The nuclear resonances of Mössbauer isotopes constitute extremely narrow energy references for high-resolution
spectroscopy, especially in the field of extreme metrology with applications to probe fundamental physics. Ultranarrow
nuclear transitions are particularly suited for such applications due to their inherent insensitivity to external perturbations
as compared to electronic levels. In this experiment we want to access for the first time the 12.4 keV nuclear
resonance of the isotope 45-Sc which exhibits a natural linewidth of 1.4 femto-eV, corresponding to a natural lifetime of
500 ms. With these values, 45-Sc surpasses all measurable Mössbauer transitions by orders of magnitude, rendering it
superior in some aspects to the 229-Th isotope. Observation of the nuclear resonance of 45-Sc via excitation from the
nuclear ground state is now within reach at the European XFEL. This will open up a new era of ultra-high precision
spectroscopy in the regime of hard x-rays