Target screening effect on the pre-emission of neutrons from Li11 halo nuclei

For the first time to our knowledge the target screening effect on the pre-emission of halo neutrons from Li11 has been quantitatively analyzed. Our work has been performed in the 7.5-15 MeV neutron energy range. In this range the target nuclei are likely to behave as opaque, and therefore the sharp-cutoff calculations are most appropriate for the target screening determination. It has been observed that the ζ probability used in the sharp-cutoff calculations is an observable of the experiment, because it can be directly obtained from measured quantities, which are the number of single detected neutrons and the number of detected neutron pairs. The value of ζexp obtained this way in the case of a Si target appears to be close to the ζ value calculated for the Li11 halo radius RH=4.8 fm, independently determined in another experiment [Phys. Lett. B287, 307 (1992)]. This property also allows investigation of Borromean halo nuclei such as He6, Be14, and B17, for which RH was not yet measured. The calculated value within the present approach of the two-neutron pre-emission yield appears to be 3.5 times larger in the case of C12 than in the case of a Si target.