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ANCIENT Egyptians may have been unwittingly emulating one of the most fundamental structures in existence. It turns out that the nuclei of some atoms may form tiny pyramids instead of spheres.

No shape is more compact than a sphere, which is why it鈥檚 so common in nature 鈥攆rom bubbles to water droplets to the Earth. Previous experiments have confirmed that although atomic nuclei can briefly form rugby ball or even egg-shaped structures, they are usually spherical or at least roughly round.

But now theoretical physicist Jerzy Dudek at the Louis Pasteur University in Strasbourg, France, and his colleagues say that it isn鈥檛 always this way. In a paper to be published in Physical Review Letters, they suggest that nuclei could in theory form stable pyramidal shapes. Dudek explains that while spheres are usually the most stable shape for a nucleus, in some heavier elements there sometimes aren鈥檛 the right number of nuclear particles to form a complete round shell. So neutrons and protons on the outer surface might bunch together, forming the rounded corners of a pyramid (see Graphic).

These triangular nuclei wouldn鈥檛 have square bases like the pyramids of Egypt, though. Instead they would be tetrahedra: four triangles joined edge to edge. From computer models based on what鈥檚 known about the behaviour of protons and neutrons in nuclei, the researchers predict that tetrahedral nuclei probably pop up all over the periodic table. Likely candidates include zirconium and ytterbium, but Dudek says dozens of other elements, such as calcium and uranium, might form less stable pyramids. There could even be other nuclear shapes, including octahedra.

Dudek鈥檚 team says that these strangely shaped nuclei may behave completely differently to spherical ones. For instance, tiny quartets of protons or neutrons would form inside the pyramids. All the members of a group would vibrate at exactly the same energy, a behaviour never seen before in atoms. The researchers also say that superheavy elements at the very end of the periodic table should be much more stable than previously thought if their nuclei are tetrahedral, making it easier for physicists to synthesise and detect these massive elements.

But first researchers need to prove the tiny pyramids really do exist. Since tetrahedral nuclei should spin very oddly, they would emit unique gamma-ray signatures that researchers could look for. 鈥淚t鈥檚 a very exciting idea, but the devil鈥檚 in the details in finding these things,鈥� says nuclear physicist Sam Tabor at Florida State University in Tallahassee. 鈥淚t will be a matter of fine instrumentation and luck.鈥� And if detectors don鈥檛 find the pyramids, physicists may have to rethink how protons and neutrons form into nuclei, he says.