Beyond the proton drip line

Abstract

Of the 2000 nuclides known to exist so far, just 300 are stable. Nuclei are usually unstable because they contain too many neutrons or too many protons. A nucleus with too many protons, for example, is unstable because of the electrostatic repulsion between the protons. It can become more stable by ejecting a proton – a process known as proton radioactivity. The limit of stability for such proton-rich nuclei is determined by the proton “drip line” – this is the line on the plot of proton number, Z, versus neutron number, N, that separates stable nuclei from their unstable relatives. (There is also a neutron drip line for neutron-rich nuclei.) The binding energy of the last proton in the nucleus is roughly zero at the appropriate drip line. All nuclei outside the proton drip line are proton radioactive in their ground state.