Revelations about radioactive zirconium forge a new path in research about the universe

Findings from MU study could hint at a wealth of neglected potential in radioactive chemical isotopes

April 29th, 2019

COLUMBIA, Mo. – Scientists have many “road maps” that guide them in respective fields of research by consolidating countless years of knowledge into a useful resource. The periodic table of elements, for example, serves as an important guide to understanding the relationships between elements and their roles in forming the universe. But imagine if one of these translational guides to nature’s elements was upended, revolutionizing our knowledge of the world at the atomic level. At the University of Missouri, researchers have done just that by revealing the largely unexplored potential of radioactive isotopes.

Knowing the probability of a chemical isotope’s nucleus “capturing” or absorbing a neutron provides researchers with fundamental information at the core of a vast number of scientific disciplines, from nuclear safety to understanding the origins of the cosmos. But this probability —known as a neutron capture cross section — is unknown for all but a few radioactive isotopes. In the case of one such isotope of zirconium, ⁸⁸Zr, researchers have found that its cross section is the highest to be discovered in the last 70 years, a finding that suggests the many other radioactive isotopes that have yet to be similarly measured are potential sources of breakthroughs in our knowledge of the origins and composition of the universe.

“Like the use of the periodic table to predict chemical reactions, we use the number of neutrons and protons in a nucleus to predict nuclear reactions,” said J. David Robertson, director of the MU Research Reactor. “Changes in that information are understandably monumental, and discovering one of the largest neutron capture cross sections from an unassuming zirconium isotope underlines the necessity of measuring more radioisotopes in this way. This isn’t just about the safety of nuclear reactions. It’s about understanding how the elements of our universe fit together and interact with each other. The question is: if we didn’t know how extraordinary ⁸⁸Zr is, what else don’t we know?”

Robertson and his colleagues faced several challenges in performing the cross section measurements due to the isotope’s radioactivity, a stumbling block that Robertson said has likely impeded similar measurements of this type of chemical. The radioactive isotope had to be produced, handled and purified with caution, with only small quantities surviving this process. The samples were then irradiated in the nuclear reactor. As they decayed they left behind a unique signature that allowed researchers to measure the neutron capture. The unexpectedly large measurement will radically change how scientists view ⁸⁸Zr and other radioactive isotopes, according to Robertson.

“An unexpected neutron capture cross section can completely change the way a radioactive isotope is used in a nuclear reaction,” Robertson said. “For example, isotopes with higher cross sections diminish the performance of a reactor, and these can be used intentionally to control a reaction. ⁸⁸Zr was never considered for this application until now. We can only wonder what we will learn as we measure the other radioactive isotopes.”

The study, “The surprisingly large neutron capture cross-section of ⁸⁸Zr,” was published in Nature. Other researchers involved in the study were: Nickie Peters of the University of Missouri; lead author Jennifer Shusterman, Keenan Thomas, Dawn Shaughnessy and Anton Tonchev of the Lawrence Livermore National Laboratory; Eric Norman of the University of California, Berkeley; and Suzanne Lapi and C. Shaun Loveless of the University of Alabama at Birmingham. Funding was provided by the U.S. Department of Energy (LLNL LDRD 16-ERD-022).