 1 MeV. Through the analysis of the angular distributions, they correspond to a 2+ state and to an L = 1 state, respectively. These new states, challenging the nuclear theories, could be used as benchmarks for checking the microscopic inputs of the newly improved structure models, and should trigger development of models including the treatments of both core excitation and continuum coupling effects

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Resonances in the unbound nucleus 11 N have been studied, using the resonance scattering reaction 10 Cϩp. The data give evidence for three states above the 10 Cϩp threshold with energies 1.30, 2.04, and 3.72 MeV. These states can be interpreted, in a potential-model analysis, as the ground state and the first two excited states with spin-parity 1/2 ϩ , 1/2 Ϫ , and 5/2 ϩ arising from the shell-model orbitals 1s 1/2 , 0p 1/2 , and 0d 5/2. A narrow state superposed on a broad structure found at higher energy could be interpreted as the mirror state of the 3/2 Ϫ in 11 Be shifted down in energy. This shift would suggest a large radius of the potential. ͓S0556-2813͑96͒50110-X͔

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Experimental search for (5)H using a secondary beam of (6)He has been performed. The transfer reaction (1)H((6)He,(2)He)(5)H was studied by detecting two protons emitted from the decay of (2)He. A peak consistent with a (5)H resonance at 1.7+/-0.3 MeV above the n+n+t threshold was observed, with a width of 1.9+/-0.4 MeV. The angular distribution of the (1)H((6)He,(2)He)(5)H reaction was measured as well as the energy correlation of the two protons.

The spectroscopy of the unstable 8 He and unbound 7 He nuclei is investigated via the p( 8 He,d) transfer reaction with a 15.7A.MeV 8 He beam from the SPIRAL facility. The emitted deuterons were detected by the telescope array MUST. The results are analyzed within the coupled-channels Born approximation framework, and a spectroscopic factor C 2 S = 4.4±1.3 for neutron pickup to the 7 Heg.s is deduced. This value is consistent with a full p3/2 subshell for 8 He. Tentative evidence for the first excited state of 7 He is found at E* = 0.9±0.5 MeV (width Γ = 1.0 ± 0.9 MeV). The second one is observed at a position compatible with previous measurements, E*=2.9±0.1 MeV. Both are in agreement with previous separate measurements. The reproduction of the first excited state below 1 MeV would be a challenge for the most sophisticated nuclear theories.

Intense and purified radioactive beam of post-accelerated 14 O was used to study the low-lying states in the unbound 15 F nucleus. Exploiting resonant elastic scattering in inverse kinematics with a thick target, the second excited state, a resonance at E R =4.757(6)(10) MeV with a width of Γ=36(5)(14) keV was measured for the first time with high precision. The structure of this narrow above-barrier state in a nucleus located two neutrons beyond the proton drip line was investigated using the Gamow Shell Model in the coupled channel representation with a 12 C core and three valence protons. It is found that it is an almost pure wave function of two quasi-bound protons in the 2s 1/2 shell.