225Ac is a valuable medical radionuclide for targeted alpha therapy, but 227Ac is an undesirable byproduct of an accelerator-based synthesis method under investigation. Sufficient detector sensitivity is critical for quantifying the trace impurity of 227Ac, with the 227Ac/225Ac activity ratio predicted to be approximately 0.15% by end-of-bombardment (EOB). Superconducting transition edge sensor (TES) microcalorimeters offer high resolution energy spectroscopy using the normal-to-superconducting phase transition to measure small change in temperature. By embedding 225Ac production samples in a gold foil thermally coupled to a TES microcalorimeter we can measure the decay energies of the radionuclides embedded with high resolution and efficiency. This technique, known as decay energy spectroscopy (DES), collapses several peaks from alpha decays into single Q-value peaks. In practice there are more complex factors in the interpretation of data using DES, which we will discuss herein. Using this technique we measured the EOB 227Ac impurity to be (0.142 +/- 0.005)% for a single production sample. This demonstration has shown that DES can distinguish closely related isotopic features and is a useful tool for quantitative measures.
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