  Advances in both instrumentation and data analysis software are now enabling
the first ultra-high-resolution microcalorimeter gamma spectrometers designed
for implementation in nuclear facilities and analytical laboratories. With
approximately ten times better energy resolution than high-purity germanium
detectors, these instruments can overcome important uncertainty limits.
Microcalorimeter gamma spectroscopy is intended to provide nondestructive
isotopic analysis capabilities with sufficient precision and accuracy to reduce
the need for sampling, chemical separations, and mass spectrometry to meet
safeguards and security goals. Key milestones were the development of the SOFIA
instrument (Spectrometer Optimized for Facility Integrated Applications) and
the SAPPY software (Spectral Analysis Program in PYthon). SOFIA is a compact
instrument that combines advances in large multiplexed transition-edge sensor
arrays with optimized cryogenic performance to overcome many practical
limitations of previous systems. With a 256-pixel multiplexed detector array
capable of 5000 counts per second, measurement time can be comparable to
high-purity germanium detectors. SAPPY was developed to determine isotopic
ratios in data from SOFIA and other microcalorimeter instruments with an
approach similar to the widely-used FRAM software. SAPPY provides a flexible
framework with rigorous uncertainty analysis for both microcalorimeter and HPGe
data, allowing direct comparison. We present current results from the SOFIA
instrument, preliminary isotopic analysis using SAPPY, and describe how the
technology is being used to explore uncertainty limits of nondestructive
isotopic characterization, inform safeguards models, and extract improved
nuclear data including gamma-ray branching ratios.
