Magic metal

2019-03-07 04:13:06

By Robert Adler THE first “doubly magic” nickel nuclei have popped into existence in France. The nuclei can be used to test competing theories of nuclear forces and even set the stage for detecting a new form of radioactivity. Light atoms tend to break apart when there is an imbalance in the numbers of protons and neutrons. But half a century ago, the German-American Nobel prizewinner Maria Goeppert-Mayer showed that their stability depends on exactly how many protons and neutrons they have. Quantum rules force protons and neutrons into nested shells inside the nucleus, and when a shell has been filled with its so-called “magic number” of protons or neutrons, it settles into a stable shape (New Scientist, 2 October, p 38). Some theories predict that because it has 28 protons and 20 neutrons—both magic numbers—nickel-48 should have a just-detectable lease on life, rather than flying apart the instant it forms. To test this, Bertram Blank and his colleagues at the Centre for Nuclear Studies in Bordeaux-Gradignan, France, bombarded a nickel target with nickel-58 nuclei. After 17 days, they had detected two nuclei of nickel-48. “It’s a highly unstable nucleus with a half-life at best of a few milliseconds,” says Blank. Now that his team can make and detect these proton-rich nuclei, Blank plans to study how they fall apart. He suspects they’ll emit two protons at the same time—a type of radioactive decay quite distinct from the alpha and beta forms familiar to scientists. Its existence was predicted nearly 40 years ago but it has never been seen. “It’s a new form of radioactivity,” says Blank. In 1991, Alex Brown of Michigan State University in East Lansing used a nuclear shell model to predict that nickel-48 would decay after between 0.01 and 2 milliseconds. “That’s what the experiments will test,” he says. He adds that knowing how such magic nuclei behave could clarify what happens in supernova explosions,