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
(firstname.lastname@example.org, 630-252-0583) says that He-6 is the lightest known nucleus
to have a neutron halo, and that the collaborationís 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