1 Historical Developments and Future Perspectives …
33
Fig. 1.12 Left panel: nested design of a four-bounce channel-cut HRM. Right panel: doubleresolution monochromator with high-energy resolution and optimized intensity: the two first crystals
in asymmetric reflections provide in two steps extreme angular collimation of the incident radiation.
The two subsequent reflections with the opposite indexes of asymmetry serve as two-step energy
analyzer
mechanics. This solution allows especially in NIS for a trade-off between energy
resolution and flux.
1.6.1.2 Backscattering Monochromator
For higher energies these silicon HRMs are no longer efficient and backscattering
monochromators may be an alternative. The classical approach with a silicon crystal [98] does not work for NRS due to the fixed energies given by the Mössbauer
levels. Other materials have to be exploited such as sapphire [21]. However, the
needed crystal’s quality is still lacking [99].
1.6.1.3 Synchrotron Mössbauer Source
For extreme monochromatization, the SMS provides a fully resonant, polarized, and
collimated γ -ray beam with an energy resolution in the neV regime at 14.4 keV [54].
Currently, SMSs are installed at SPring-8 [56] and the ESRF [57] (see Fig. 1.13
as an example at the ESRF). Details of the SMS, theoretical and technical, are laid
down in several articles [52, 54, 57, 58, 100–102].
The key element of the SMS is an iron borate
57 FeBO 3 single crystal enriched
in the
57 Fe isotope up to 95%. Iron borate is a canted antiferromagnet with a Néel
temperature of 348.35 K. The crystal is set to electronically forbidden but nuclear
allowed [111]- or [333]-reflections to extract the purely scattered nuclear signal.
At room temperature
57 FeBO 3 shows in this diffraction geometry with polarized
SR a four line spectrum due to hyperfine splitting of the nuclear levels with a flux
33
Fig. 1.12 Left panel: nested design of a four-bounce channel-cut HRM. Right panel: doubleresolution monochromator with high-energy resolution and optimized intensity: the two first crystals
in asymmetric reflections provide in two steps extreme angular collimation of the incident radiation.
The two subsequent reflections with the opposite indexes of asymmetry serve as two-step energy
analyzer
mechanics. This solution allows especially in NIS for a trade-off between energy
resolution and flux.
1.6.1.2 Backscattering Monochromator
For higher energies these silicon HRMs are no longer efficient and backscattering
monochromators may be an alternative. The classical approach with a silicon crystal [98] does not work for NRS due to the fixed energies given by the Mössbauer
levels. Other materials have to be exploited such as sapphire [21]. However, the
needed crystal’s quality is still lacking [99].
1.6.1.3 Synchrotron Mössbauer Source
For extreme monochromatization, the SMS provides a fully resonant, polarized, and
collimated γ -ray beam with an energy resolution in the neV regime at 14.4 keV [54].
Currently, SMSs are installed at SPring-8 [56] and the ESRF [57] (see Fig. 1.13
as an example at the ESRF). Details of the SMS, theoretical and technical, are laid
down in several articles [52, 54, 57, 58, 100–102].
The key element of the SMS is an iron borate
57 FeBO 3 single crystal enriched
in the
57 Fe isotope up to 95%. Iron borate is a canted antiferromagnet with a Néel
temperature of 348.35 K. The crystal is set to electronically forbidden but nuclear
allowed [111]- or [333]-reflections to extract the purely scattered nuclear signal.
At room temperature
57 FeBO 3 shows in this diffraction geometry with polarized
SR a four line spectrum due to hyperfine splitting of the nuclear levels with a flux