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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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.
The authors declare no competing financial interests.
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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Extended data figures and tables
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.
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.
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.
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.
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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
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