2 Synchrotron-Radiation-Based Energy-Domain Mössbauer …
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with reasonable absorption depth. If the scatterer is too thick, the absorption depth
becomes shallow. Another condition concerns the detector. The detector usually
detects scattered γ-rays, whose energy is the same as the nuclear resonant energy,
and fluorescent X-rays following the internal conversion process, whose energy is
lower than that of the γ-rays, as delayed nuclear resonant scattering. The detection of fluorescent X-rays becomes important in SR-based Mössbauer spectroscopy
using high-resonant-energy isotopes. This is because the detection efficiency of APD
becomes very low when detecting high-energy X-rays. For example, the energy of
γ-rays at
61 Ni Mössbauer spectroscopy is 67.4 keV, while that of fluorescent Kα Xrays of Ni atom is 7.5 keV. The detection efficiency of APD, whose depletion layer is
150 μm, is 93% for the 7.5-keV X-rays and 0.9% for the 67.4-keV γ-rays. In the latest
measurement system, the scatterer and detector are packed in a vacuum chamber to
detect internal conversion electrons [8]. The internal conversion coefficients of many
Mössbauer nuclides are higher than 1. Furthermore, the detection efficiency of the
APD detector is usually 100% against electrons with energies below 100 keV. Therefore, the detection rate of the detector becomes many times higher with the electron
detection in Mössbauer spectroscopy using those nuclides. For example, when a
sample under an extreme condition is studied by SR-based Mössbauer spectroscopy
using high-resonant-energy isotopes, the sample is often arranged in a large environmental cell and cannot be moved by a velocity transducer. Additionally, this cell
blocks the internal conversion electrons. Even in that case, arranging the sample as
the transmitter and the energy reference substance as the scatterer should support the
following three conditions: velocity control, low temperature, and vacuum condition
for the electron detection. In those cases, heat transfer by thin copper wires or springs
satisfies the conditions to some extent. However, the efficiency of heat transfer by
this method is usually inferior to that by the helium exchange gas technique.
2.2.3 Analysis of the Spectra
The spectra obtained are the velocity-dependent intensities and are thus very similar
to the conventional Mössbauer spectra using RI. One example is shown in Fig. 2.5. A
clear difference is observed between the spectra, and a simple Lorentzian evaluation
was enough to see the transition in valence. In many simple cases, this kind of
Lorentzian evaluation is sufficient. For this kind of rough evaluation, programs for
analyzing the conventional Mössbauer spectra using RI can be applied.
However, the exact line shape of SR-based Mössbauer spectra is not Lorentzian. A
deviation between the Lorentzian evaluation lines and experimental data is observed
around the shoulders of absorption profiles in Fig. 2.5. Furthermore, you can see a
zigzag shape at the background in the experimental data. They do not correspond to
the minor component in the sample and are characteristics of SR-based Mössbauer
spectra, caused by the measurement method described above. They depend on some
factors, such as the effective thickness of the transmitter and the scatterer, chemical
composition, and time window at the APD detector. Now, we consider the detailed
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with reasonable absorption depth. If the scatterer is too thick, the absorption depth
becomes shallow. Another condition concerns the detector. The detector usually
detects scattered γ-rays, whose energy is the same as the nuclear resonant energy,
and fluorescent X-rays following the internal conversion process, whose energy is
lower than that of the γ-rays, as delayed nuclear resonant scattering. The detection of fluorescent X-rays becomes important in SR-based Mössbauer spectroscopy
using high-resonant-energy isotopes. This is because the detection efficiency of APD
becomes very low when detecting high-energy X-rays. For example, the energy of
γ-rays at
61 Ni Mössbauer spectroscopy is 67.4 keV, while that of fluorescent Kα Xrays of Ni atom is 7.5 keV. The detection efficiency of APD, whose depletion layer is
150 μm, is 93% for the 7.5-keV X-rays and 0.9% for the 67.4-keV γ-rays. In the latest
measurement system, the scatterer and detector are packed in a vacuum chamber to
detect internal conversion electrons [8]. The internal conversion coefficients of many
Mössbauer nuclides are higher than 1. Furthermore, the detection efficiency of the
APD detector is usually 100% against electrons with energies below 100 keV. Therefore, the detection rate of the detector becomes many times higher with the electron
detection in Mössbauer spectroscopy using those nuclides. For example, when a
sample under an extreme condition is studied by SR-based Mössbauer spectroscopy
using high-resonant-energy isotopes, the sample is often arranged in a large environmental cell and cannot be moved by a velocity transducer. Additionally, this cell
blocks the internal conversion electrons. Even in that case, arranging the sample as
the transmitter and the energy reference substance as the scatterer should support the
following three conditions: velocity control, low temperature, and vacuum condition
for the electron detection. In those cases, heat transfer by thin copper wires or springs
satisfies the conditions to some extent. However, the efficiency of heat transfer by
this method is usually inferior to that by the helium exchange gas technique.
2.2.3 Analysis of the Spectra
The spectra obtained are the velocity-dependent intensities and are thus very similar
to the conventional Mössbauer spectra using RI. One example is shown in Fig. 2.5. A
clear difference is observed between the spectra, and a simple Lorentzian evaluation
was enough to see the transition in valence. In many simple cases, this kind of
Lorentzian evaluation is sufficient. For this kind of rough evaluation, programs for
analyzing the conventional Mössbauer spectra using RI can be applied.
However, the exact line shape of SR-based Mössbauer spectra is not Lorentzian. A
deviation between the Lorentzian evaluation lines and experimental data is observed
around the shoulders of absorption profiles in Fig. 2.5. Furthermore, you can see a
zigzag shape at the background in the experimental data. They do not correspond to
the minor component in the sample and are characteristics of SR-based Mössbauer
spectra, caused by the measurement method described above. They depend on some
factors, such as the effective thickness of the transmitter and the scatterer, chemical
composition, and time window at the APD detector. Now, we consider the detailed