1.3 Observation of a Very Weak Uranium NMR (Nuclear Magnetic Resonance) Spectrum

Fig. 1-5 Principle of 235U nuclear magnetic resonance (NMR)
Nuclei having nuclear spin absorb microwaves in a magnetic field, the frequency of the microwaves depending on the magnetic field strength. The magnetic field is applied externally as in usual NMR experiments. In our 235U experiments, however, an internal, local magnetic field due to the surrounding electrons is used for the observation of microwave absorption.

Fig. 1-6 235U NMR signal observed in the 150-250 MHz region
The energy state of 235U (nuclear spin: I=7/2) splits into 8 (=2I+1) levels in an applied magnetic field. Resonant absorption of microwaves occurs between adjacent levels. Their frequency appears in seven lines due to the gradient at the 235U nuclei in electric field made by 5f-electrons.

Nuclear magnetic resonance (NMR) spectroscopy, which observes microwave absorption by nuclei having nuclear spin in an applied magnetic field, provides useful information on the local electronic configuration around the nuclei.
Actinide elements with 5f-electrons reveal various unusual electric and magnetic properties, e.g. anisotropic superconductivity and have now become widely studied. The NMR experiments have been difficult because the 238U nuclei, which amount to 99.3% of natural uranium, have no nuclear spin and the 235U nuclei with nuclear spin of 7/2 have only a poor natural abundance of 0.7% and also a weak sensitivity.
We have prepared a highly (93%) 235U enriched and sintered sample (1 g) of UO2 having its exact stoichiometry (U/O=1/2). It is known that UO2 below 30.8 K (Neel temperature) is in the antiferromagnetic state where magnetic moments of neighboring uranium atoms are aligned antiparallel. The NMR experiments in the antiferromagnetic states were carried out at 1.5 K and seven resonance lines, as expected from the 235U nuclear spin, have been clearly observed.
As NMR spectroscopy gives us much information about the environment at the probe nucleus, it is expected to become a powerful tool in the material science of actinide compounds.

Reference
K. Ikushima et al., Observation of 235U NMR in the Antiferromagnetic State of UO2, J. Phys. Soc. Jpn., 67 (1), 65 (1998).

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