Investigation of correlations in light neutron-rich nuclei

Correlations play a crucial role in understanding the structure of light nuclei at and beyond the neutron drip-line. In this context, the two-neutron halo nucleus He6 and the unbound systems H5, He7,9 and Li10 have been studied via measurements of the breakup of beams of He6 and Be11,12. The CHARISSA and DEMON detector arrays were employed. The interpretation was facilitated by a simulation code (SILLAGE) which provided for the setup. In the case of He7, the existence of an excited state with Er ∼ 1 MeV and gamma ∼ 0.75 MeV was confirmed. The virtual character of the s-wave ground state of Li10 was also confirmed and a scattering length of as ∼ -16 fm deduced. The results obtained for He9 suggest that a virtual s-wave state may exist just above threshold. The study of the three-body breakup of He6 found that the decay of the first 2+ state is essentially direct, while the decay of the remaining continuum strength is sequential - passage via He5. Using the technique of intensity interferometry an rms separation between the halo neutrons of 7.7 +- 0.8 fm was derived. This result was confirmed by a complementary method utilizing Dalitz plots. In the case of H5, the invariant mass spectrum was found to exhibit a broad (gamma ∼ 2 MeV) structure some 1.8 MeV above threshold. Comparison with recent three-body model calculations suggest that this corresponds to the predicted 1/2+ ground state. An rms valence neutron separation of some 5.5 fm was estimated. A search was also carried out for the 4n system using the Be12* (2 alpha + Xn decay channel). No signal was observed beyond that expected on the basis of the known background processes. (author)