The surprisingly large neutron capture cross-section of 88Zr

Abstract

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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Data availability

The data supporting the findings of this study are presented within this Letter and its Extended Data.

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Acknowledgements

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.

Reviewer information

Nature thanks S. Heinitz, R. Rundberg and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Authors

Contributions

J.A.S., N.D.S. and E.B.N. prepared most of the manuscript. J.A.S. performed the chemistry, target preparations, nuclear counting and data analysis. C.S.L. and S.E.L. performed the irradiation of the Y-sputtered Nb. N.J.P. carried out MCNP modelling. J.D.R. helped with the irradiation at MURR. K.J.T. did nuclear counting, reduction of γ-ray spectra and assisted with data analysis. J.A.S., E.B.N., N.D.S., K.J.T., A.P.T. and D.A.S. contributed to discussions on the results and interpretation of the data. All authors commented on the manuscript.

Corresponding author

Correspondence to Jennifer A. Shusterman.

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Extended data figures and tables

Extended Data Fig. 1 Neutron energy spectrum calculated using MCNP5.

The spectrum corresponds to the location where the samples were irradiated in the graphite reflector at MURR (position G1). The average thermal and resonance-region neutron fluxes were measured to be 7.1 × 1013 n cm−2 s−1 and 2.7 × 1012 n cm−2 s−1, respectively. The flux of neutrons with energies above 12.5 MeV was determined using the monitor reaction 90Zr(n,2n)89Zr to be 4.5 × 109 n cm−2 s−1.

Extended Data Fig. 2 Example γ-ray spectra for a 88Zr sample, collected before and after neutron irradiation.

a, Prior to irradiation, the γ-ray spectrum is dominated by the 393-keV line from the decay of 88Zr accompanied by its daughter, 88Y, which is formed between stock preparation and target encapsulation. b, The same sample, counted seven days after the 10.35-h irradiation at MURR, after dissolution. Following irradiation, there is a considerable amount of 89Zr present, whereas the 88Zr activity has decreased, indicating conversion of 88Zr to 89Zr. The other small γ-ray lines in a are from 56Co impurities and in b are from activation products (82Br and 187W) of impurities present in the sample.

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Shusterman, J.A., Scielzo, N.D., Thomas, K.J. et al. The surprisingly large neutron capture cross-section of 88Zr. Nature 565, 328–330 (2019). https://doi.org/10.1038/s41586-018-0838-z

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