Intruder state

An "intruder state" is a concept used primarily in nuclear physics and many-body physics, specifically in the context of nuclear structure and shell models. It refers to a state of a nucleus that involves the excitation of nucleons (protons and neutrons) to higher energy levels outside the standard shell model predictions. In the shell model of the nucleus, nucleons occupy discrete energy levels or "shells" analogous to electrons in atomic orbitals.