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

Abstract

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