New Cranked-Up Nuclear States

Some nuclear physicists seek to make new elements by fusing two nuclei and hope the amalgamated body will hold together at least for a while. Other researchers explore the nuclear world by creating new spin states. A highly spinning nucleus is not "excited" in the usual sense of possessing a lot of internal energy, but allows nevertheless the nucleus constituent protons and neutrons to deploy themselves in new ways.

This high-spin universe is reached in off-center smashups of two nuclei. In new experimental work at the Lawrence Berkeley National Laboratory in Berkeley, Calif., erbium-158 nuclei were spun up to very high rates and then closely observed as they slowed down by offloading high energy photons. These gamma rays, each carrying off two units of angular momentum (each unit equals 2 times pi times Planck's constant), are observed in the Gammasphere detector surrounding the collision site; the number of the gamma rays provides information about nuclear spin.

So, for example, a nucleus spun up to a level of 40 units would, by relaxing back to normal, throw off about 20 gammas; other forms of nuclear radioactive relaxation -- throwing out electrons or alpha particles -- take too long to come about. Theorists believe that above a spin value of about 46, the entire erbium-158 nucleus cannot be spun up any further without a drastic rearrangement of the entire state of the nucleus. Instead a spherical core of nuclear particles (constituting a gadolinium-146 nucleus) rotates no more while a fleet of 12 "valence" particles (neutrons and protons) orbits the core at ever higher spin values (see this progression of spin states at Physics News Graphics).

Eddie Paul of the University of Liverpool (esp@ns.ph.liv.ac.uk), in England, and his colleagues have been able to discover new pathways to a higher spin regime by observing the pattern of gamma rays thrown out. They find evidence that the core observed in previous experiments can occasionally break up a bit, allowing collective rotation of all the nucleons to resume, permitting the total spin of the nucleus to attain higher values. The highest value observed in this way was a state with 65 units of spin.

The researchers hope to explore even higher values of spin, maybe so high that the nucleus approaches the fission limit. At this point the nucleus does not de-excite by losing gammas, but by actually fissioning -- that is by coming apart into large nuclear fragments.

Paul et al., Physical Review Letters, upcoming article