Electromagnetic excitation of 11Li.

Abstract

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 ${\mathrm{\ensuremath{\sigma}}}_{\mathit{E}1}$(E) and the dipole strength function dB(E1)/dE were determined from the Coulomb dissociation cross section. ${\mathrm{\ensuremath{\sigma}}}_{\mathit{E}1}$(E) has a peak at E=1.0 MeV and a width \ensuremath{\Gamma}=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 ${\mathrm{\ensuremath{\sigma}}}_{9}$=18\ifmmode\pm\else\textpm\fi{}4 MeV/c and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{n}}$=13\ifmmode\pm\else\textpm\fi{}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.