The specific objectives of the project are: To improve experimental techniques for spectrometry using novel cryogenic detectors based on MMCs and MCRs for radionuclide metrology in the energy range of 20 eV – 100 keV. To determine fractional EC probabilities of selected radionuclides by means of spectrometry based on novel cryogenic detectors with high energy resolution and very low energy threshold using sources embedded in the detector absorber. To measure absolute X-ray emission intensities of selected radionuclides by using a combination of high-resolution spectrometry based on novel cryogenic detectors using external sources and accurate primary activity determination. To improve theoretical models and ab initio calculations of the EC process and subsequent atomic relaxation and to validate them with the high-precision experimental data from this project. To facilitate the take-up of the technology and measurement infrastructure developed in the project by radionuclide metrologists, nuclear physicists and other researchers.