The Helium-6 Nucleus

The helium-6 nucleus consists of a He-4 nucleus (two protons plus two neutrons) surrounded by a halo cloud consisting of two more neutrons. The charge radius for He-6 has been now measured for the first time. The experimental value, 2.1 fm (2.1 x 10^-15 m), is larger than the radius for He-4, 1.7 fm, the reason being that the halo neutrons in He-6 cause the core portion of the nucleus to inflate somewhat (see figure at http://www.aip.org/png/2004/222.htm). The He-6 nuclei are made at a special beamline at Argonne National Lab by smashing a beam of lithium ions into a target. The stray He-6 atoms made in the process (about a million per second) are drawn into and lodged within a trap at a rate of about one a minute. This is sufficient to do laser spectroscopy on the atoms. The charge radius of the nucleus can be deduced from the way in which the frequency of the light corresponding to an internal atomic transition from one quantum state to another in the atoms is shifted in going from He-6 to He-4. Zheng-Tian Lu of Argonne (lu@anl.gov, 630-252-0583) says that He-6 is the lightest known nucleus to have a neutron halo, and that the collaboration’s next experimental quarry, He-8, represents the most neutron-rich (highest neutron-to-proton ratio) nuclear matter in the world. (Wang et al., Physical Review Letters, 1 October 2004; lab website at www-mep.phy.anl.gov/atta/)