The double-gamma (γγ)-decay of a quantum system in an excited state is a fundamental second-order process of quantum electrodynamics. In contrast to the well-known single-gamma (γ)-decay, the γγ-decay is characterized by the simultaneous emission of two γ quanta, each with a continuous energy spectrum. In nuclear physics, this exotic decay mode has only been observed for transitions between states with spin-parity quantum numbers Jπ = 0+ (refs 1, 2, 3). Single-gamma decays—the main experimental obstacle to observing the γγ-decay—are strictly forbidden for these 0+ → 0+ transitions. Here we report the observation of the γγ-decay of an excited nuclear state (Jπ = 11/2−) that is directly competing with an allowed γ-decay (to ground state Jπ = 3/2+). The branching ratio of the competitive γγ-decay of the 11/2− isomer of 137Ba to the ground state relative to its single γ-decay was determined to be (2.05 ± 0.37) × 10−6. From the measured angular correlation and the shape of the energy spectra of the individual γ-rays, the contributing combinations of multipolarities of the γ radiation were determined. Transition matrix elements calculated using the quasiparticle–phonon model reproduce our measurements well. The γγ-decay rate gives access to so far unexplored important nuclear structure information, such as the generalized (off-diagonal) nuclear electric polarizabilities and magnetic susceptibilities3.
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This work was supported by the State of Hesse under the Helmholtz International Center for FAIR (HIC for FAIR) and by the German Research Council (DFG) under grant no. SFB 634. We thank D. J. Millener, R. J. Sutter for an open discussion and for generously sharing their preliminary results before publication, and C. J. Lister for discussions. H.S. thanks Dirk Schwalm for raising interest in this topic.
The authors declare no competing financial interests.
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Walz, C., Scheit, H., Pietralla, N. et al. Observation of the competitive double-gamma nuclear decay. Nature 526, 406–409 (2015). https://doi.org/10.1038/nature15543
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