Why do Xe-135 and Zr-88 have surprisingly large thermal neutron capture cross-sections?
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 defense 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. There is 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 $861,000 \pm 69,000$ barns ($1\sigma$ uncertainty), which is five orders of magnitude larger than the theoretically predicted value of $10$ barns. 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 are $^{135}Xe$ ($2.6 \times 10^6$ barns), a fission product that was first discovered as a poison in early reactors, and $^{157}Gd$ ($2.5 × 10^5$ barns).
My question is why do these isotopes have such a large thermal neutron capture cross-section? What is the reason behind this?
