  The nuclear two-photon or double-gamma ($2\gamma$) 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 $2\gamma$ 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 $\sim100$\,keV and half-lives as short as
$\sim10$\,ms. The half-life for the $2\gamma$ decay of the first-excited $0^+$
state in bare $^{72}\mathrm{Ge}$ ions was determined to be
$23.9\left(6\right)$\,ms, which strongly deviates from expectations.
