Neutron-Rich Isotopes Discovered

Carbon-12 and -14. Oxygen-16 and -18. Phosphorus-31 and -32. Add one or two neutrons to an element, and it creates another isotope.

But what about adding 14 or even 16 neutrons to a light atom? That brings it to the "neutron drip line," the limiting number of neutrons that can be bound within an atomic nucleus. Such neutron-rich isotopes have fleeting lifetimes but nonetheless may play a critical role in stellar nuclear reactions. Understanding their properties could help deepen scientists' understanding of the atomic nucleus.

Researchers at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University have now expanded the neutron drip lines of magnesium and aluminum by adding two neutron-rich isotopes to the list of those previously observed: ⁴⁰Mg and ⁴²Al (Nature 2007, 449, 1022).

Led by Thomas Baumann, a physicist at NSCL, the researchers did the experiments by accelerating a beam of ⁴⁸Ca atoms and shooting it at a tungsten target. When the calcium atoms hit the target, they fragmented, losing protons and neutrons to become various isotopes of other atoms, including the two newly observed ones. The researchers then separated the collision products magnetically and identified them with a tandem mass spectrometer.

That ⁴⁰Mg could exist had been predicted by theoretical analysis. ⁴²Al, however, was a surprise. Protons and neutrons reside in orbitals analogous to electron orbitals, and they also pair up like electrons. With 29 neutrons, ⁴²Al has an odd, unpaired neutron, and for isotopes at the edge of the drip line, researchers thought such a lone neutron couldn't be bound.

"This discovery that ⁴⁰Mg and ⁴²Al are stable is a major advance in our understanding of the limits of nuclear binding," says Richard F. Casten, a professor of physics and director of the A. W. Wright Nuclear Structure Laboratory at Yale University. "This alters the landscape of known nuclei," Casten adds, referring to ⁴²Al. "It alters our understanding of the forces that bind nuclei into stable objects, and it has important implications for future attempts to map the evolution of nuclear structure and existence into the most weakly bound nuclei."