  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.
