Atom's core gets pear-shaped

Atomic nuclei come in many shapes and sizes, and scientists have now obtained precise measurements of an elusive form: pear-shaped. Studying these exotic nuclei, which are described in the May 9 Nature, could allow physicists to better understand subatomic structure and to find new particles and forces.

“It’s a beautiful, clear-cut result of a very careful experiment,” says Christopher Lister, a physicist at the University of Massachusetts in Lowell.

Diagrams in middle school textbooks depict atomic nuclei as spherical, but the real story is a lot more complex. Protons and neutrons are jam-packed into a space just 10^-15 meters wide, held together by a crushing force that dwarfs that of gravity. At the same time, the subatomic particles constantly move, shifting around and sometimes warping the nucleus into the shape of a football or even a flattened disk.

All those shapes are symmetrical vertically and horizontally. Physicists want to find asymmetrical nuclei because some theories predict that such deformed nuclei could exhibit strange new physical properties. Experiments over the last few decades have hinted that certain arrangements of protons and neutrons result in a pear-shaped nucleus, narrow on one side and bulging on the other.

For the new study, physicist Peter Butler of the University of Liverpool in England and an international team probed two potentially pear-shaped nuclei: radon-220, which is made up of 86 protons and 134 neutrons, and radium-224, with its 88 protons and 136 neutrons. Scientists can determine the shape of nuclei by measuring the pattern of radiation they emit.

At the On-Line Isotope Mass Separator facility at CERN outside Geneva, Butler’s team fired protons at a thick slab of uranium carbide. The high-energy protons shattered the atoms in the block, producing a cornucopia of exotic atoms, including radon-220 and radium-224. The physicists filtered out the isotopes they wanted and stripped away the atoms’ electrons, leaving behind a stockpile of nuclei. Then the researchers used magnets to accelerate the nuclei to nearly 10 percent of the speed of light toward a thin layer of metal foil. The nuclei interacted with stationary atoms in the foil as they passed through, resulting in the emission of a measurable stream of gamma radiation.

The radiation measurements confirmed that both radon-220 and radium-224 nuclei have an asymmetric pear shape. The radium appears to maintain a rigid pear shape, while the radon is shiftier: Its mass continuously jiggles around so that the fat and narrow ends trade places.

“It oscillates like a ball of jelly,” says Matt Dietrich, a physicist at Argonne National Laboratory in Illinois.

Dietrich is impressed that Butler’s team achieved such precise measurements of the nuclei’s dimensions, which are essential for in-depth analyses of the physics at work inside. In his own work, Dietrich searches for a phenomenon called an electric dipole moment, in which the center of positive charge of a particle or atom lies at a different point than its center of negative charge.

In every atom ever measured, no such dipole moment exists — the positively charged nucleus sits at the very center of a negatively charged cloud of electrons. Dietrich and colleagues believe that the uneven distribution of positive charge in an asymmetrical nucleus makes it a good candidate for exhibiting a dipole moment. The standard model of particle physics predicts that atoms should have virtually nonexistent electric dipole moments, so finding one could mean that an undiscovered particle or force is at work.