1 Historical Developments and Future Perspectives …
21
Normally, spectroscopy is carried out in transmission geometry as in conventional
Mössbauer spectroscopy with radioactive sources. In both cases the energy of the
γ -quanta is varied via Doppler shift in the neV to μeV regime by moving the source
and after the sample the energy spectrum is recorded. The excellent collimation of
the γ -ray beam from the SMS allows as well for scattering experiments, which were
very challenging with radioactive sources due to their isotropic emission spectrum
and only exceptionally done.
Sofar the SMS has been developed for the case of
57 Fe, which was favoured due
to the availability of highly perfect single crystals enriched in the Mössbauer isotope. However, generally speaking, this may be achieved for any Mössbauer isotope
provided one is able to grow perfect single crystals or structures, which show pure
nuclear reflections and preferably a single line.
1.3.2 Synchrotron Radiation Based Mössbauer Spectroscopy
Another technique in the energy domain was recently presented by Seto et al. [59]:
Synchrotron radiation based Mössbauer spectroscopy (SRMS), see Fig. 1.7. The
idea is as follows: The sample under investigation (in the beam) “modulates” the
synchrotron radiation, which is then analyzed (demodulated)—by time integration
and an efficient resonance detector—in the energy domain utilizing a conventional
Mössbauer driving system. In order to improve the signal-to-noise-ratio timing mode
is still required for the suppression of the prompt radiation by time-gating. That
implies that the time integration can only be carried out over a finite time window,
which in turn influences the energy spectrum. A detailed description will be given
elsewhere in this book by Seto et al. [60].
1.3.3 Nuclear Forward Scattering
Nuclear forward scattering [61, 62] may be considered as the time analog to Mössbauer spectroscopy; in fact it is its scattering variant. The ‘white’ synchrotron radiation excites all Mössbauer levels in the sample and creates a coherent collective
nuclear state. In the static case this nuclear state will decay in the forward direction giving rise to an excess of intensity at delayed times (see Fig. 1.7 NFS). The
time scale is determined by the lifetime τ 0 of the involved nuclear level. Multiple
scattering may influence the measured time response giving rise to dynamical beats.
Furthermore, in case of split nuclear levels due to hyperfine interaction (electric,
magnetic) an additional interference pattern from all involved nuclear levels, the
so-called quantum beat structure is superimposed.
An important variant of NFS is reflectometry or grazing incidence nuclear resonance scattering (GINRS) for the investigation of thin films, surfaces etc. Even
when the scattering angle in GINRS is very small (mrad) one has to account for it in
21
Normally, spectroscopy is carried out in transmission geometry as in conventional
Mössbauer spectroscopy with radioactive sources. In both cases the energy of the
γ -quanta is varied via Doppler shift in the neV to μeV regime by moving the source
and after the sample the energy spectrum is recorded. The excellent collimation of
the γ -ray beam from the SMS allows as well for scattering experiments, which were
very challenging with radioactive sources due to their isotropic emission spectrum
and only exceptionally done.
Sofar the SMS has been developed for the case of
57 Fe, which was favoured due
to the availability of highly perfect single crystals enriched in the Mössbauer isotope. However, generally speaking, this may be achieved for any Mössbauer isotope
provided one is able to grow perfect single crystals or structures, which show pure
nuclear reflections and preferably a single line.
1.3.2 Synchrotron Radiation Based Mössbauer Spectroscopy
Another technique in the energy domain was recently presented by Seto et al. [59]:
Synchrotron radiation based Mössbauer spectroscopy (SRMS), see Fig. 1.7. The
idea is as follows: The sample under investigation (in the beam) “modulates” the
synchrotron radiation, which is then analyzed (demodulated)—by time integration
and an efficient resonance detector—in the energy domain utilizing a conventional
Mössbauer driving system. In order to improve the signal-to-noise-ratio timing mode
is still required for the suppression of the prompt radiation by time-gating. That
implies that the time integration can only be carried out over a finite time window,
which in turn influences the energy spectrum. A detailed description will be given
elsewhere in this book by Seto et al. [60].
1.3.3 Nuclear Forward Scattering
Nuclear forward scattering [61, 62] may be considered as the time analog to Mössbauer spectroscopy; in fact it is its scattering variant. The ‘white’ synchrotron radiation excites all Mössbauer levels in the sample and creates a coherent collective
nuclear state. In the static case this nuclear state will decay in the forward direction giving rise to an excess of intensity at delayed times (see Fig. 1.7 NFS). The
time scale is determined by the lifetime τ 0 of the involved nuclear level. Multiple
scattering may influence the measured time response giving rise to dynamical beats.
Furthermore, in case of split nuclear levels due to hyperfine interaction (electric,
magnetic) an additional interference pattern from all involved nuclear levels, the
so-called quantum beat structure is superimposed.
An important variant of NFS is reflectometry or grazing incidence nuclear resonance scattering (GINRS) for the investigation of thin films, surfaces etc. Even
when the scattering angle in GINRS is very small (mrad) one has to account for it in