Neutrons cross the line

Abstract

For most atomic nuclei, the maximum number of neutrons that can be bound is unknown. The discovery of two neutron-rich nuclei — and the confirmed absence of others — might help solve this conundrum. The limit of how many neutrons can be bound by a given number of protons — called the neutron drip line — is of fundamental importance in nuclear physics. It is unknown for all but the lightest elements, because of the complex interplay between single particle and collective quantum effects, but the detection of two new neutron-rich isotopes of magnesium and aluminium is a significant step towards establishing this limit. 40Mg and 42Al, predicted to be 'drip-line' nuclei on the edge of stability, were produced at the coupled cyclotron facility of the US National Superconducting Cyclotron Laboratory. The debut of 42Al is the first experimental indication that the stability of very neutron-rich aluminium isotopes is enhanced relative to magnesium, and indicates that the neutron drip line may be farther from stable isotopes than expected. The findings suggest that the drip line may be beyond the reach of most current and projected nuclear science facilities for nuclei with atomic numbers over 12.