Electromagnetic character of the competitive $\gamma\gamma/\gamma$-decay from $^{137\mathrm{m}}$Ba

TL;DR

This paper confirms the observation of double-gamma decay in $^{137m}$Ba with high significance, revealing a dominant octupole-dipole character and challenging previous interpretations, thus advancing understanding of second-order nuclear processes.

Contribution

The study provides an improved measurement of the double-photon branching ratio and uncovers a dominant octupole-dipole component, contradicting earlier quadrupole-quadrupole assumptions.

Findings

Confirmed double-gamma decay with 8.7σ significance

Found a branching ratio of 2.62×10⁻⁶

Identified a dominant octupole-dipole decay component

Abstract

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-$\beta$ decay, and stimulated atomic two-photon transitions. For the electromagnetic nuclear interaction, the observation of the competitive double-$\gamma$ decay from $^{137\mathrm{m}}$Ba 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 $8.7\sigma$ significance, and an improved value on the double-photon versus single-photon branching ratio as $2.62\times10^{-6}(30)$. 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 $^{137\mathrm{m}}$Ba 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.