Identification of radionuclides in mixtures, and their accurate quantification (in units of Bq) through full characterization of each decay, remains an open challenge that must be met to support the increasing uses of radioactivity in medicine and to meet demands in manufacturing, health care, and nuclear security applications. No instrument today directly measures all decays in a sample with sufficient energy resolution to uniquely identify each radionuclide. NIST is developing a 4-pi decay-energy spectrometery (DES) system using ultra-low temperature Transition Edge Sensor (TES) micro-calorimeters to identify and quantify radionuclide samples embedded in the detector. The research associate would primarily work on commissioning a DES system using TES-based detectors being made in the NIST microelectronics cleanroom facility. The associate would have the opportunity to work on a collaboration with experts across multiple Laboratories at NIST involving detector-response modelling, next-generation TES sensor design, and quantitative sample-preparation methods.