Discovery of 40Mg and 42Al suggests neutron drip-line slant towards heavier isotopes

The edge of stability 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. 40 Mg and 42 Al, 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 42 Al 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. The paper reports a significant advance in the determination of the limit of how many neutrons a given number of protons can bind to (known as the neutron drip line), with the discovery of two new neutron-rich isotopes predicted to be drip line nuclei. Nuclei at the drip line gain stability from an unpaired proton, which narrows the shell gaps and provides the opportunity to bind many more neutrons. A fundamental question in nuclear physics is what combinations of neutrons and protons can make up a nucleus. Many hundreds of exotic neutron-rich isotopes have never been observed; the limit of how many neutrons a given number of protons can bind is unknown for all but the lightest elements 1 , owing to the delicate interplay between single particle and collective quantum effects in the nucleus. This limit, known as the neutron drip line, provides a benchmark for models of the atomic nucleus. Here we report a significant advance in the determination of this limit: the discovery of two new neutron-rich isotopes— 40 Mg and 42 Al—that are predicted to be drip-line nuclei 2 . In the past, several attempts to observe 40 Mg were unsuccessful 3 , 4 ; moreover, the observation of 42 Al provides an experimental indication that the neutron drip line may be located further towards heavier isotopes in this mass region than is currently believed. In stable nuclei, attractive pairing forces enhance the stability of isotopes with even numbers of protons and neu