The use of radioactive nuclei as projectiles in nuclear reaction studies at energies ranging typically from 30 to I000 MeV/nucleon has yielded interesting results in many areas of nuclear physics, see e.g. [1][2][3][4]. In the following, I shall restrict my comments almost exclusively to single-particle and giant-resonance phenomena in the lightest nuclei, a subject that has been discussed by a number of speakers at this conference. There are two main points that I would like to make. The first is that neutron and proton halo states offer examples of an extreme singleparticle behavior, and the second is that their reactions in many cases tend to be dominated by the special structure of the initial state, Theoretical calculations of the isovector response of neutron-rich systems including the hypothetical drip-line nucleus 280 were presented by Sagawa and also by Reinhard, Co16, and Lanza. The main feature is the appearance of dipole strength at low energies which appears to be largely non-collective. An experimental search for high-energy gamma rays representing the 11Be giant dipole resonance was presented by Beene. He and also Austin pointed out that the sequence of the stable oxygen isotopes 16'17'180 already show an increase of E1 strength below the giant resonance. During the conference, new data for the oxygen isotopes with masses 17-20 were presented by Aumann together with results for the two-neutron halo nucleus SHe.
There is already much experimental evidence on the one-and two-neutron halos of l~Be and ~Li which have one-and two-neutron separation energies of 0.5 and 0.3 MeV, respectively. The data seem to suggest that we are faced with a more extreme situation in which the halo neutron(s) are decoupled almost completely from the giant resonance of the core. This could mean that there is less to be learned from studies of giant resonances of halo systems than is often assumed.
We consider first the status for ~tBe. The first identification of its neutron halo by Millener et al. [5] was based on the surprisingly short lifetime of the Β½ excited state, which decays to the ground state, a Β½* intruder state. This transition probability has been re-determined in intermediate-energy Coulomb excitation experiments, see [6] and references therein, which confirm that the reduced transition probability B(E1) indeed is as large as 0.1 e2fm 2. By normal standards this is a very large value for a transition between low-lying bound states. Still, it is more than an order of magnitude smaller than that of the E1 transitions of low energy leading from the ground state to the lΒ°Be+n continuum. This absolute differential cross section has also been measured. It can be accounted for essentially quantitatively by a model [7] in which both