The surprisingly large neutron capture cross-section of 88 Zr

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 uncertainties 1 . There are almost no nuclear data for neutron-induced reactions of the radioactive nucleus 88 Zr, despite its importance as a diagnostic for nuclear security. Here, by exposing 88 Zr to the intense neutron flux of a nuclear reactor, we determine that 88 Zr has a thermal neutron capture cross-section of 861,000 ± 69,000 barns (1 σ uncertainty), which is five orders of magnitude larger than the theoretically predicted value of 10 barns 2 . 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 10 5 barns 3 – 6 are 135 Xe (2.6 × 10 6 barns), a fission product that was first discovered as a poison in early reactors 7 , 8 , and 157 Gd (2.5 × 10 5 barns), which is used as a detector material 9 , 10 , a burnable reactor poison 11 and a potential medical neutron capture therapy agent 12 . In the case of 88 Zr neutron capture, both the target and the product ( 89 Zr) 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.