A Surprise for Nuclear Chemists

An exploding supernova

In nuclear reactors as well certain stars and supernovae — places where there are many free neutrons around — an atom’s nucleus and a free neutron can merge. An isotope’s ability to do this is measured by its neutron capture cross section. A high number means the isotope is good at taking neutrons in. Scientists use cross-section numbers in areas including astrophysics, nuclear power, and defense applications.
A team of scientists including Professor Jennifer Shusterman (GC/Hunter College, Chemistry) recently measured the neutron capture cross section for the isotope zirconium-88 (88Zr). The resulting number was 100,000 larger than expected.
“There is nothing to make you think it would be very large,” Shusterman says. “Scientists have found three cross sections on this scale — it's a rare occurrence. People have postulated about the statistical nature of these occurrences.”
Shusterman is the lead author on the paper, which appears in Nature.
Neutron capture is responsible for producing atoms of elements heavier than iron inside some types of stars and supernovae. Knowing cross sections of different isotopes helps astrophysicists unravel exactly how these elements came to be. Since the U.S. stopped full-scale nuclear weapons tests, scientists have also used cross sections in calculations to monitor the safety and efficacy of the country’s stockpile.
In nuclear reactors, which help provide 20 percent of the electricity in the U.S. and produce nuclides for medical use, isotopes that are good at capturing neutrons can be both beneficial and detrimental. They use up neutrons that could otherwise be involved in nuclear fission, which can poison a reactor. On the other hand, species that do this well can be used to control these reactions.
“Neutron capture cross sections are important for various areas of nuclear science, from astrophysics to nuclear energy.” Shusterman says. “The results suggest that there are potentially more of these cross sections that we haven’t found, and that we should continue to measure more.
If researchers continue to find more surprise cross sections, it could change the way scientists think about neutron capture reactions.
“Scientists have measured relatively few cross sections of radioactive nuclei because they present additional challenges,” Shusterman says, “but there are new facilities coming online that will make this easier.”
The high number is partly mystifying, Shusterman says, because the isotope has an even number of both neutrons and protons in its nucleus. “You would expect this if you had an odd number of neutrons, because an even-even nucleus is more stable.”
Shusterman’s collaborators in the study included researchers from Lawrence Livermore National Laboratory, where she completed her postdoctoral studies, and the University of California, Berkeley, where Shusterman received her Ph.D.
“I wasn't introduced to nuclear or radiochemistry in undergrad classes,” Shusterman says, “but during my undergrad I attended a nuclear chemistry summer school and found the field fascinating.”

Submitted on: FEB 6, 2019

Category: Chemistry | Faculty | General GC News