The three-body breakup 6 He→ 4 Heϩnϩn is studied experimentally, using a secondary 6 He ion beam of 240 MeV/nucleon incident on carbon and lead targets. Integrated cross sections for one-and two-neutron knockout and differential cross sections d/dE* and d/d for inelastic nuclear or electromagnetic excitations into the 6 He continuum are presented. The E1-strength distribution is deduced from electromagnetic cross sections and is found to exhaust (10Ϯ2)% of the energy-weighted Thomas-Reiche-Kuhn sum rule or (40Ϯ8)% of the cluster sum rule for excitation energies below 5 MeV. Both the energy-weighted and non-energy-weighted dipole cluster sum rules are almost exhausted integrating the strength up to 10 MeV, a fact from which the root-mean-square distance between the ␣ core and the two valence neutrons of r ␣Ϫ2n ϭ(3.36Ϯ0.39) fm is derived. The known I ϭ2 ϩ ͑1.80 MeV͒ resonance in 6 He is observed in nuclear inelastic scattering; model-dependent values of the quadrupole deformation parameter ␦ 2 ϭ(1.7Ϯ0.3) fm or B(E2,0 ϩ →2 ϩ )ϭ(3.2Ϯ0.6)e 2 fm 4 are derived. No clear signature could be obtained for predicted higherlying 2 ϩ resonances, but low-lying continuum strength of multipolarity other than dipole, likely of monopole and quadrupole multipolarity, is indicated by the data. Two-body correlations in the decaying 4 Heϩnϩn system are investigated. The astrophysical relevance of the data with regard to the two-neutron capture process 4 He(2n,␥) 6 He is briefly discussed.

The ground and first excited states in 15 F were studied in resonant elastic scattering using the thick ͑CH 4 ͒ gas target method in inverse kinematics with a separated 14 O beam. An analysis of the excitation functions of the elastic scattering was carried out with the potential model. The quantum numbers 1 / 2 + (ground state) and 5/2 + (first excited state) were assigned to the lowest two states in 15 F. Also, the widths and the proton decay energies of the unbound levels were obtained. The analysis of the data indicates that a large diffuseness is needed in the Woods-Saxon potential in order to describe single-particle features in drip-line nuclei. Over the past decade it has become clear that drip-line nuclei demonstrate a number of phenomena which are not found in nuclei close to the line of stability. One such feature is the change in magic numbers, which are generated by a conventional Woods-Saxon potential with parameters fitted for stable nuclei [1][2][3]. One example is the intruder singleparticle 2s 1/2 state, which appears to be the ground state in 11 Be [4] and 11 N [5] instead of the 1p 1/2 state. As another example, it has been predicted [1-3] that the diffuseness of nuclear densities for intermediate mass nuclei increases dramatically when approaching the neutron drip line. These phenomena indicate that the parameters of the shell model potential can be unusual for nuclei at the borders of nuclear stability. Figure 1 shows how, for a light nucleus, the energies of the shell model levels in a Woods-Saxon potential depend on the ratio of the diffuseness to radius parameters. It can be seen in Fig. 1 that the 1p 1/2 and 2s 1/2 levels approach each other as the ratio increases. One way to explain the phenomena observed near the drip line is that the singleparticle potential changes its shape giving way to new shell structure. This effect can be tested by an analysis of the nucleon widths of the single-particle states in drip-line nuclei, which are mainly dependent upon the geometrical parameters of the well.15 F is a good system to check the considerations above. The lowest states in 15 F are unstable to proton decay and should have dominantly single-particle structure. The theoretical predictions for the single-particle spectroscopic factors are 0. O on hydrogen with the thick target inverse kinematics method [5,10,11]. An approach similar to Ref.[9] was used in the present experiment. However, there are important differences in the details of the two measurements. In the present work, a gas target, CH 4 , was used instead of a solid CH 2 target. The gas target results in a drastic decrease of the background (see below). Also, only the excitation function at 180°͑c.m.͒ in arbitrary units was measured in Ref. [9], while in the experiment reported here measurements were made at several angles. As a result, we have a better determination of the positions and the widths of the levels which allows us to make conclusions about the parameters of the interaction potential between 14 O and protons. The expe...

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

The 15 N(p, γ ) 16 O reaction provides a path from the CN cycle to the CNO bi-cycle and CNO tri-cycle. The measured astrophysical factor for this reaction is dominated by resonant capture through two strong 16 O reaction. The results indicate that the direct capture contribution was previously overestimated. We find the astrophysical factor to be S(0) = 36.0 ± 6.0 keV b, which is about a factor of 2 lower than the presently accepted value. We conclude that for every 2200 ± 300 cycles of the main CN cycle one CN catalyst is lost due to this reaction.