222
C. McCammon
SPring-8
Super photon ring-8 GeV
XANES
X-ray absorption near-edge structure
5.1 Introduction
Mössbauer spectroscopy is practically unique among other methods used in
geoscience. The signal is specific to the nucleus being examined, so only those
phases containing the Mössbauer isotope give a signal. It is one of the few methods
that is able to distinguish different valence states, and one of the even fewer that
require no calibration to determine accurate values of relative abundance. Mössbauer spectroscopy provides structural information, such as the coordination and
geometry of crystallographic sites. The method is a short range probe, so it allows
the study of poorly crystallised materials such as glasses and provides information
complementary to data from long-range probes such as X-ray diffraction. Finally,
there is practically no limitation on pressure and also to a certain extent temperature,
which means that Mössbauer spectroscopy can be carried out on samples at most
conditions within Earth’s interior.
One aspect of Mössbauer spectroscopy that has improved greatly throughout its
history is spatial resolution. Technical advances have reduced the beam size dramatically, initially as radioactive point sources became commercially available and subsequently through advances in focussing capabilities at synchrotron facilities. The main
driver of these developments has been research questions involving high pressure,
where the quest for ever higher pressures has driven the need to measure ever smaller
samples. High-pressure studies have not been the sole benefactor of higher spatial
resolution, however. Research questions in geoscience are usually best addressed
using analytical methods with high spatial resolution, since natural samples may be
small and their history can often be deciphered from inhomogeneous variations in
composition and oxidation state.
The Mössbauer effect has been observed in a large number of elements (more
than 40) in about 80 different nuclides for a total of nearly 100 different nuclear
transitions, e.g., [1]. Applications of Mössbauer spectroscopy are concentrated on
isotopes with convenient transitions, and by far the most popular (particularly for
geoscience applications) has been the 14.4 keV transition in
57 Fe. Iron is a popular
Mössbauer isotope because the half-life of its common parent,
57 Co, is reasonably
long (270 days), recoil-free fractions are generally high at room temperature and the
absorption cross section is sufficiently large that reasonable spectra can be obtained
even for absorbers with low iron concentrations. For
57 Fe, the lifetime of the excited
state is 98 ns, which gives a natural linewidth of 4.7 neV. Compared to the energy
of the γ-ray, the energy resolution is roughly one part in 10
13 , which enables very
small changes in the local atomic environment to be detected. The natural abundance
of
57 Fe is 2.14%, which provides the possibility to enhance the Mössbauer signal
C. McCammon
SPring-8
Super photon ring-8 GeV
XANES
X-ray absorption near-edge structure
5.1 Introduction
Mössbauer spectroscopy is practically unique among other methods used in
geoscience. The signal is specific to the nucleus being examined, so only those
phases containing the Mössbauer isotope give a signal. It is one of the few methods
that is able to distinguish different valence states, and one of the even fewer that
require no calibration to determine accurate values of relative abundance. Mössbauer spectroscopy provides structural information, such as the coordination and
geometry of crystallographic sites. The method is a short range probe, so it allows
the study of poorly crystallised materials such as glasses and provides information
complementary to data from long-range probes such as X-ray diffraction. Finally,
there is practically no limitation on pressure and also to a certain extent temperature,
which means that Mössbauer spectroscopy can be carried out on samples at most
conditions within Earth’s interior.
One aspect of Mössbauer spectroscopy that has improved greatly throughout its
history is spatial resolution. Technical advances have reduced the beam size dramatically, initially as radioactive point sources became commercially available and subsequently through advances in focussing capabilities at synchrotron facilities. The main
driver of these developments has been research questions involving high pressure,
where the quest for ever higher pressures has driven the need to measure ever smaller
samples. High-pressure studies have not been the sole benefactor of higher spatial
resolution, however. Research questions in geoscience are usually best addressed
using analytical methods with high spatial resolution, since natural samples may be
small and their history can often be deciphered from inhomogeneous variations in
composition and oxidation state.
The Mössbauer effect has been observed in a large number of elements (more
than 40) in about 80 different nuclides for a total of nearly 100 different nuclear
transitions, e.g., [1]. Applications of Mössbauer spectroscopy are concentrated on
isotopes with convenient transitions, and by far the most popular (particularly for
geoscience applications) has been the 14.4 keV transition in
57 Fe. Iron is a popular
Mössbauer isotope because the half-life of its common parent,
57 Co, is reasonably
long (270 days), recoil-free fractions are generally high at room temperature and the
absorption cross section is sufficiently large that reasonable spectra can be obtained
even for absorbers with low iron concentrations. For
57 Fe, the lifetime of the excited
state is 98 ns, which gives a natural linewidth of 4.7 neV. Compared to the energy
of the γ-ray, the energy resolution is roughly one part in 10
13 , which enables very
small changes in the local atomic environment to be detected. The natural abundance
of
57 Fe is 2.14%, which provides the possibility to enhance the Mössbauer signal