Measurement of fundamental nuclear decay data using metallic magnetic calorimeters The overall objective of this project is to improve the knowledge of electron capture (EC) decay and subsequent atomic relaxation processes. New theoretical calculation techniques and extensive experiments using specially adapted metallic magnetic calorimeters (MMCs) will be developed to determine important decay data which are relevant when studying the influence of EC decay in cancer therapy on the DNA level or the early history of the solar system as well as for primary activity standardisations in radionuclide metrology. The experimental parts will be complemented with a new approach based on microwave coupled resonators (MCRs). The need Determining the age of the solar system or how cancer treatments damage DNA are two research areas that both rely on very precise nuclear decay probabilities produced by EC during radioactive decay. Atomic data for EC decays have been derived from measurements and calculations performed more than 20 years ago, and these are now causing significant measurement problems. Compiled data, for example, are based on the frozen core approximation with no explicit description of multiple ionisation processes. Therefore, accurate experimental X-ray emission intensities are needed to establish consistent EC decay schemes and theoretical models of subsequent relaxation processes. The precise knowledge of EC probabilities is pivotal when calculating the electron and photon emissions resulting from EC decay. The accurate knowledge of these emission spectra is a prerequisite for state-of-the art liquid scintillation counting (LSC) techniques which are frequently used for primary activity determination in radionuclide metrology. The uncertainties of fractional EC probabilities define the resulting uncertainty of the activity determined by LSC, e.g. the triple-to-double coincidence ratio (TDCR) method. A sound improvement of the quality of LSC measurements therefore requires improved computation methods of emission spectra which, in turn, can only be developed based on new theoretical approaches and experimentally determined EC probabilities of the highest achievable accuracy. In addition, X-ray emission intensities are key data used to quantify the activity of a radioactive material by X-ray spectrometry. Some of the technical developments are currently being carried out within the EMPIR project 15SIB10 MetroBeta for pure beta-emitting isotopes with endpoint energies in the range from 70 keV to 700 keV. To study EC decays and X-ray emissions, new developments for MMC-based techniques are required for high precision measurements with both internal and external sources.