  Isospin is an approximate symmetry in atomic nuclei, arising from the rather
similar properties of protons and neutrons. Perhaps the clearest manifestation
of isospin within nuclei is in the near-identical structure of excited states
in mirror nuclei: nuclei with inverted numbers of protons and neutrons. Isospin
symmetry, and therefore mirror-symmetry, is broken by electromagnetic
interactions and the difference in the masses of the up and down quarks. A
recent study by Hoff and collaborators presented evidence that the ground-state
spin of $^{73}$Sr is different from that of its mirror, $^{73}$Br, due to an
inversion of the ground- and first-excited states, separated by only 27 keV in
the $^{73}$Br system. In this brief note, we place this inversion within the
necessary context of the past half-century of experimental and theoretical
work, and show that it is entirely consistent with normal behaviour, and
affords no new insight into isospin-symmetry breaking. The essential point is
that isospin-breaking effects due to the Coulomb interaction frequently vary
from level to level within a given medium-mass nucleus by as much as 200 keV.
Any level splitting smaller than this is liable to manifest a level inversion
in the mirror partner which, absent disagreement with an appropriate nuclear
model, does not challenge our understanding. While we note the novelty of an
inversion in nuclear ground states, we emphasize that in the context of isospin
there is nothing specifically illuminating about the ground state, or a level
inversion.
