High-resolution monochromator for the 8.41 keV 169 Tm nuclear resonance

Rare-earth elements are key components in a wide range of functional materials, including solid-state lasers, superconductors and high-performance permanent magnets, owing to their localized 4 f electrons that give rise to distinctive electronic and magnetic properties. Nuclear resonant scattering of synchrotron radiation using Mössbauer isotopes provides a powerful isotope-specific probe of these properties, enabling access to vibrational dynamics as well as electronic and magnetic hyperfine interactions. Its application, however, requires dedicated high-resolution monochromators tailored to the ultra-narrow nuclear resonances of individual Mössbauer isotopes. The 8.41 keV nuclear resonance of 169 Tm was among the first resonances excited at synchrotron sources, but has not been revisited since the early pioneering experiments. Here, a narrow-band high-resolution monochromator optimized for the 169 Tm nuclear resonance at the High-Resolution Dynamics Beamline P01 of PETRA III (DESY, Hamburg) is presented. The monochromator provides a tunable energy resolution between 1.4 and 0.33 meV. Its performance is demonstrated by nuclear forward scattering and nuclear inelastic scattering measurements on Tm 2 O 3 and metallic Tm, allowing the determination of hyperfine parameters and the phonon density of states, and derived thermodynamic quantities. The instrument establishes 169 Tm nuclear resonant scattering as a probe of vibrational, electronic and magnetic properties in Tm-based materials.