Deformed nuclei spit out protons.

A rare type of radioactive decay can signal the shape of an atom's nucleus.

By determining the rate at which the unstable nuclei of two elements emit protons, researchers have for the first time obtained experimental evidence that these particular nuclei look more like flattened globes than true spheres.

"We haven't actually measured the deformation, but our results show these nuclei to be highly deformed," says Cary N. Davids of Argonne (Ill.) National Laboratory. Davids and an international team of researchers describe their findings in the March 2 Physical Review Letters.

"It's a new example of proton radioactivity and, more than that, an excellent example of the ability to deduce something about the shape of a nucleus from the properties of the decay," says Richard F. Casten of Yale University.

Atomic nuclei are made up of protons and neutrons. Normally, neutrons help prevent the electrostatic repulsion between the positively charged protons from splitting the nucleus apart.

Many nuclei are unstable, however, because they contain too many protons for the number of neutrons present. Such proton-rich nuclei sometimes become more stable by spontaneously ejecting a proton.

In proton radioactivity, a proton deep inside the nucleus penetrates an outer surface shell of protons. To get through the barrier, it takes advantage of a quantum effect known as tunneling.

During the last few years, Davids and his colleagues have been smashing stable nuclei together to create unstable, proton-rich nuclei and looking for evidence of proton radioactivity among these short-lived isotopes. Nearly all of their proton decay results were consistent with tunneling out of a spherical nucleus.

Recent improvements in detector sensitivity have enabled the researchers to detect proton radioactivity among certain unstable isotopes of elements containing between 55 and 69 protons. Theorists had predicted that many of these nuclei would be nonspherical.

Davids and his team recently created the isotopes holmium-141 (67 protons and 74 neutrons) and europium-131 (63 protons and 68 neutrons). In both cases, the measured half-lives and energies associated with proton decay indicated that the protons must have tunneled through a barrier that could not be uniform in all directions. A spherical model of the nucleus failed to fit the data.

"Such proton-emitting nuclei are fantastic laboratories for testing our understanding of quantum tunneling through a deformed barrier," comments Witold Nazarewicz of Oak Ridge (Tenn.) National Laboratory.

Davids and his team aim to detect proton radioactivity in unstable isotopes of other elements that contain an odd number of protons, from which a single proton is most likely to escape. Such results, Casten says, will help physicists refine their models of the forces that shape a nucleus.