2 Synchrotron-Radiation-Based Energy-Domain Mössbauer …
63
After the transmitter, SR has an energy profile reflecting the hyperfine structure of
the transmitter. This situation is shown in Fig. 2.3c. Subsequently, SR is incident
on the scatterer, and the scattering from the scatterer is detected by the detector,
arranged just above (or below) the scatterer to cover a large solid angle from the
scatterer. Here, either the transmitter or the scatterer is connected with a velocity
transducer to control the nuclear resonant energy by the Doppler effect of light.
Figure 2.3a shows the case in which the transducer is connected with the scatterer,
and Fig. 2.3d shows the energy profile of the scattering from the scatterer, in which
the resonant energy is controlled by the velocity transducer. The scatterer is usually
in the shape of a plate or film and inclined to minimize the self-absorption of the
scattering. At the detector, both high time response of the sub-nanosecond order and
high dynamic range are required to distinguish the delayed nuclear resonant scattering from the prompt electronic scattering by setting an appropriate time window.
This is because the incoherent nuclear resonant scattering was emitted with some
delay corresponding to the lifetime of the excited state of the probe nuclide. Thus, we
usually use an avalanche photodiode (APD) detector [24] (See 1.6.2 in this book).
A simple description of the instrumentation was presented in the previous text.
In advanced cases, we have some additional components and some desirable conditions. As for monochromator, we can arrange other sets of monochromator(s) to
obtain higher energy resolution. A resolution of some milli electron volts is possible
by a Si crystal monochromator using higher index. Such a high-resolution monochromator drastically improves the efficiency in this method because it yields the deeper
absorption spectra by suppressing non-Mössbauer nuclear resonant absorption, that
is, nuclear resonant absorption with recoil. Furthermore, it also alleviates the damage
to the sample. The high-resolution monochromators are available for X-rays with the
energy below typically 40 keV. However, the reflectivity of such a high-resolution
monochromator becomes low in higher-energy range, especially over 60 keV. Thus,
we cannot always use such a monochromator. As for the transmitter and scatterer,
there are some conditions. One condition is that they should not undesirably vibrate.
In the measurement system, the precise control of the relative velocity between the
transmitter and scatterer corresponds to the energy scan, and such undesirable vibration spoils the control. Nevertheless, the actual measurement system often includes
the sources of vibration, such as vacuum pumps, refrigerators, and air compressors.
Vibration from them increases the apparent energy width of the spectra and reduces
their absorption depth because the spectra obtained is the convolution of the vibrational profile and the real Mössbauer spectra. When the hyperfine structure to be
measured is sufficiently larger than the effect of vibration, the former effect on width
might not be quite problematic, but the latter on the absorption depth is still very
problematic. Therefore, the vibration should be sufficiently less than the bandwidth
of nuclear levels in the velocity expression: v = cΓ /E res . Here, c denotes the speed
of light, Γ denotes the natural width of the nuclear level, and E res denotes the nuclear
resonant energy of the nuclide. Next, we discuss the arrangement of the sample under
study. It is usually more convenient to arrange the sample at the transmitter because
the limitation of the transmitter in addition to the vibration control is milder than
that of the scatterer. The important limitation of the transmitter is that SR penetrates
63
After the transmitter, SR has an energy profile reflecting the hyperfine structure of
the transmitter. This situation is shown in Fig. 2.3c. Subsequently, SR is incident
on the scatterer, and the scattering from the scatterer is detected by the detector,
arranged just above (or below) the scatterer to cover a large solid angle from the
scatterer. Here, either the transmitter or the scatterer is connected with a velocity
transducer to control the nuclear resonant energy by the Doppler effect of light.
Figure 2.3a shows the case in which the transducer is connected with the scatterer,
and Fig. 2.3d shows the energy profile of the scattering from the scatterer, in which
the resonant energy is controlled by the velocity transducer. The scatterer is usually
in the shape of a plate or film and inclined to minimize the self-absorption of the
scattering. At the detector, both high time response of the sub-nanosecond order and
high dynamic range are required to distinguish the delayed nuclear resonant scattering from the prompt electronic scattering by setting an appropriate time window.
This is because the incoherent nuclear resonant scattering was emitted with some
delay corresponding to the lifetime of the excited state of the probe nuclide. Thus, we
usually use an avalanche photodiode (APD) detector [24] (See 1.6.2 in this book).
A simple description of the instrumentation was presented in the previous text.
In advanced cases, we have some additional components and some desirable conditions. As for monochromator, we can arrange other sets of monochromator(s) to
obtain higher energy resolution. A resolution of some milli electron volts is possible
by a Si crystal monochromator using higher index. Such a high-resolution monochromator drastically improves the efficiency in this method because it yields the deeper
absorption spectra by suppressing non-Mössbauer nuclear resonant absorption, that
is, nuclear resonant absorption with recoil. Furthermore, it also alleviates the damage
to the sample. The high-resolution monochromators are available for X-rays with the
energy below typically 40 keV. However, the reflectivity of such a high-resolution
monochromator becomes low in higher-energy range, especially over 60 keV. Thus,
we cannot always use such a monochromator. As for the transmitter and scatterer,
there are some conditions. One condition is that they should not undesirably vibrate.
In the measurement system, the precise control of the relative velocity between the
transmitter and scatterer corresponds to the energy scan, and such undesirable vibration spoils the control. Nevertheless, the actual measurement system often includes
the sources of vibration, such as vacuum pumps, refrigerators, and air compressors.
Vibration from them increases the apparent energy width of the spectra and reduces
their absorption depth because the spectra obtained is the convolution of the vibrational profile and the real Mössbauer spectra. When the hyperfine structure to be
measured is sufficiently larger than the effect of vibration, the former effect on width
might not be quite problematic, but the latter on the absorption depth is still very
problematic. Therefore, the vibration should be sufficiently less than the bandwidth
of nuclear levels in the velocity expression: v = cΓ /E res . Here, c denotes the speed
of light, Γ denotes the natural width of the nuclear level, and E res denotes the nuclear
resonant energy of the nuclide. Next, we discuss the arrangement of the sample under
study. It is usually more convenient to arrange the sample at the transmitter because
the limitation of the transmitter in addition to the vibration control is milder than
that of the scatterer. The important limitation of the transmitter is that SR penetrates