A Double Dose of Nuclear Magic

Scientists have discovered an isotope of nickel that should not exist. Normally, nuclei as light as nickel have roughly equal numbers of protons and neutrons, but the new nucleus has 28 protons and 20 neutrons. The repulsion of such an excess of positive charge should make the nucleus fly apart, if it were not for a dose of nuclear numerology.

The numbers 20 and 28 are what nuclear physicists call "magic numbers," and nuclei with this many protons or neutrons get an extra measure of stability. The rare occurrence of two magic numbers--in both protons and neutrons--means that even such a lopsided nucleus as nickel-48 could hold itself together for a few microseconds. Indeed, when the team collided a beam of nickel-58 ions with a target made of a natural mixture of nickel isotopes at the National Heavy Ion Accelerator (GANIL) in Caen, France, the researchers managed to spot just two fleeting nickel-48 nuclei.

The concept of magic numbers is derived from the shell model of the atom nucleus, explains Bertram Blank of the Center for Nuclear Studies in Bordeaux, the spokesperson of the international collaboration. Nuclear theorists believe that the combined quantum mechanical effect of the protons and neutrons in a nucleus create a set of energy levels akin to the levels that govern electrons orbiting the nucleus. Groups of levels with similar energies are referred to as shells and each can hold a specific number of protons or neutrons. Magic numbers are the sum of particles required to fill a shell. The established magic numbers are 2, 8, 20, 28, 50, 82, and 126.

Up to now, only nine doubly magic isotopes were known. The last and most massive one to be discovered was tin-100, with 50 protons and 50 neutrons, at GANIL in 1994. The discovery of such isotopes helps to fine-tune current models of the atomic nucleus, especially because the extreme nature of the doubly magic nuclei allow theorists to make more precise predictions than for other nuclei, says Blank. "There are models that predict that [nickel-48] has such a short lifetime that we should not have been able to see it, and other models agree with our observations because they predict that it lives at least a few microseconds."

Blank expects that nickel-48 will be the last doubly magic nucleus to be discovered for many years, because others are simply beyond the reach of today's experimental techniques. Nuclear theorist Alex Brown of Michigan State University in East Lansing says it is now important to find out how this nucleus decays.

According to predictions, a small number of proton-rich nuclei should be able to decay by emitting a pair of protons, a process not yet observed. "This nucleus is very special, there are only about three or four of these nuclei predicted so far, and none have been seen," says Brown. The collaboration is already planning experiments to observe nickel-48 decay. Watch this space.