Isomer depletion as experimental evidence of nuclear excitation by electron capture

  • Nature volume 554, pages 216218 (08 February 2018)
  • doi:10.1038/nature25483
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The atomic nucleus and its electrons are often thought of as independent systems that are held together in the atom by their mutual attraction. Their interaction, however, leads to other important effects, such as providing an additional decay mode for excited nuclear states, whereby the nucleus releases energy by ejecting an atomic electron instead of by emitting a γ-ray. This ‘internal conversion’ has been known for about a hundred years and can be used to study nuclei and their interaction with their electrons1,2,3. In the inverse process—nuclear excitation by electron capture (NEEC)—a free electron is captured into an atomic vacancy and can excite the nucleus to a higher-energy state, provided that the kinetic energy of the free electron plus the magnitude of its binding energy once captured matches the nuclear energy difference between the two states. NEEC was predicted4 in 1976 and has not hitherto been observed5,6. Here we report evidence of NEEC in molybdenum-93 and determine the probability and cross-section for the process in a beam-based experimental scenario. Our results provide a standard for the assessment of theoretical models relevant to NEEC, which predict cross-sections that span many orders of magnitude. The greatest practical effect of the NEEC process may be on the survival of nuclei in stellar environments7, in which it could excite isomers (that is, long-lived nuclear states) to shorter-lived states. Such excitations may reduce the abundance of the isotope after its production. This is an example of ‘isomer depletion’, which has been investigated previously through other reactions8,9,10,11,12, but is used here to obtain evidence for NEEC.

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  1. 1.

    . (ed.) Internal Conversion Processes 1–13 (Academic Press Inc., 1966)

  2. 2.

    & Theoretical Nuclear Physics 614–622 (John Wiley & Sons, 1952)

  3. 3.

    , , , & Evaluation of theoretical conversion coefficients using BrIcc. Nucl. Instrum. Methods A 589, 202–229 (2008)

  4. 4.

    & On the excitation of isomeric nuclear levels by laser radiation through inverse internal electron conversion. Phys. Lett. B 62, 393–394 (1976)

  5. 5.

    ., ., ., & Nuclear excitation by electronic processes: NEEC and NEET effects. AIP Conf. Proc. 769, 1085–1088 (2005)

  6. 6.

    , & Isomer triggering via nuclear excitation by electron capture. Phys. Rev. Lett. 99, 172502 (2007)

  7. 7.

    & Enhanced nuclear level decay in hot dense plasmas. Phys. Rev. C 70, 064603 (2004)

  8. 8.

    et al. Photoactivation of 180Tam and its implications for the nucleosynthesis of nature’s rarest naturally occurring isotope. Phys. Rev. Lett. 83, 5242–5245 (1999)

  9. 9.

    . et al. Nuclear structure and depletion of nuclear isomers using electron linacs. AIP Conf. Proc. 1525, 586–594 (2013)

  10. 10.

    et al. Direct evidence for inelastic neutron “acceleration” by 177Lum. Phys. Rev. C 83, 064617 (2011)

  11. 11.

    & Cross section for inelastic neutron “acceleration” by 178Hfm2. Phys. Rev. C 83, 024604 (2011)

  12. 12.

    et al. Coulomb excitation of 68,70Cu: first use of postaccelerated isomeric beams. Phys. Rev. Lett. 98, 122701 (2007)

  13. 13.

    & Calculated yield of isomer depletion due to NEEC for 93mMo recoils. Phys. At. Nucl. 75, 1362–1367 (2012)

  14. 14.

    Nuclear data sheets for A = 93. Nucl. Data Sheets 112, 1163–1389 (2011)

  15. 15.

    & Improved charge-state formulas. Nucl. Instrum. Methods B 175–177, 125–131 (2001)

  16. 16.

    et al. Resonance conditions for 93mMo isomer depletion via nuclear excitation by electron capture in a beam-based scenario. Phys. Rev. C 95, 034312 (2017)

  17. 17.

    The Gammasphere. Nucl. Phys. A 520, c641–c655 (1990)

  18. 18.

    et al. A digital data acquisition system for the detectors at Gammasphere. In IEEE Nuclear Science Symposium and Medical Imaging Conference 1536–1540 (IEEE, 2012)

  19. 19.

    et al. Low-lying states near the Iπ = 6+ isomer in 108Ag. J. Phys. G 43, 015103 (2016)

  20. 20.

    , & A comment on “nuclear excitation by target electron capture”. Phys. Lett. A 152, 367–370 (1991)

  21. 21.

    & First-principles calculation of the cross sections for nuclear excitation by electron capture of channeled nuclei. Phys. Rev. C 47, 323–328 (1993)

  22. 22.

    , , & Dominant secondary nuclear photoexcitation with the x-ray free-electron laser. Phys. Rev. Lett. 112, 082501 (2014)

  23. 23.

    , , , & Direct and secondary nuclear excitation with x-ray free-electron lasers. Phys. Plasmas 22, 112706 (2015)

  24. 24.

    ., ., . & Tailoring laser-generated plasmas for efficient nuclear excitation by electron capture. Preprint at (2017)

  25. 25.

    Coupled reaction channels calculations in nuclear physics. Comput. Phys. Rep. 7, 167–212 (1988)

  26. 26.

    et al. GOSIA user manual for simulation and analysis of Coulomb excitation experiments, (2012)

  27. 27.

