Researchers observe the competitive double-gamma nuclear decay

October 15, 2015 by Bob Yirka, report

(—A team of researchers with Technische Universität Darmstadt, in Germany and an associate with the University of Saskatchewan in Canada has detected, for the first time, the double gamma decay of a nuclear quantum transition that vies with the allowed single γ-decay. In their paper published in the journal Nature, the team describes how they managed to capture the one in 487,805 chances phenomenon. Alexandra Gade with Michigan State University offers a News & Views piece on the work done by the team in the same journal issue, along with a brief history of the study and application of double-gamma nuclear decay.

Double gamma rays, denoted as γγ-gama rays, occur when two γ-rays are emitted as the excited quantum state of atomic nuclei decays, simultaneously, rather than the normal one at a time. The idea was first put forth by Maria Göppert-Mayer back in the late 1930's. A decade later famed physicists Edward Teller and colleague Gregory Breit suggested that double-gamma nuclear decay was likely the mode at play during certain states of hydrogen, when single photon emission was not possible due to angular-momentum rules. It took forty more years for the rare process to be verified in the lab. In this new work, the researchers have succeeded in actually observing the process as it occurred with a barium nucleus.

To make the observation, the team used an established means for detecting single γ-rays, but added five scintillation detectors and lead lining in the chamber to reduce scattering of the rays from one of the detectors to any of the others. Then, they ran the experiment for 50 days, taking measurements along the way. It was at that point that they finally received a verifiable (by being observed by more than one of the detectors at the same time) signal at the predicted energy levels, the first ever, though other teams have been trying for several years.

In addition to serving as what Gade describes as an experimental tour de-force, the work likely will be used in the future to further study the nature of atomic nuclei. Next up is to find a way to measure the energy distribution of the γγ-gama rays.

Explore further: Researchers observe a new kind of disbandment in the atomic nuclei rich in protons

More information: Observation of the competitive double-gamma nuclear decay, Nature 526, 406–409 (15 October 2015) DOI: 10.1038/nature15543

The double-gamma (γγ)-decay of a quantum system in an excited state is a fundamental second-order process of quantum electrodynamics. In contrast to the well-known single-gamma (γ)-decay, the γγ-decay is characterized by the simultaneous emission of two γ quanta, each with a continuous energy spectrum. In nuclear physics, this exotic decay mode has only been observed for transitions between states with spin-parity quantum numbers Jπ = 0+. Single-gamma decays—the main experimental obstacle to observing the γγ-decay—are strictly forbidden for these 0+ 0+ transitions. Here we report the observation of the γγ-decay of an excited nuclear state (Jπ = 11/2−) that is directly competing with an allowed γ-decay (to ground state Jπ = 3/2+). The branching ratio of the competitive γγ-decay of the 11/2− isomer of 137Ba to the ground state relative to its single γ-decay was determined to be (2.05 ± 0.37) × 10−6. From the measured angular correlation and the shape of the energy spectra of the individual γ-rays, the contributing combinations of multipolarities of the γ radiation were determined. Transition matrix elements calculated using the quasiparticle–phonon model reproduce our measurements well. The γγ-decay rate gives access to so far unexplored important nuclear structure information, such as the generalized (off-diagonal) nuclear electric polarizabilities and magnetic susceptibilities.

Related Stories

Recommended for you

Coffee-based colloids for direct solar absorption

March 22, 2019

Solar energy is one of the most promising resources to help reduce fossil fuel consumption and mitigate greenhouse gas emissions to power a sustainable future. Devices presently in use to convert solar energy into thermal ...

Physicists reveal why matter dominates universe

March 21, 2019

Physicists in the College of Arts and Sciences at Syracuse University have confirmed that matter and antimatter decay differently for elementary particles containing charmed quarks.

ATLAS experiment observes light scattering off light

March 20, 2019

Light-by-light scattering is a very rare phenomenon in which two photons interact, producing another pair of photons. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of ...

How heavy elements come about in the universe

March 19, 2019

Heavy elements are produced during stellar explosion or on the surfaces of neutron stars through the capture of hydrogen nuclei (protons). This occurs at extremely high temperatures, but at relatively low energies. An international ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.