New 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 more 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, ⁴⁰Mg and ⁴²Al, to the list of those previously observed (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.

The newly identified isotopes come on the heels of the discovery of ⁴⁴Si earlier this year from the same group at NSCL, led by physicist Oleg B. Tarasov (Phys. Rev. C 2007, 75, 064613). Prior to ⁴⁴Si, the last neutron-rich isotopes discovered were ³⁴Ne, ³⁷Na, and ⁴³Si in 2002 (Phys. Lett. B 2002, 542, 49) and ³¹F in 1999 (Phys. Lett. B 1999, 448, 180).

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. Notably, ³⁰F, ³³Ne, and ³⁶Na, all with odd numbers of neutrons, have not been observed experimentally, although ³¹F, ³⁴Ne, and ³⁷Na, with even numbers, have been documented.

The researchers also observed one instance of ⁴³Al, compared with three of ⁴⁰Mg and 23 of ⁴²Al. A single, unreproduced observation of an isotope is normally not a reportable result, but the fact that ⁴²Al was documented makes the existence of ⁴³Al credible. With an even number of neutrons, ⁴³Al should be more stable than ⁴²Al.

"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. Casten notes that the full impact of the newly identified isotopes is not yet understood. For example, scientists don't know whether the ability to pack in more neutrons than expected is limited to a small range of elements, including magnesium and aluminum, or whether it's a characteristic that eventually will be observed across the entire periodic table.

NEVERTHELESS, the observation of ⁴²Al, with its unpaired neutron, "alters the landscape of known nuclei," Casten says. "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."

The latest NSCL experiment stretches the limits of current particle accelerators, according to Paul-Henri Heenen, a professor of nuclear physics at Université Libre, in Brussels, who wrote a commentary on the work. Further experimental input on the location of the neutron drip line may require new accelerators, such as a facility for rare-isotope beams, identified as a priority for the long-range plan for nuclear science being developed by the Department of Energy and the National Science Foundation. In the meantime, Heenen says, NSCL's results will likely be used to improve the accuracy of current theoretical models of nuclear interactions.