14
R. Rüffer and A. I. Chumakov
However, the time discrimination technique has widely been used as an additional
option in the quest of the resonant quanta.
Similary, one might use the different polarization behaviour of electronic and
nuclear resonance scattering. As discussed above, SR is linearly polarized in the plane
of the synchrotron, i.e., the electric wave vector is parallel to the acceleration and
perpendicular to the velocity of the particles. For horizontal (in-plane) 90
◦ -scattering
the electronic scattering (electric dipole (E1)) would be “zero”, whereas e.g. in case
of
57 Fe with its 1/2–3/2 magnetic dipole transition (M1) the nuclear scattering could
stay due to optical activity. In the early days this could not efficiently be accomplished
due to technical challenges. However, meanwhile with state-of-the-art SR sources
and crystal optics this endeavour was successful reaching polarization purities in
the 10
−9 to 10
−10 regime, see e.g. [32].
1.2.2.2 Thin Films and Total Reflection
Another method to suppress the electronic towards the nuclear scattering lies in
the different strength of their scattering amplitudes. In case of
57 Fe the intensity of
nuclear scattering is about 400 times bigger than the electronic one.
Trammell et al. [33] have calculated that in case of a 10
3 layer thick iron crystal
only about three successive (332)-reflections would be sufficient to filter a pure γ -ray
beam out of the SR. They assumed 90
◦ -scattering in order to benefit as well from
the different scattering properties, E1 and M1. However, to our knowledge no such
experiment has been successfully conducted.
Staying with thin films, however now in total reflection geometry, one may even
omit the need for single crystals. Then the key parameter is the different index of
refraction for electronic and nuclear scattering. Further, one may coat the surface
with a λ/4 anti-reflection layer, such as in optical applications, for an additional
suppression of the electronic scattering. These are the so-called “Grazing Incidence
Anti-Reflection” (GIAR) films [34]. Theory predicts suppression factors of 10
3 –10
4 .
Various solutions have been discussed and laid down in several publications [35–38].
In practice those suppression values could not be reached at that time due to
the rather big divergence and beam size of the SR and the non perfect big mirrors
needed for total reflection. Nowadays with the improved and dedicated SR sources
those ideas flourish e.g. in γ -optical applications, see Röhlsberger and Evers [39].
1.2.2.3 Pure Nuclear Reflections
The most promising avenue seemed to be nuclear Bragg diffraction (NBD) and the
use of so-called pure nuclear reflections.
In contrast to the well-known Bragg diffraction at the electrons, electric charge
scattering, now the scattering proceeds resonantly via the interaction of the radiation
with the excited nuclear levels (Mössbauer levels). In consequence nuclear Bragg
R. Rüffer and A. I. Chumakov
However, the time discrimination technique has widely been used as an additional
option in the quest of the resonant quanta.
Similary, one might use the different polarization behaviour of electronic and
nuclear resonance scattering. As discussed above, SR is linearly polarized in the plane
of the synchrotron, i.e., the electric wave vector is parallel to the acceleration and
perpendicular to the velocity of the particles. For horizontal (in-plane) 90
◦ -scattering
the electronic scattering (electric dipole (E1)) would be “zero”, whereas e.g. in case
of
57 Fe with its 1/2–3/2 magnetic dipole transition (M1) the nuclear scattering could
stay due to optical activity. In the early days this could not efficiently be accomplished
due to technical challenges. However, meanwhile with state-of-the-art SR sources
and crystal optics this endeavour was successful reaching polarization purities in
the 10
−9 to 10
−10 regime, see e.g. [32].
1.2.2.2 Thin Films and Total Reflection
Another method to suppress the electronic towards the nuclear scattering lies in
the different strength of their scattering amplitudes. In case of
57 Fe the intensity of
nuclear scattering is about 400 times bigger than the electronic one.
Trammell et al. [33] have calculated that in case of a 10
3 layer thick iron crystal
only about three successive (332)-reflections would be sufficient to filter a pure γ -ray
beam out of the SR. They assumed 90
◦ -scattering in order to benefit as well from
the different scattering properties, E1 and M1. However, to our knowledge no such
experiment has been successfully conducted.
Staying with thin films, however now in total reflection geometry, one may even
omit the need for single crystals. Then the key parameter is the different index of
refraction for electronic and nuclear scattering. Further, one may coat the surface
with a λ/4 anti-reflection layer, such as in optical applications, for an additional
suppression of the electronic scattering. These are the so-called “Grazing Incidence
Anti-Reflection” (GIAR) films [34]. Theory predicts suppression factors of 10
3 –10
4 .
Various solutions have been discussed and laid down in several publications [35–38].
In practice those suppression values could not be reached at that time due to
the rather big divergence and beam size of the SR and the non perfect big mirrors
needed for total reflection. Nowadays with the improved and dedicated SR sources
those ideas flourish e.g. in γ -optical applications, see Röhlsberger and Evers [39].
1.2.2.3 Pure Nuclear Reflections
The most promising avenue seemed to be nuclear Bragg diffraction (NBD) and the
use of so-called pure nuclear reflections.
In contrast to the well-known Bragg diffraction at the electrons, electric charge
scattering, now the scattering proceeds resonantly via the interaction of the radiation
with the excited nuclear levels (Mössbauer levels). In consequence nuclear Bragg