Nuclear Pyramids

Physicists normally think of atomic nuclei as being something like a droplet with a roughly spherical shape. But if atoms can assemble into tiny pyramid structures (such as the ammonia molecule, NH₄), why not nuclei? It all depends on how the nuclear forces act in a nucleus.

A group of physicists from the Universite Louis Pasteur (Strasbourg, France) and the Marie Curie University (Lublin, Poland) have, for the first time, tried to imagine how stable nuclei could form with pyramid, or even cubic or octahedral shapes.

In chemistry many configurations are possible because the interactions (e.g., Van der Waals, covalent, or hydrogen bonding) can extend over considerable distances.

The nuclear force, by contrast, is attenuated, and acts not much further than the size of nucleons (the protons and neutrons making up the nucleus). An excited pyramidal nucleus would turn in space, every now and then throwing out a high-energy photon (gamma ray). This would make for a characteristic spectrum, but one which would most likely require a gamma detection sensitivity only now being planned for experiments in the US and Europe.

Jerzy Dudek (jerzy.dudek@res.in2p3.fr, 33-388-10-6498) and his colleagues have worked out the "magic numbers" for those elements and isotopes most likely to be sustainable in tetrahedral form, nuclei with certain numbers of protons (e.g., 20, 32, 40, 55/58, 70) and neutrons.

For example, barium-126 (56 protons, 70 neutrons) and barium-146 (56 protons, 90 neutrons) have promise, whereas Ba-114 or Ba-168 do not. (Dudek et al., Physical Review Letters, 24 June 2002.)