Probing the 11 Li halo structure by two neutrons interferometry experiments

The neutron halo nuclei discovered by Tanihata and co-workers, are characterized by very large matter radii, small separation energies, and small internal momenta of valence neutrons. Recently, it was predicted that, due to the very large dimension of 11 Li, one may expect that in a fusion process on a light target, the valence neutrons may not be absorbed together with the 9 Li core, but may be emitted in the early stage of the reaction. Indeed, the experimental investigations of neutron pre-emission in the fusion of 11 Li halo nuclei with Si targets, have shown that a fair amount of fusions, are preceded by one or two halo neutron pre-emission. It was also found that in the position distribution of the pre-emitted neutrons, a very narrow neutron peak, leading to transverse momentum distribution much narrower that that predicted by COSMA model is present. Following the supposition that this narrow distribution could be produced by two neutron interferometry and following the preliminary observation of coincident neutrons, it was decided to perform a new experiment aiming to investigate the neutron pair pre-emission in conditions of much higher statistics, by mean of a neutron array detector. This array detector consisting of 81 modules, was built in cooperation between IPNE-Romania and RIKEN-Japan. The experiment has been performed at the RIKEN-RIPS radioactive beam separator facility. The new measurement has confirmed with higher precision the presence of the very narrow peak. The FWHM of this spectrum is ∼13 cm at 138 cm from the target and corresponds to a solid angle of ∼ 9 msr. Within this narrow peak, a large number of n-n coincidences were observed for 11 Li, by comparing the data obtained with 11 Li and with 9 Li beams. Therefore a neuron correlation analysis, based on event by event investigation has been undertaken. Soon it was observed that there are serious problems with the building of the correlation function due to the presence of residual correlation of single detected halo neutrons. We obtained the correlation function by using (b) denominator (see M. Petrascu Yad. Phys. 8 2003, in press). A similar rrms (as given by K. Yeki et al., Phys. Rev. Lett. 70 (1993) 130) was obtained with a correlation function with (a) denominator. This value of rrms is inflated due to residual correlation. The correlation function is closer to COSMAI, but the denominator (b) by which this was built may still contain correlated background. An intrinsic correlation function could be obtained in a combined experiment with 11 Li and 11 Be. This new experiment according to the authors' estimation should be performed on a 12 C target. There are many very challenging points in this project based on 11 Li, 11 Be and 12 C targets. One, is to pave the way for obtaining the real intrinsic n-n correlation (interferometry) peak, with a two times higher luminosity than for Si target, what seems to be of certain interest. (authors)