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The low-energy behavior of the strength function for the $1^-$ soft dipole excitation in $^{6}$He is studied theoretically. Use of very large basis sizes and well-grounded extrapolation procedures allows to move to energies as small as 1 keV, at which the low-energy asymptotic behavior of the E1 strength function seems to be achieved. It is found that the low-energy behavior of the strength function is well described in the effective three-body "dynamical dineutron model". The astrophysical rate for the $\alpha$+$n$+$n \rightarrow ^6$He+$\gamma$ is calculated. Comparison with the previous calculations is performed., Comment: 6 pages, 9 figures

Properties of positronium (Ps), its negative ion, and the recently discovered positronium molecule are briefly reviewed. Emphasis is on the recent progress and on quantities of current experimental interest, including the Ps atomic and molecular spectrum, and the ion lifetime.

Cross-sections for dipole excitation of low energy (0.1–0.3 eV) electron–hole pairs at the (111) and (100) surfaces of aluminium have been measured with use of electron energy loss spectroscopy. We observe a substantial crystal-face effect at low incident electron energies (

Highlights of RIKEN radioactive beam experiments are reviewed. In particular the phenomena of giant soft dipole excitation of halo nuclei are closely discussed.

Coulomb excitations using radioactive beams are discussed. Applications of such reactions are described including soft dipole excitation of neutron halo nuclei, collective E2 excitation of 32 Mg and astrophysically interesting 8 B breakup.

A general method for the calculation of second order effects in the Coulomb dissociation is presented in the small ξ-limit avoiding the introduction of intermediate states. The amplitudes for a dipole excitation are analytically calculated in a simple model for the projectile with a zero-range force. The model is applied to the excitation of 11 Li at projectile energies of 28 and 800 MeV nucleon . The angular and energy distribution of the fragments and post-acceleration effects are examined in first and second order perturbation theory as well as the sudden approximation.

Cross sections for the electromagnetic dissociation of 197 Au targets by 1.7 A GeV 20 Ne and 1 A GeV 88 Kr, 197 Au and 209 Bi beams have been measured by an activation method. Weizsacker-Williams calculations show that the experimental 1n cross sections can be understood by first order excitation of the giant dipole and the isoscalar giant quadrupole resonance. In contrast, the 3n cross sections show a much stronger Z proj - dependence, which can only be explained by a dominant contribution from two-phonon giant dipole excitation.

A brief description of exotic states of nuclei involving radioactive beam technology is presented with the emphasis being on the nucleus 11 Li. The status of experiment and theory associated with soft dipole excitation are discussed and suggestions for theoretical improvements are given.

Inelastic excitations have been studied in 48 MeV α-scattering from 208 Pb. A concentration of isoscalar L = 1 strength is observed at E x ≈ 5.5 Mev which is interpreted as a dipole excitation mode representing a dipole oscillation of the nuclear diffuseness. A consistent description of inelastic excitations and electromagnetic widths is obtained.

Electron energy loss measurements and concommitant RPAE calculations are reported of the valence-shell dipole excitation spectrum of molecular fluorine. The measured spectrum is dominated by a series of strong features in the 12–16 eV interval which are in accord with X 1 Σ g + → 1 Σ u + bands assigned in a previously reported high-resolution optical study. These features are attributed on basis of the present RPAE calculations to configuration mixing between 1π g → n pπ u Rydberg and 3σ g →3σ u intravalence excitations. A depleted X→V σ charge-transfer excitation is correspondingly observed at ≈17 eV, in good accord with the calculated values. The appearance of the σ→σ* transition in F 2 below the 3σ g −1 threshold is in marked contrast to the situation in other light diatomic molecules, in which cases σ→σ* transitions appear as intravalence shape resonances in photoionization continua. Assignments are also provided of weak, irregularly spaced X 1 Σ g + → 1 Π u excitations the origins of which are attributed to configuration mixing between 1π g → n pσ u and 1π u → n sσ g Rydberg series.

The parent analog of the giant dipole resonance in 28 Si is studied by means of the (n, p) reaction. The continuum contribution to the giant dipole region was estimated by a phenomenological parameterization. The observed cross section exhausts 31% of the SJ sum rule; 88% of the GT sum rule or 67% of the Myers-Swiatecki prediction. A comparison is made to the giant resonance region observed in the 28 Si(p, p') reaction which was recently used, via comparison to 28 Si(α, α'), to infer that the GDR in 28 Si is not excited or at least not seen in (p, p').

Bremsstrahlung-weighted cross-section (bwcs) measurements may be a decisive step to obtain information about the difference between neutron and proton radii. A model-independent relation is discussed between the total bwcs, the relative intensity of the bwcs of the upper ( T > ) fragment of the dipole excitation and the neutron and proton radii.

It is pointed out that, in addition to direct capture, semi-direct capture through the dipole states of nuclei is important in the region of energies near that of the dipole excitation. A schematic calculation shows that the semi-direct capture can enhance the cross sections by a factor of ∼ 10 in the region of the giant resonance, and that its effects persist to relatively high energies.