Measured and Theoretical Kα X-Ray Emission Linewidths of U, Np, and Pu

We present measurements of the Kα_{1} and Kα_{2} natural x-ray linewidths of uranium, neptunium, and plutonium (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 Kα x-rays overlap with diagnostic gamma lines. We also present multiconfiguration Dirac-Fock calculations of the Kα 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.