Electromagnetic excitation of ¹¹Li

We have performed a kinematically complete measurement of the Coulomb dissociation of 28 MeV/nucleon ¹¹Li into ⁹Li and two neutrons by a Pb target. From the energies and angles of the emitted neutrons and of ⁹Li, the excitation energy E of ¹¹Li 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 σ_E1(E) and the dipole strength function dB(E1)/dE were determined from the Coulomb dissociation cross section. σ_E1(E) has a peak at E=1.0 MeV and a width Γ=0.8 MeV. These parameters are consistent with the picture of a soft dipole mode. However, a significant post-breakup Coulomb acceleration of ⁹Li suggests instead a direct breakup. The complete kinematical measurement also allowed neutron and ⁹Li momentum distributions to be constructed in the rest frame of the ¹¹Li. The momentum distributions were fitted with Gaussian functions, yielding width parameters σ₉=18±4 MeV/c and σ_n=13±3 MeV/c. A more general feature of the breakup mechanism of ¹¹Li could be deduced from these measurements. It was found that the ⁹Li and neutron momentum distributions and the neutron-neutron relative momentum distribution could be reproduced if the ¹¹Li excitation energy was partitioned between the ⁹Li 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 ⁹Li core.