Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Observation of the competitive double-gamma nuclear decay


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+ (refs 1, 2, 3). 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 susceptibilities3.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: The two main experimental obstacles to measuring the γγ/γ-decay.
Figure 2: Energy-sum spectrum and energy-gated time spectra of the 72°-group.
Figure 3: Energy-sum spectrum of the 144°-group.
Figure 4: Measured energy spectrum and angular correlation.


  1. Watson, B. A., Bardin, T. T., Becker, J. A. & Fisher, T. R. Two-photon decay of the 6.05-MeV state of 16O. Phys. Rev. Lett. 35, 1333–1336 (1975)

    Article  CAS  ADS  Google Scholar 

  2. Schirmer, J. et al. Double gamma decay in 40Ca and 90Zr. Phys. Rev. Lett. 53, 1897–1900 (1984)

    Article  CAS  ADS  Google Scholar 

  3. Kramp, J. et al. Nuclear two-photon decay in 0+ → 0+ transitions. Nucl. Phys. A 474, 412–450 (1987)

    Article  ADS  Google Scholar 

  4. Mokler, P. H. & Dunford, R. W. Two-photon decay in heavy atoms and ions. Phys. Scr. 69, C1–C9 (2004)

    Article  CAS  Google Scholar 

  5. Ilakovac, K., Uroic, M., Majer, M., Pasic, S. & Vukovic, B. Two-photon decay of k-shell vacancy states in heavy atoms. Radiat. Phys. Chem. 75, 1451–1460 (2006)

    Article  CAS  ADS  Google Scholar 

  6. Göppert, M. Über die Wahrscheinlichkeit des Zusammenwirkens zweier Lichtquanten in einem Elementarakt. Naturwissenschaften 17, 932 (1929)

    Article  ADS  Google Scholar 

  7. Göppert-Mayer, M. Über Elementarakte mit zwei Quantensprüngen. Ann. Phys. 401, 273–294 (1931)

    Article  Google Scholar 

  8. Göppert-Mayer, M. Double beta-disintegration. Phys. Rev. 48, 512–516 (1935)

    Article  ADS  Google Scholar 

  9. Hayes, A. C. et al. Two-photon decay of the first excited 0+ state in 16O. Phys. Rev. C 41, 1727–1735 (1990)

    Article  CAS  ADS  Google Scholar 

  10. Beusch, W. Über Zweiquanten-Übergänge an Ba137. Helv. Phys. Acta 33, 363–394 (1960)

    CAS  Google Scholar 

  11. Basenko, V. K., Berlizov, A. N. & Prokopets, G. A. Estimation of the probability of two-photon decay of the 0.662 MeV 137Ba state. Bull. Russ. Acad. Sci. Phys. 56, 94 (1992)

    Google Scholar 

  12. Millener, D. J., Sutter, R. J. & Alburger, D. E. 2-gamma decay of the 662-keV isomer in 137Ba. Bull. Am. Phys. Soc. 56, DNP.CF.8 (2011); available at (2011)

  13. Lister, C. J. et al. A search for 2-photon emission from the 662 keV state in 137Ba. Bull. Am. Phys. Soc. 58, DNP.CE.3 (2013); available at (2013)

  14. Moran, K. et al. E5 decay from the Jπ = 11/2 isomer in 137Ba. Phys. Rev. C 90, 041303 (2014)

    Article  ADS  Google Scholar 

  15. Friar, J. L. Low-energy theorems for nuclear Compton and Raman scattering and 0+ → 0+ two-photon decays in nuclei. Ann. Phys. 95, 170–201 (1975)

    Article  CAS  ADS  Google Scholar 

  16. Soloviev, V. G. Theory of Atomic Nuclei: Quasiparticles and Phonons (Institute of Physics Publishing, Bristol, 1992)

    Google Scholar 

Download references


This work was supported by the State of Hesse under the Helmholtz International Center for FAIR (HIC for FAIR) and by the German Research Council (DFG) under grant no. SFB 634. We thank D. J. Millener, R. J. Sutter for an open discussion and for generously sharing their preliminary results before publication, and C. J. Lister for discussions. H.S. thanks Dirk Schwalm for raising interest in this topic.

Author information

Authors and Affiliations



C.W. and H.S. performed the data analysis and derived the equations given in Supplementary Information. C.W., H.S. and N.P. contributed to the interpretation of the results. C.W. and R.L. were responsible for the set-up of the LaBr3:Ce detector array and the data acquisition system. V.Yu.P. performed the QPM calculation. C.W., H.S., N.P. and T.A. prepared the manuscript. All authors discussed the results, commented on and contributed to the manuscript.

Corresponding author

Correspondence to H. Scheit.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Text and Data, Supplementary Figure 1, Supplementary Tables 1-4 and Supplementary References. (PDF 273 kb)

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Walz, C., Scheit, H., Pietralla, N. et al. Observation of the competitive double-gamma nuclear decay. Nature 526, 406–409 (2015).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing