Double Magic for New Nickel Nucleus

In what you might call a smashing success, physicists have chiseled out an atomic nucleus laden with a record eight more protons than neutrons. The new nucleus, nickel-48, self-destructs in a fraction of a second, but it may help researchers study a form of radioactive decay that they have long sought but not yet observed.

As with fraternity brothers in a phone booth, you can cram only so many protons into a nucleus. There, protons and neutrons stick to one another with a peculiar pull called the strong force. But the like-charged protons also push against one another with an electric force. If the protons outnumber the uncharged neutrons by too wide a margin, the electric force wins out and the nucleus falls apart. That's why almost every nucleus in nature has more neutrons that protons.

But proton-rich nuclei can stabilize themselves by playing a shell game. Like the electrons buzzing around in their orbital shells, the protons and neutrons in the nucleus pile into distinct nuclear shells. And just as elements with full electron shells are inert, nuclei with full nuclear shells tend to be stable. So nuclei that are "doubly magic"--that possess a full shell of neutrons and a full shell of protons--may be stable, even if the protons outnumber the neutrons. Knowing this, nuclear physicist Bertram Blank and his team at the Grand Accélérateur National d'Ions Lourds, in Caen, France, spent the last 10 years hunting for nickel-48, a doubly magic nucleus with 28 protons and only 20 neutrons.

To make the novel nucleus, the researchers smashed a high-energy beam of naturally occurring nickel-58 nuclei (28 protons and 30 neutrons) into a nickel-58 target. The collision knocked bits and pieces off the passing particles, producing a shower of odd nuclei. The team measured the charge and mass of every one with a sophisticated spectrometer. After sifting through millions of nuclei, they found four nickel-48 nuclei, as the researchers report in the February 7 Physical Review Letters.

The barely stable nucleus is a prime candidate for two-proton decay, a form of nuclear decay that until now has been strictly theoretical, says P. Gregers Hansen, a nuclear physicist at Michigan State University in East Lansing. Because of peculiarities of the strong force, nickel-48 cannot eject one proton, but it might just spit out two at once, thereby emitting a kind of radiation that has never been detected. "It's wonderful to know [nickel-48] exists," Hansen says. "Someday we may do some experiments with it."