Transition-edge sensors operating just above absolute zero measured the fundamental radiation linewidths of uranium, neptunium and plutonium with relative uncertainties of 0.5% to 1.5%. Results three to eight times more precise than earlier values reduce systematic error from overlapping radiation in gamma-ray assays of plutonium-bearing materials and provide a tighter test for theoretical atomic calculations.
Transition-edge sensors are superconducting films held a tiny fraction of a degree above absolute zero. When a high-energy photon hits the film, its deposited heat sharply raises electrical resistance, with the increase proportional to the photon's energy. Arrays of these detectors separated the fundamental lines of uranium, neptunium and plutonium in finer detail than earlier measurements.[1], [2]
The peer-reviewed paper reports natural linewidths with relative uncertainties of 0.5% to 1.5%, making the measurements three to eight times more precise than previous values. Researchers also compared the experimental results with multiconfiguration Dirac-Fock calculations. The improved accuracy exposed a discrepancy between theory and experiment that had not been resolvable before, creating a new benchmark for atomic calculations.[2], [1]
Gamma-ray patterns used to identify and account for nuclear material can overlap with K-alpha X-rays in the same energy range. Knowing the linewidths more precisely helps analysts subtract that background and reduce systematic error, particularly in assays of plutonium-bearing material. NIST says detectors are installed at three Department of Energy laboratories, while the refrigeration hardware remains too bulky for a handheld instrument.[1], [2]
