Electromagnetic character of the competitive γγ/γ-decay from 137mBa

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-β decay, and stimulated atomic two-photon transitions. For the electromagnetic nuclear interaction, the observation of the competitive double-γ decay from 137m Ba has opened up the nuclear structure field for detailed investigation of second-order processes through the manifestation of off-diagonal nuclear polarisability. 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 component in the decay, hindered due to an evolution of the internal nuclear structure. The implied strongly hindered double-photon branching in 137m Ba opens up the possibility of the double-photon branching as a feasible tool for nuclear-structure studies on off-diagonal polarisability in nuclei where this hindrance is not present.

$γγ$ decay as a probe of neutrinoless $ββ$ decay nuclear matrix elements

“…Second-order EM decays are naturally suppressed with respect to first-order ones. Nevertheless, γγ transitions have been measured between 0 + first-excited states and GSs, where single-γ decay is forbidden [51][52][53], and, recently, among general nuclear states in competition with γ transitions [54,55]. Future DIAS to GS γγ decay measurements, combined with the good linear correlation between NMEs presented in this work, show as a promising tool to give insights on 0νββ NMEs.…”

The model for describing the width of double-gamma decay of the quadrupole state of spherical nuclei

“…The branching ratio of the competitive γγ-decay of the 11/2 − isomer of the odd-even nucleus 137 Ba to the ground state relative to its single γ-decay was determined to be (2.05 ± 0.37) × 10 −6 . This discovery has very recently been confirmed and the data were made more precise, in particular with respect to the contributing multipolarities [2].…”

Formal theory of heavy ion double charge exchange reactions

“…The DSCE rank-2 nuclear polarization tensors occur also in 2ν2β decay. Moreover, they establish an interesting connection to double-gamma decay [17].…”