Large-scale shell-model calculations across the full odd-A indium chain from 99In to 131In show that the 9/2+ ground state and the low-lying 1/2-, 3/2- and 5/2- levels are not the pure single-proton-hole states the naive picture makes them. Away from the N=50 and N=82 shell closures they are strong configuration mixtures, and electromagnetic moments, spectroscopic factors and seniority decomposition all register the same departure. The specific claim is about the 1/2- level, which had resisted explanation: its excitation energy follows the effective single-particle energy of the proton p1/2 orbital, built from the fractional occupancies of the neutron orbitals through central- and tensor-force monopole matrix elements, but reproducing where the level actually sits requires mixing with the proton p3/2 and f5/2 orbitals. Effective single-particle energies account for the trend; only configuration mixing accounts for the values.
Indium has one proton fewer than tin, so an indium nucleus can be pictured as a doubly-magic tin core with a single proton missing — a proton hole. Which orbital that hole sits in fixes the nuclear s…