ELECTROMAGNETIC-EXCITATION OF LI-11

We have performed a kinematically complete measurement of the Coulomb dissociation of 28 MeV/nucleon Li-11 into Li-9 and two neutrons by a Pb target. From the energies and angles of the emitted neutrons and of Li-9, the excitation energy E of Li-11 was determined on an event-by-event basis, and the Coulomb dissociation cross section as a function of excitation energy was constructed. The photonuclear cross section sigma(E1)(E) and the dipole strength function dB(E1)/dE were determined from the Coulomb dissociation cross section. sigma(E1)(E) has a peak at E = 1.0 MeV and a width GAMMA = 0.8 MeV. These parameters are consistent with the picture of a soft dipole mode. However, a significant post-breakup Coulomb acceleration of Li-9 suggests instead a direct breakup. The complete kinematical measurement also allowed neutron and Li-9 momentum distributions to be constructed in the rest frame of the Li-11. The momentum distributions were fitted with Gaussian functions, yielding width parameters sigma9 = 18 +/- 4 MeV/c and sigma(n) = 13 +/- 3 MeV/c. A more general feature of the breakup mechanism of Li-11 could be deduced from these measurements. It was found that the Li-9 and neutron momentum distributions and the neutron-neutron relative momentum distribution could be reproduced if the Li-11 excitation energy was partitioned between the Li-9 and the neutrons by a three-body phase space distribution. This indicates there is no directional correlation between the halo neutrons, and shows that the halo neutrons do not exist as a dineutron bound to a Li-9 core.