  The precision of existing decay data of radionuclides for activity
determination is often a limitation for actual applications in science,
society, and industry. For this reason, the EMPIR project PrimA-LTD aims to
introduce an advanced primary activity standardization technique that is based
on magnetic microcalorimeters (MMCs) and that will offer very low energy
threshold of few eV and a decay scheme independent detection efficiency close
to 100%. As a proof of concept, we developed two MMC-based detector types in
order to standardize an $\alpha$-decaying, a $\beta$-decaying and an electron
capture decaying isotope. One detector type aims to introduce a reusable
detector setup, while the other aims to provide highly accurate decay spectra
by high resolution measurements with high statistics. We present the designs,
fabrication status and first characterization measurements of both detectors
types and outline next steps.
