Pre-emission of correlated neutrons in the fusion of 11 Li halo nuclei with Si targets

The Borromean two-neutron halo nuclei, much far from the stability, as 11 Li, 6 He, 14 Be, are three-body systems (core-n-n), in which the two-body subsystems are unbound. Such behaviour gives rise to the question of the n-n and core-n correlations in the exit channel, between the constituents of these three-body systems. The results of a recent experiment performed at RIKEN-RIPS radioactive beam separator, using a new array detector, built for the investigation of halo neutron pair pre-emission in the fusion of 11 Li (∼13 AMeV) with Si targets, in collaboration HH-NIPNE, Romania - RIKEN, Japan, are presented. The experimental arrangement with three main components is shown. It was found that the n-n coincidences for the neutron pair pre-emission are concentrated in a very narrow forward cone of 9 msrad, much narrower than that predicted (150 msrad) by the COSMA model. The position spectrum along the central horizontal line of detectors is shown. The neutron energy was selected between 6 and 16 MeV. The FWHM of this spectrum is ∼ 13 cm and corresponds to a solid angle of 9 msrad. Within this narrow peak and a large number of n-n coincidences were observed for 11 Li. The mechanism determining the appearance of this peak is assumed to be the final state n-n interaction of the two valence neutrons, which plays the role of a convergent lens. Therefore a neutron correlation based on event by event investigations has been undertaken. In order to obtain the n-n correlation function, C(q), given by the ratio between the true n-n coincidences and uncorrelated events, computing procedures and the programmes: prelnta, spectrum, comb with procos subroutine and fc, in FORTRAN language, have been worked out. The critical problem of the denominator construction was carefully analysed. By applying the single neutron product technique, followed by cross-talk rejection, we have built the denominator and we have obtained an experimental evidence for residual correlations of single detected halo neutrons. The correlation functions C(q) obtained with a resolution of 0.5 MeV/c, by using a sample of all 204 real coincidences is presented. The n-n separation rnn between the halo neutrons is then a Gaussian function with the variance = 20.5 r0 and rnnrms 60.5 r0. The correlation function of the first order coincidences, built with a resolution of 0.5 MeV/c points out the strength of the correlation and the fact that with our neutron array we can obtain a large range beginning with low values ( 2.25 MeV/c) of the relative momentum q. Also, the nn correlation function obtained constitutes a way to estimate the separation distance rnnrms between the two halo neutrons. The present correlation data for Si (11 Li, fusion) suggest a root mean square separation of the two halo neutrons in the moment of pre-emission between 7.5 and 11 fm, nearer to the value of 8.3 fm predicted by COSMAI model. (authors)