As a consequence of the Pauli exclusion principle, nuclei with an excess of protons or neutrons have a higher average energy per nucleon. Nuclei with a sufficient excess of neutrons have a greater energy than the combination of a free neutron and a nucleus with one less neutron, and therefore can decay by neutron emission. Nuclei which can decay by this process are described as lying beyond the neutron drip line.
Two examples of isotopes that emit neutrons are beryllium-13 (decaying to beryllium-12 with a mean life ) and helium-5 (helium-4, ).
In tables of nuclear decay modes, neutron emission is commonly denoted by the abbreviation n.