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 (email@example.com), 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,