Fig. 1: Illustration of 8B breaking up into 7Be and proton.
From: Breakup of the proton halo nucleus 8B near barrier energies
In the initial state, the nucleons in 8B occupy bound single particle orbits in the potential well. Although located in the potential well, the halo proton is extremely weakly-bound in an extended distribution around the centrum 7Be core. It hence can be regarded as a semi-open quantum system. During the collision, 8B is excited to the unbound states above the breakup threshold (either resonant or non-resonant), forming an open quantum system.
