Valence neutrons focusing effect at the fusion of 13 MeV/n 11 Li

The fusion of 9 Li and 11 Li with Si targets has been experimentally investigated in a global approach. The neutrons resulting from Si (9,11 Li, fusion) were measured with 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 kinds 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, the radius of the spot on the 'wall' for these neutrons being of the order of 10 cm. This observation of 'focused' neutrons, confirms the recent prediction of a 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 π of the halo neutrons could be obtained. This value of π turns out to be 11.4 ± 0.6 MeV/c (author)