High-purity X-ray polarimetry at high energies for Mössbauer spectroscopy

High-purity X-ray polarimetry has so far been established only in the low-energy range up to 14.4 keV and has therefore been applied exclusively to the Mössbauer isotope ⁵⁷Fe. Here we extend high-purity X-ray polarimetry to photon energies between 15 and 30 keV. Using asymmetric channel-cut Si crystals operated close to the Brewster angle, we design and realize a polarimeter optimized for synchrotron-based Mössbauer spectroscopy in this range of transition energies. As a demonstration, we implement a four-bounce Si (5 7 11) polarimeter at a photon energy of 22.494 keV at PETRA III and achieve a polarization purity of (1.2 ± 0.5) × 10^-8. This establishes the polarimeter performance required for synchrotron Mössbauer spectroscopy at this energy and shows, supported by dynamical-diffraction calculations, that the method can be extended to nuclear transitions up to about 30 keV-opening access to isotopes such as ¹⁵¹Eu, ¹⁴⁹Sm, ¹¹⁹Sn, and ¹⁶¹Dy. A nuclear-resonant measurement on these isotopes is the natural next step at a dedicated high-brilliance beamline.