Electromagnetic excitation ofLi11

We have performed a kinematically complete measurement of the Coulomb dissociation of 28 MeV/nucleon ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ into ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ and two neutrons by a Pb target. From the energies and angles of the emitted neutrons and of ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$, the excitation energy E of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ 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 ${\mathrm{\sigma }}_{\mathit{E}1}$(E) and the dipole strength function dB(E1)/dE were determined from the Coulomb dissociation cross section. ${\mathrm{\sigma }}_{\mathit{E}1}$(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 ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ suggests instead a direct breakup. The complete kinematical measurement also allowed neutron and ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ momentum distributions to be constructed in the rest frame of the ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$. The momentum distributions were fitted with Gaussian functions, yielding width parameters ${\mathrm{\sigma }}_9$=18±4 MeV/c and ${\mathrm{\sigma }}_{\mathit{n}}$=13±3 MeV/c. A more general feature of the breakup mechanism of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ could be deduced from these measurements. It was found that the ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ and neutron momentum distributions and the neutron-neutron relative momentum distribution could be reproduced if the ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ excitation energy was partitioned between the ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ 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 ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ core.