Plutonium-239 yields to nuclear magnetic resonance

Plutonium-239 yields to nuclear magnetic resonance. An appropriately tuned RF wave will excite a nucleus whose energy levels have been split by an externally applied magnetic field; that’s the basis for nuclear magnetic resonance (NMR). The precise excitation frequency gives information about the nucleus’s local magnetic environment, which perturbs the field experienced by the nucleus. For more than 50 years, ²³⁹Pu stymied efforts to probe it with NMR, but Hiroshi Yasuoka and colleagues at the Japan Atomic Energy Agency and Los Alamos National Laboratory have now coaxed a signal from the iconic nucleus. The group had to overcome two principal difficulties. First, the hyperfine interaction between electron and nuclear spins is unusually strong in ²³⁹Pu, which means the NMR signal decays extremely quickly. Second, the gyromagnetic ratio γ of ²³⁹Pu, which relates external field and resonance frequency, was poorly known; the Yasuoka team had only a rough idea where to look for the resonance. The first problem was solved by preparing the oxide PuO₂ (see the figure) and maintaining it at 4 K to force the Pu⁴⁺ ions into their electronic ground state. That state is nonmagnetic, so there’s no hyperfine interaction. The second problem was solved by care and patience; the group set a specific radio frequency and swept the magnetic field from 3 to 8 tesla. Once they found the resonance, experimental refinements yielded a precise value for γ. The group also observed two distinct signals for a sample in which the ²³⁹Pu exists in two oxidation states. That environmental sensitivity, say Yasuoka and company, could be particularly useful for understanding the consequences of long-term ²³⁹Pu storage. (H. Yasuoka et al., Science 336, 901, 2012.)