Measurement of the Isolated Nuclear Two-Photon Decay in Ge 72

The nuclear two-photon or double-gamma ( 2 γ ) decay is a second-order electromagnetic process whereby a nucleus in an excited state emits two gamma rays simultaneously. To be able to directly measure the 2 γ decay rate in the low-energy regime below the electron-positron pair-creation threshold, we combined the isochronous mode of a storage ring with Schottky resonant cavities. The newly developed technique can be applied to isomers with excitation energies down to ∼ 100 keV and half-lives as short as ∼ 10 ms . The half-life for the 2 γ decay of the first-excited 0 + state in bare Ge 72 ions was determined to be 23.9(6) ms, which strongly deviates from expectations.