A two-in-a-million double-photon nuclear decay

In her 1930 dissertation, Maria Goeppert Mayer used perturbation theory to derive the probability for an atom to change its electronic state by absorbing or emitting two photons of appropriate combined energy. Such two-photon processes are today being routinely used in spectroscopy and other applications. Although atomic nuclei can undergo similar transitions involving two gamma-ray photons, they are usually overwhelmed by single-gamma transitions. Two-gamma decays have been observed only for three isotopes in which quantum mechanics forbids single-gamma decay. Now a team at the Darmstadt University of Technology in Germany reports the first clear observation of two-photon emission from excited nuclei for which the single-photon decay is allowed—and 5 × 10⁵ more likely—a situation the researchers call competitive double-gamma decay. As sketched here, the researchers deployed five detectors (red) to record pairs of gammas (blue) emitted in the two-photon decay of excited barium-137 nuclei. However, a lone gamma...