We have performed a kinematically complete measurement of the Coulomb dissociation of 28 MeV/nucleon into and two neutrons by a Pb target. From the energies and angles of the emitted neutrons and of , the excitation energy E of 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 (E) and the dipole strength function dB(E1)/dE were determined from the Coulomb dissociation cross section. (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 suggests instead a direct breakup. The complete kinematical measurement also allowed neutron and momentum distributions to be constructed in the rest frame of the . The momentum distributions were fitted with Gaussian functions, yielding width parameters =18±4 MeV/c and =13±3 MeV/c. A more general feature of the breakup mechanism of could be deduced from these measurements. It was found that the and neutron momentum distributions and the neutron-neutron relative momentum distribution could be reproduced if the excitation energy was partitioned between the 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 core.
- Received 30 November 1992
- Published in the issue dated July 1993
© 1993 The American Physical Society