Second-order processes in physics is a research topic focusing attention from
several fields worldwide including, for example, non-linear quantum
electrodynamics with high-power lasers, neutrinoless double-β decay, and
stimulated atomic two-photon transitions. For the electromagnetic nuclear
interaction, the observation of the competitive double-γ decay from
137mBa has opened up the nuclear structure field for detailed
investigation of second-order processes through the manifestation of
off-diagonal nuclear polarizability. Here we confirm this observation with an
8.7σ significance, and an improved value on the double-photon versus
single-photon branching ratio as 2.62×10−6(30). Our results, however,
contradict the conclusions from the original experiment, where the decay was
interpreted to be dominated by a quadrupole-quadrupole component. Here, we find
a substantial enhancement in the energy distribution consistent with a
dominating octupole-dipole character and a rather small quadrupole-quadrupole
element in the decay, hindered due to an evolution of the internal nuclear
structure. The implied strongly hindered double-photon branching in
137mBa opens up the possibility of the double-photon branching as
a feasible tool for nuclear-structure studies on off-diagonal polarizability in
nuclei where this hindrance is not present.Comment: 12 pages, 5 figures, 2 tabel