1. National Superconducting Cyclotron Laboratory,
   
2. Department of Physics and Astronomy,
   
3. Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA
   
4. Flerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Moscow region, Russian Federation

Correspondence to: T. Baumann¹ Correspondence and requests for materials should be addressed to T.B. (Email: baumann@nscl.msu.edu).

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 elements¹, 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—⁴⁰Mg and ⁴²Al—that are predicted to be drip-line nuclei². In the past, several attempts to observe ⁴⁰Mg were unsuccessful^{3, 4}; moreover, the observation of ⁴²Al 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 neutrons. In contrast, the present work shows that 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^{5, 6}.