Skip to main content

Thank you for visiting nature.com. 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.

Isomer depletion as experimental evidence of nuclear excitation by electron capture

Matters Arising to this article was published on 02 June 2021

Abstract

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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Figure 1: Relevant part of the 93Mo decay scheme.
Figure 2: Spectra demonstrating the signature of NEEC in 93Mo.
Figure 3: Spectra used to determine the NEEC probability in 93Mo.

References

  1. 1

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

  2. 2

    Blatt, J. M. & Weisskopf, V. F. Theoretical Nuclear Physics 614–622 (John Wiley & Sons, 1952)

  3. 3

    Kibédi, T., Burrows, T. W., Trzhaskovskaya, M. B., Davidson, P. M. & Nestor, C. W. Jr. Evaluation of theoretical conversion coefficients using BrIcc. Nucl. Instrum. Methods A 589, 202–229 (2008)

    ADS  Article  Google Scholar 

  4. 4

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

    ADS  Article  Google Scholar 

  5. 5

    Morel, P ., Daugas, J. M ., Gosselin, G ., Méot, V. & Gogny, D. Nuclear excitation by electronic processes: NEEC and NEET effects. AIP Conf. Proc. 769, 1085–1088 (2005)

    CAS  ADS  Article  Google Scholar 

  6. 6

    Pálffy, A., Evers, J. & Keitel, C. H. Isomer triggering via nuclear excitation by electron capture. Phys. Rev. Lett. 99, 172502 (2007)

    ADS  Article  Google Scholar 

  7. 7

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

    ADS  Article  Google Scholar 

  8. 8

    Belic, D. 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)

    CAS  ADS  Article  Google Scholar 

  9. 9

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

    CAS  ADS  Article  Google Scholar 

  10. 10

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

    ADS  Article  Google Scholar 

  11. 11

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

    ADS  Article  Google Scholar 

  12. 12

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

    CAS  ADS  Article  Google Scholar 

  13. 13

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

    CAS  Article  Google Scholar 

  14. 14

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

    CAS  ADS  Article  Google Scholar 

  15. 15

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

    ADS  Article  Google Scholar 

  16. 16

    Polasik, M. 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)

    ADS  Article  Google Scholar 

  17. 17

    Lee, I.-Y. The Gammasphere. Nucl. Phys. A 520, c641–c655 (1990)

    ADS  Article  Google Scholar 

  18. 18

    Anderson, J. T. 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)

    Google Scholar 

  19. 19

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

    ADS  Article  Google Scholar 

  20. 20

    Kimball, J. C., Bittel, D. & Cue, N. A comment on “nuclear excitation by target electron capture”. Phys. Lett. A 152, 367–370 (1991)

    CAS  ADS  Article  Google Scholar 

  21. 21

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

    CAS  ADS  Article  Google Scholar 

  22. 22

    Gunst, J., Litvinov, Y. A., Keitel, C. H. & Pálffy, A. Dominant secondary nuclear photoexcitation with the x-ray free-electron laser. Phys. Rev. Lett. 112, 082501 (2014)

    ADS  Article  Google Scholar 

  23. 23

    Gunst, J., Wu, Y., Kumar, N., Keitel, C. H. & Pálffy, A. Direct and secondary nuclear excitation with x-ray free-electron lasers. Phys. Plasmas 22, 112706 (2015)

    ADS  Article  Google Scholar 

  24. 24

    Wu, Y ., Gunst, J ., Keitel, C. H . & Pálffy, A. Tailoring laser-generated plasmas for efficient nuclear excitation by electron capture. Preprint at https://arxiv.org/abs/1708.04826 (2017)

  25. 25

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

    CAS  ADS  Article  Google Scholar 

  26. 26

    Cline, D. et al. GOSIA user manual for simulation and analysis of Coulomb excitation experiments, http://www.pas.rochester.edu/~cline/Gosia/Gosia_Manual_20120510.pdf (2012)

  27. 27

    Hayes, A. B. & Cline, D. RACHEL graphical interface to GOSIA, https://github.com/adamhayes/Rachel (2017)

  28. 28

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

    CAS  ADS  Article  Google Scholar 

  29. 29

    McCutchan, E. A., Lister, C. J. & Greene, J. P. A target vacuum interlock system for Gammasphere. Nucl. Instrum. Methods A 607, 564–567 (2009)

    CAS  ADS  Article  Google Scholar 

  30. 30

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

    CAS  ADS  Article  Google Scholar 

  31. 31

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

    ADS  Article  Google Scholar 

  32. 32

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

    CAS  ADS  Article  Google Scholar 

  33. 33

    Hasegawa, M ., Sun, Y ., Tazaki, S ., Kaneko, K. & Mizusaki, T. Characteristics of the 21/2+ isomer in 93Mo: toward the possibility of enhanced nuclear isomer decay. Phys. Lett. B 696, 197–200 (2011)

    CAS  ADS  Article  Google Scholar 

  34. 34

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

    CAS  ADS  Article  Google Scholar 

  35. 35

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

  36. 36

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

    CAS  ADS  Article  Google Scholar 

Download references

Acknowledgements

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

Affiliations

Authors

Contributions

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.

Corresponding author

Correspondence to C. J. Chiara.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Additional information

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

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Figure 1 Configuration of the target used in the experiment.

a, Schematic of the target construction showing the layers in which 93Mo production occurs (Li), NEEC can occur (C) and the backing that stops all recoils (208Pb), in addition to the important gap of about 3 mm that is needed to accommodate the effective half-life for the decay of the 4,900-keV level. Relative dimensions are not to scale. The beam is incident on the Li surface. b, Photograph of the target positioned inside the Gammasphere target chamber. The beam enters from the lower right side and is parallel to the double rods shown in the upper left part of the photograph.

Extended Data Figure 2 Spectra showing the line shape of the 2,475-keV transition in 93Mo.

a, b, The spectra are from the detectors in ring 9 of Gammasphere at 90° (a) and ring 7 at 79° (b). The spectra in red were recorded while using a Li target backed with 208Pb and with no gap in between. The blue spectra were obtained with a modified target configuration with a gap of about 3 mm. The 2,361-keV peak corresponds to a transition in 92Mo that lies below a level with a half-life of t1/2 = 35 ps. The similar line shapes of these two transitions support the estimate of a delay of tens of picoseconds in the 2,475-keV emission and therefore the rationale for the final target construction.

Extended Data Figure 3 Spectra used to determine background contributions.

Component spectra for the double gate on the Doppler-shifted 2,475-keV γ-ray (1) and the unshifted 1,478-keV γ-ray (2), where gates on the peak and background regions are denoted as ‘g’ and ‘b’, respectively (see text). a, g1g2. b, g1b2. c, b1g2. d, b1b2. Only those γ-rays in 93Mo relevant to the discussion are labelled, with the dashed lines marking their energies. We note that the 770-keV peak in a is a multiplet with the 773- and 777-keV transitions in 92Mo and 97Ru, respectively; only the last two peaks appear in b and d.

Extended Data Figure 4 Calculations of possible competing processes.

The inelastic-scattering cross-sections for exciting 93mMo to the intermediate state, calculated with the code FRESCO, are plotted versus the energy of recoiling 93Mo ions traversing the 7Li (blue) and 12C (red) target layers. The initial energy is the average recoil energy corresponding to 93Mo production at the centre of the Li target.

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Chiara, C., Carroll, J., Carpenter, M. et al. Isomer depletion as experimental evidence of nuclear excitation by electron capture. Nature 554, 216–218 (2018). https://doi.org/10.1038/nature25483

Download citation

Further reading

Comments

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.

Search

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