    & RACHEL graphical interface to GOSIA, (2017)

  28. 28.

    & Development of the program LISE: application to fusion-evaporation. Nucl. Instrum. Methods B 204, 174–178 (2003)

  29. 29.

    , & A target vacuum interlock system for Gammasphere. Nucl. Instrum. Methods A 607, 564–567 (2009)

  30. 30.

    Background subtraction from in-beam HPGe coincidence data sets. Nucl. Instrum. Methods A 361, 306–316 (1995)

  31. 31.

    , & Quantum interference between nuclear excitation by electron capture and radiative recombination. Phys. Rev. A 75, 012709 (2007)

  32. 32.

    et al. High-spin isomer in 93Mo. Eur. Phys. J. A 24, 249–257 (2005)

  33. 33.

    ., ., ., & Characteristics of the 21/2+ isomer in 93Mo: toward the possibility of enhanced nuclear isomer decay. Phys. Lett. B 696, 197–200 (2011)

  34. 34.

    Nuclear data sheets for A = 92. Nucl. Data Sheets 113, 2187–2389 (2012)

  35. 35.

    et al. (eds) Table of Isotopes 8th edn, Vol. II (John Wiley & Sons, 1996)

  36. 36.

    ESCL8R and LEVIT8R: software for interactive graphical analysis of HPGe coincidence data sets. Nucl. Instrum. Methods A 361, 297–305 (1995)

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C.J.C. and J.J.C. thank A. D. Ayangeakaa for input on the potential contributions of Coulomb excitations to the background and M. S. Litz and N. R. Pereira for discussions. We also thank J. Rohrer for assistance in setting up the Gammasphere experiment and the ATLAS operations staff for their efforts. This work was initiated under the US Army Research Laboratory (ARL) Director’s Research Initiative, award number DRI-FY14-SE-022. Further support was provided by ARL Cooperative Agreements W911NF-12-2-0019 and W911NF-16-2-0034, the US Department of Energy (DOE), Office of Science, Office of Nuclear Physics under contract number DE-AC02-06CH11357, the National Science Foundation under grant number PHY-1203100, the Australian Research Council under grant number FT100100991, and the Polish National Science Centre under grants 2011/01/D/ST2/01286 and 2017/25/B/ST2/00901. M.P., J.R. and A.B.H. received support through Ecopulse, Inc. under ARL contract number W911QX09D0016-0004. This research used resources of Argonne National Laboratory’s ATLAS facility, which is a DOE Office of Science User Facility, and of the HIAF at ANU.

Author information

Author notes

    • R. V. F. Janssens
    • , J. C. Marsh
    •  & S. Bottoni

    Present addresses: Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3255, USA and Triangle Universities Nuclear Laboratory, Duke University, Durham, North Carolina 27708-2308, USA (R.V.F.J.); Frontier Technology, Inc., Space and Naval Warfare Systems Command, Systems Center Pacific, 49599 Lassing Road, San Diego, California 92152, USA (J.C.M.); Dipartimento di Fisica, Università degli Studi di Milano and INFN sez. Milano, I-20133, Milano, Italy (S.B.).

    • S. A. Karamian



  1. Oak Ridge Associated Universities Fellowship Program, US Army Research Laboratory, Adelphi, Maryland 20783, USA

    • C. J. Chiara
    •  & J. C. Marsh
  2. US Army Research Laboratory, Adelphi, Maryland 20783, USA

    • J. J. Carroll
  3. Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

    • M. P. Carpenter
    • , J. P. Greene
    • , R. V. F. Janssens
    • , D. Seweryniak
    • , S. Zhu
    •  & S. Bottoni
  4. Department of Physics, US Naval Academy, Annapolis, Maryland 21402, USA

    • D. J. Hartley
  5. Department of Nuclear Physics, Research School of Physics and Engineering, Australian National University, Canberra, Australian Capital Territory 0200, Australia

    • G. J. Lane
  6. Defense Threat Reduction Agency, Fort Belvoir, Virginia 22060, USA

    • D. A. Matters
  7. Faculty of Chemistry, Nicolaus Copernicus University in Toruń, 87-100 Toruń, Poland

    • M. Polasik
  8. National Centre for Nuclear Research, 05-400 Otwock, Poland

    • J. Rzadkiewicz
  9. Institute of Optics, University of Rochester, Rochester, New York 14627, USA

  10. Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna 141980, Russia

    • A. B. Hayes
    •  & S. A. Karamian


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C.J.C. led the experimental effort. The experiment was conceptualized by S.A.K. and J.J.C., with the final design provided by C.J.C. and J.J.C. with input from D.J.H. and G.J.L. The targets were prepared by J.P.G. All authors, except S.B., A.B.H. and S.A.K., participated in the experiment. C.J.C. analysed the data, with substantial input from J.J.C. Guidance on the atomic conditions for NEEC was provided by M.P. and J.R. The calculations of inelastic-scattering cross-sections with FRESCO were performed by S.B. and those of Coulomb excitation with GOSIA by A.B.H. We wish to call attention to the role of our late colleague S.A.K., who provided the initial impetus for this work but sadly did not see these results.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to C. J. Chiara.

Reviewer Information Nature thanks O. Kocharovskaya and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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