The probability that a nucleus will absorb a neutron—the neutron capture cross-section—is important to many areas of nuclear science, including stellar nucleosynthesis, reactor performance, nuclear medicine and defence applications. Although neutron capture cross-sections have been measured for most stable nuclei, fewer results exist for radioactive isotopes, and statistical-model predictions typically have large uncertainties1. There are almost no nuclear data for neutron-induced reactions of the radioactive nucleus 88Zr, despite its importance as a diagnostic for nuclear security. Here, by exposing 88Zr to the intense neutron flux of a nuclear reactor, we determine that 88Zr has a thermal neutron capture cross-section of 861,000 ± 69,000 barns (1σ uncertainty), which is five orders of magnitude larger than the theoretically predicted value of 10 barns2. This is the second-largest thermal neutron capture cross-section ever measured and no other cross-section of comparable size has been discovered in the past 70 years. The only other nuclei known to have values greater than 105 barns3,4,5,6 are 135Xe (2.6 × 106 barns), a fission product that was first discovered as a poison in early reactors7,8, and 157Gd (2.5 × 105 barns), which is used as a detector material9,10, a burnable reactor poison11 and a potential medical neutron capture therapy agent12. In the case of 88Zr neutron capture, both the target and the product (89Zr) nuclei are radioactive and emit intense γ-rays upon decay, allowing sensitive detection of miniscule quantities of these radionuclides. This result suggests that as additional measurements with radioactive isotopes become feasible with the operation of new nuclear-science facilities, further surprises may be uncovered, with far-reaching implications for our understanding of neutron capture reactions.
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The data supporting the findings of this study are presented within this Letter and its Extended Data.
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We thank N. Gharibyan, K. Moody, P. Grant, R. Henderson, G. Severin, G. Peaslee and M. Stoyer for discussions. We thank T. Wooddy for nuclear counting support and P. Spackman for inductively coupled plasma mass-spectrometry analysis. We also thank the operators and radiation safety staff of the University of Alabama at Birmingham Cyclotron for assistance in 88Zr production and the irradiation services staff at MURR for experimental support at the reactor. This work was funded through LLNL LDRD 16-ERD-022 and was performed under the auspices of the US Department of Energy by LLNL under contract DE-AC52-07NA27344.
Nature thanks S. Heinitz, R. Rundberg and the other anonymous reviewer(s) for their contribution to the peer review of this work.