Strong focusing by final state interaction of the halo neutrons pre-emitted in the fusion of 11 Li with Si targets

The pre-emission of neutron pairs in the fusion of 11 Li halo nuclei with Si targets, was first mentioned in 1997 by Petrascu et al. However the statistics of n-n coincidences was rather low due to the fact that the detection system was not optimal for such measurements. The results of a recent experiment performed by the aid of a new array detector, revealed a notable n-n coincidence effect. It was found that the n-n coincidences are concentrated in a very narrow forward cone of 9 msrad. The first correlation results for the coincidences between adjacent detectors (denoted here as 'first order coincidences') pointing out to a large correlation strength were presented at the EXON Symp. in 2001. Here presented are the results of recent correlation investigation performed over a large range of the relative momentum q between 2.25 and 18.25 MeV/c. In this investigation, besides first order coincidences also second to fourth order coincidences were observed. In constructing the denominator of the correlation function, samples of single detected neutrons were used. We found that the denominator built by using single neutrons from the central array detector zone, shows large fluctuations. We interpret these fluctuations, as an effect of the residual correlation of single neutrons. On the contrary, the denominator built by using single neutrons from the outer array circle is smooth, due to the fact that the number of n-n coincidences are much less in that zone. The C(q) is obtained. The first 4 points represent the correlation function for the first order coincidences. The much lower values of the points numbers 2-4 are due to cross-talk rejection. The point number 1 not affected by c-t, suggests a correlation strength near 11. The rest of the points correspond to coincidences of the second to fourth orders. Two curves were calculated with the method by Lednicky and Lyuboshits corresponding to r0 values equal respectively to 3 fm and to 4.5 fm. From these values one can calculate drms separation of the two neutrons in the moment of pre-emission: drms = 60.5r0. It follows that one curve points out to a separation drms = 7.5 fm and the other one to a separation drms = 11 fm. Consequently, the present correlation data suggest a separation of the two neutrons in the moment of pre-emission between 7.5 and 11 fm. In a work by Marques et al where the breakup of 11 Li in Coulomb field of the target was investigated, a reduction of the r0 inflation was tried by means of an iterative calculation, considering that this inflation is due to the residual correlation of single neutrons. In our case this effect is automatically compensated by the use of a smooth denominator as mentioned above. Therefore there is no reason to apply to the present data an iterative calculation. One may think to a difference between the Coulomb breakup and the pre-emission process. In the last case the separation of the 2 halo neutrons takes place very close to the target and a damping of large qs. may occur. A testing for this would be an experiment on C-target, where the screening of the target is reduced by a factor 2. (authors)