Neutron pair pre-emission in the fusion of 11 Li halo nuclei with light targets

The pre-emission of neutron pairs in the fusion of 11 Li halo nuclei with Si targets was first mentioned in 1997 in a previous work. However the statistics of n-n coincidences was rather low due to the fact that the detection system was not optimal for such measurements. Here, we report results of a recent experiment performed using of a new array detector, revealing a notable n- n coincidence effect. After subtraction of the cross-talk background, a number of about 300 n-n coincidences remained within the range of the narrow neutron group. The measurement with the array detector allowed a new determination with increased resolution of the transverse momentum distribution for the neutrons within the narrow group. The best fit of the experimental data was obtained by including a background term in the Gauss function. In this case the transverse momentum ptrans expressed by σ of the distribution is ptrans ≅ 6 ± 1.2 MeV/c. About 2 x larger a value of σ was obtained by forcing the fit without a background term. The n-n coincidence data processed by considering a background on which the narrow neutron group is superimposed allowed obtaining Pnn, the percentage for the neutron pair pre-emission, with the value Pnn = (72 ± 10)%. Without subtraction of this background Pnn turns out to be Pnn = (22 ± 3)%. From the coincidence results, also the differential cross-section d2σnn(θ1,θ2)/dΩ1dΩ2 , for 2 neutron pre-emission, was obtained. It was found that the d2σnn(θ1,θ2)/dΩ1dΩ2 values are close to the slope characterizing the neutron transverse momentum distribution. In contrast to the n-n coincidence data, the cross-talk data processed in same manner for getting the cross-sections, are totally in disagreement with the slope of the neutron transverse momentum distribution. It was also observed a change in the slope of n-n coincidence data, by interchanging det number 1 with det number 2,4,6,8 and 3,5,7,9. This may be an indication for n-n correlation. It is estimated that a 2-3 x larger n-n coincidence effect may be expected in the case of 11 Li + C detector-target fusion interaction. The present approach may be of interest also for the borromean 6 He nucleus investigation. (authors)