Resumen de Observation of the competitive double-gamma nuclear decay
C. Walz, H. Scheit, N. Pietralla, T. Aumann, R. Lefol, Yu. V. Ponomarev
The double-gamma ([gamma][gamma])-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 ([gamma])-decay, the [gamma][gamma]-decay is characterized by the simultaneous emission of two [gamma] 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[pi] = 0+ (refs 1, 2, 3). Single-gamma decays—the main experimental obstacle to observing the [gamma][gamma]-decay—are strictly forbidden for these 0+ -> 0+ transitions. Here we report the observation of the [gamma][gamma]-decay of an excited nuclear state (J[pi] = 11/2-) that is directly competing with an allowed [gamma]-decay (to ground state J[pi] = 3/2+). The branching ratio of the competitive [gamma][gamma]-decay of the 11/2- isomer of 137Ba to the ground state relative to its single [gamma]-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 [gamma]-rays, the contributing combinations of multipolarities of the [gamma] radiation were determined. Transition matrix elements calculated using the quasiparticle–phonon model reproduce our measurements well. The [gamma][gamma]-decay rate gives access to so far unexplored important nuclear structure information, such as the generalized (off-diagonal) nuclear electric polarizabilities and magnetic susceptibilities3.
