First observation of the competitive double-gamma (γγ/γ) decay process

First observation of the competitive double-γ decay process is presented. It is a second-order electromagnetic decay mode. The 662-keV transition from the 11/2⁻ isomer of ¹³⁷Ba to its ground state proceeds at a fraction of 2 x 10⁻⁶ by simultaneous emission of two γ quanta instead of one. The angular correlation and energy distribution of coincident γ quanta are reproduced by a dominant M2-E2 and a minor E3-M1 contribution to the double-γ decay branch. The data are well accounted for by a calculation in the Quasiparticle Phonon Model.