This Isotope Grabs A Lot of Neutrons
Out in the cosmos, certain stars and supernovas forge elements heavier than iron through radioactive decay and a process called neutron capture. The process works just as it sounds: An atom’s nucleus merges with a free neutron.
An atom’s tendency to absorb new neutrons is measured by a number called its “neutron capture cross-section.” Back on Earth, scientists use cross-sections as a tool in nuclear medicine, power, and defense applications.
Scientists recently measured the neutron capture cross-section of the zirconium-88 isotope, and found it to be 100,000 times larger than expected. Professor Jennifer Shusterman (Hunter College and The Graduate Center, CUNY) was lead author on the paper, which appears in the journal Nature.
About 20 percent of electricity in the U.S. comes from nuclear power plants, and reactors also generate isotopes used in medicine. Isotopes with large cross-sections can disrupt nuclear fission, but they can also help control reactivity. Since the U.S. stopped full-scale nuclear weapons tests, scientists have used cross-sections in calculations that track the safety and efficacy of the country’s stockpile.
For an astrophysicist, these numbers can help figure out the origins of heavy elements formed within stars.
Researchers expected a smaller cross-section for Zr-88 partly because it has an even number of neutrons and protons in its nucleus. Isotopes with this property often have smaller cross-sections than those with odd numbers. “There is nothing to make you think it would be very large,” Shusterman said. “Scientists have found three cross-sections on this scale — it’s a rare occurrence. People have postulated about the statistical nature of these occurrences.”
If researchers continue to find more surprise cross-sections, it could change the way scientists think about these reactions. “The results suggest that there are potentially more of these that we haven’t found, and that we should continue to measure more,” Shusterman said.