Number 115 (Story #2), February 25, 1993 by Phillip F. Schewe and Ben Stein|
THE LITHIUM-11 NUCLEUS consists of nine nucleons surrounded at some distance by a pair of neutrons constituting a weakly-bound halo. The existence of the halo was first suspected when, five years ago, nuclear scattering experiments showed that the reaction cross section for Li-11 was significantly larger than expected; this suggested that the nuclear size was larger than normal. A new experiment has now actually measured the size of the halo in the act of breaking up (Nature, 11 Feb. 1993). At the National Superconducting Cyclotron Lab at Michigan State, a beam of radioactive Li-11 is created by sending oxygen-18 ions into a thin lithium foil. The Li-11 is then scattered from a target of lead nuclei, whose electric fields cause the relatively gentle breakup of the Li-11 into Li-9 and a pair of neutrons (I. Ieki et al., 8 Feb. 1993 Physical Review Letters). When Li-11 is scattered instead from light nuclei, such as Be-9, fragmentation occurs through the agency of the strong nuclear force (N.A. Orr et al., 5 Oct. 1992 Phys. Rev. Lett.). Both types of scattering suggest that the Li-11 halo is five times larger than the size of the Li-9 nucleus. Since the halo represents a sort of "neutron matter," experiments at MSU and several other labs, such as GANIL and SACLAY in France and RIKEN in Japan, hope to examine the interactions between the halo neutrons and to seek out other nuclei with halos ("Physics News in 1992," in the March 1993 APS News).