We present measurements of the $\mathrm{K}{\alpha }_1$ and $\mathrm{K}{\alpha }_2$ natural x-ray linewidths of uranium, neptunium, and plutonium ($\mathrm{Z}=92$, 93, 94) obtained using a superconducting transition-edge sensor microcalorimeter array. The relative uncertainties range from 0.5% to 1.5%, improving upon previous values by factors ranging from 3 to 8. These refined linewidths reduce systematic errors in gamma-ray spectroscopy assays of Pu-bearing materials, where $\mathrm{K}\alpha$ x-rays overlap with diagnostic gamma lines. We also present multiconfiguration Dirac-Fock calculations of the $\mathrm{K}\alpha$ linewidths for U, Np, and Pu, providing a benchmark for theory and experiment. These calculations reveal a discrepancy between ab initio theory and experiment possible to observe only with the improved experimental accuracy achieved in this work.

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• Atomic & molecular structure
• Atomic spectra
• Energy levels
• Lifetimes & widths
• Nuclear decay
• First-principles calculations
• Hartree-Fock methods
• X-ray emission spectroscopy