To be more precise, the charge radius of this heaviest of helium isotopes (containing two protons and six neutrons) has been measured for the first time. The charge radius tells you how widely the proton charge is spread out in space. The new work, conducted by a Argonne-Chicago-GANIL-Windsor (Canada)-Los Alamos collaboration, arrives at a value of 1.93 fm (1 fermi equals 10^-15 m). For comparison, the charge radius of the He-6 isotope, is 2.068 fm; that is, the lighter isotope actually has a larger charge radius, the result of the binding effect of the strong nuclear force. He-8 is very rare, hard to make, and represents the most neutron-rich material known on Earth. Still heavier helium groupings, such as He-10, are not really bound entities-they can only be considered as “resonances.”
For the new experiment, He-8 was produced by bombarding a carbon target with 1-GeV beam of C-13 ions. The charge radius of the respective isotopes-He-4, He-6, and He-8-is determined by comparing the subtle shifting of the atomic spectra from the three different species of helium atom. The spectroscopy measurements involve only the electromagnetic force between the electrons and the nucleus in these atoms, and not the strong nuclear force that holds each nucleus together. However, once the charge distribution is determined, it can be used to infer things about the binding force operating in the nucleus.
The current thinking on the distribution of protons and neutrons (illustrated in the figure at http://www.aip.org/png/2007/291.htm) suggests that the He-4 nucleus, composed of two protons and two neutrons ( a unit usually referred to as an alpha particle), forms the default nucleus, while in He-6 the extra two neutrons are thought to orbit the core as a sort of “halo.” In this model, the alpha core wobbles a bit around the joint center of mass with the halo neutron pair. In He-8, the halo consists of two two-neutron pairings. This actually allows the core to wobble a bit less than in the case of He-6, allowing the charge radius of He-8 to be a bit less. One of the researchers, Peter Mueller (630-252-7267, firstname.lastname@example.org), says that the current nuclear theory did an excellent job of predicting the charge radius for He-8, giving confidence to those who model heavier nuclei. (Mueller et al., Physical Review Letters, 21 December 2007)