  Decay energy spectrometry (DES) is a novel radiometric technique for
high-precision analysis of nuclear materials. DES employs the unique thermal
detection physics of cryogenic microcalorimeters with ultra-high energy
resolution and 100$\%$ detection efficiency to accomplish high precision decay
energy measurements. Low-activity nuclear samples of 1 Bq or less, and without
chemical separation, are used to provide elemental and isotopic compositions in
a single measurement. Isotopic ratio precisions of 1 ppm - 1,000 ppm (isotope
dependent), which is close to that of the mass spectrometry, have been
demonstrated in 12-hour DES measurements of ~5 Bq samples of certified
reference materials of uranium (U) and plutonium (Pu). DES has very different
systematic biases and uncertainties, as well as different sensitivities to
nuclides, compared to mass-spectrometry techniques. Therefore, the accuracy and
confidence of nuclear material assays can be improved by combining this new
technique with existing mass-spectrometry techniques. Commercial-level DES
techniques and equipment are being developed for the implementation of DES at
the Nuclear Material Laboratory (NML) of International Atomic Energy Agency
(IAEA) to provide complementary measurements to the existing technologies. The
paper describes details of DES measurement methods, as well as DES precision
and accuracy to U and Pu standard sources to discuss its capability in analysis
of nuclear safeguards samples.
