A nuclear drip line marks a boundary beyond which one- or few-nucleon emission becomes energetically allowed. One- and two-particle nuclear separation energies can give distinct boundaries because of nuclear pairing.
The neutron drip line is the neutron-rich boundary at which one- or two-neutron emission becomes energetically possible. Neutrons beyond that boundary are not stably bound; the absence of a Coulomb barrier often makes their emission prompt, subject to angular-momentum barriers.
The proton drip line is the proton-rich boundary at which one- or two-proton emission becomes energetically possible. It is governed by proton separation energies, rather than the energetics of beta decay alone.
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The term "nuclear drip line" refers to the boundaries in the map of nuclear stability that separate regions of nuclear existence and non-existence for isotopes of elements. Specifically, there are two main drip lines: 1. **Neutron Drip Line**: This boundary represents the limit where adding one more neutron to a nucleus makes it unstable and causes it to decay or break apart.