Fusion of halo nuclei with light nuclear targets

A specific mechanism for the 11 Li fusion with light nuclear target, has been emphasised. Due to the very large dimension of 11 Li one may expect that the valence neutrons will not be absorbed in the fusion process together with 9 Li core, but will be emitted in the early stage of the reaction, with practically non altered beam velocity energy. The fusion of 9 Li and 11 Li with Si target has been experimentally investigated in a global approach. The neutrons resulting from Si (9,11 Li, fus) were measured by the aid of the RIKEN 'long neutron wall'. While for the energy range 0.4 - 5 MeV the neutrons show the same behaviour as concerns their number and position spectra, for the range 5 - 22 MeV there is a dramatic difference between the two kind of beams, for both mentioned parameters. Thus in the case of 11 Li an important fraction of the neutrons are concentrated in a very narrow region, according to the results of position measurement by the central 7, 8, 9, detectors of the wall. The radius of the spot on the 'wall' for this neutrons is of the order of 10 cm. For the rest of detectors, the position spectra appear to be broader but essentially narrower than in the case of 9 Li beam. From a correlation analysis it follows as a preliminary conclusion, that in central spot, the neutrons are emitted in pairs, presumably after a smooth breakup from the colliding cores. Outside this zone they are emitted as single neutrons or in pairs with a much larger correlation lengths, following a 1 step interaction with the colliding cores. This observation of 'focused' neutron confirms the recent prediction of the mechanism consisting in the pre-emission of one or two halo neutrons in the early stage of the fusion process. From the width of the central neutron spot, an estimate of the internal momentum pi of the halo neutrons could be obtained. This value of pi turns out to be 11.4±0.6 MeV/c. (author)