Observation of the competitive double-gamma nuclear decay

The exotic double-gamma nuclear decay has been observed in cases where the usual single-gamma decay is forbidden, but now a double-gamma decay of excited 137 Ba is reported that is in competition with a single-gamma decay. Competitive double-gamma decay Most decays of quantum states involve the emission of one photon, but in processes first predicted by Nobel laureate Maria Goeppert-Mayer more than 75 years ago, exotic decays emitting two photons can occur. An analogous exotic decay process in atomic nuclei is the double-gamma decay, which has been observed in cases where the usual single-gamma decay is forbidden. In an experimental tour de force, Heiko Scheit and colleagues observe a double-gamma decay which is in competition with a single-gamma decay. Through decay of caesium-137, the first excited state of barium-137 is populated. Monitoring the emissions for more than 50 days and carefully excluding alternative explanations for their data, the authors are able to unambiguously measure the competitive double-gamma decay. Precise measurements of the double-gamma decay rate promise to give access to nuclear structure information that is elusive by other methods. 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 J π = 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 ( J π = 11/2 − ) that is directly competing with an allowed γ -decay (to ground state J π = 3/2 + ). The branching ratio of the competitive γγ -decay of the 11/2 − isomer of 137 Ba 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 nuclea