Observation of the competitive double-gamma nuclear decay

Abstract

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 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.