Plutonium Analysis Advances

Researchers have detected the long-sought nuclear magnetic resonance signal of plutonium-239, one of the common radioactive elements used in nuclear fuel and weapons.

The discovery will allow scientists to use NMR to directly probe plutonium coordination, electronic structure, and nuclear spin relaxation processes in nuclear fuels and waste, as well as in plutonium-containing superconductors, says Melissa A. Denecke. She is the head of actinide speciation at Karlsruhe Institute of Technology’s Institute for Nuclear Waste Disposal, in Germany, and was not involved in the work.

²³⁹Pu has been difficult to study directly because of its radioactivity and complex oxidation chemistry. Additionally, its most common oxidation state, Pu(IV), is nonmagnetic, which means that techniques such as magnetic susceptibility or electron spin resonance yield little information. And NMR experiments are challenging because coupling between unpaired electrons and the nuclear spin leads to rapid relaxation and thus loss of signal.

The ²³⁹Pu signal was located by a group of researchers from Los Alamos National Laboratory (LANL) and Japan’s Atomic Energy Agency (JAEA). Led by LANL postdoctoral researcher Georgios Koutroulakis and JAEA scientific counselor Hiroshi Yasuoka, the group studied samples of ²³⁹PuO₂ that were cooled to 4 K to slow spin relaxation. In contrast to standard NMR experiments, in which the magnetic field is held constant while the radiowave frequency is varied, the researchers held the radio frequency constant and swept the magnetic field from 3 to 8 tesla. They found that the gyromagnetic ratio, which predicts where the signal will appear for a particular frequency and magnetic field combination, is 2.856 × 2π MHz/tesla for ²³⁹PuO₂.

Additional study of a mixed-oxide sample revealed two distinct NMR signals, indicating that ²³⁹Pu NMR should be sensitive to different chemical environments around the Pu atom. Koutroulakis notes, however, that fast relaxation times may still be a problem for other Pu complexes, even at 4 K.