230
C. McCammon
require nearly 400 days if the count rate remained constant. The count rate decreases,
however, and source thickness would increase substantially (Fig. 5.2), necessitating
the replacement of the point source several times during the measurement. However
if a lower quality spectrum were sufficient for the research question being addressed,
this exercise demonstrates the possibilities for collecting Mössbauer data with beam
sizes as small as 50 μm diameter using a point source.
There are a number of geoscience applications for which a point source is not
appropriate and the only choice is synchrotron radiation. These cases include very
small samples or when high spatial resolution is needed (<50 μm diameter for
samples with high dimensionless effective thickness but much larger if samples
contain less
57 Fe), as well as experiments where time is an important parameter. Examples include measurements during laser heating at high pressure [19],
time-differentiated measurements [20], and in situ kinetic studies [21].
5.3 Measurement Approach
Mössbauer spectra collected using a radioactive source are traditionally measured
by changing the velocity of the sample relative to the source (or vice versa) and
measuring counts as a function of velocity (v), which can be converted to energy
through the Doppler shift:
E = E 0 v/c
(5.3)
where E 0 is the unperturbed energy of the γ-ray (14.4 keV) and c is the velocity of
the γ-ray (i.e., the speed of light = 2.998 × 10
11 mm/s). The variable is therefore
energy, and such measurements are said to be carried out in the energy domain.
Mössbauer spectra collected using a synchrotron source can be carried out in the
energy domain using SMS, but can also be carried out in the time domain using NFS.
In the latter method all resonant nuclear transitions are excited at time zero by a sharp
pulse, after which interference between the decaying energy levels in the quiet time
between pulses produces an intensity spectrum as a function of time. More details are
available in the literature, for example [17], but for the purpose of the present chapter,
Sect. 5.3 focuses specifically on aspects of energy and time domain measurements
that are relevant to small or heterogeneous samples in geoscience investigations,
namely spectral deconvolution and counting time.
C. McCammon
require nearly 400 days if the count rate remained constant. The count rate decreases,
however, and source thickness would increase substantially (Fig. 5.2), necessitating
the replacement of the point source several times during the measurement. However
if a lower quality spectrum were sufficient for the research question being addressed,
this exercise demonstrates the possibilities for collecting Mössbauer data with beam
sizes as small as 50 μm diameter using a point source.
There are a number of geoscience applications for which a point source is not
appropriate and the only choice is synchrotron radiation. These cases include very
small samples or when high spatial resolution is needed (<50 μm diameter for
samples with high dimensionless effective thickness but much larger if samples
contain less
57 Fe), as well as experiments where time is an important parameter. Examples include measurements during laser heating at high pressure [19],
time-differentiated measurements [20], and in situ kinetic studies [21].
5.3 Measurement Approach
Mössbauer spectra collected using a radioactive source are traditionally measured
by changing the velocity of the sample relative to the source (or vice versa) and
measuring counts as a function of velocity (v), which can be converted to energy
through the Doppler shift:
E = E 0 v/c
(5.3)
where E 0 is the unperturbed energy of the γ-ray (14.4 keV) and c is the velocity of
the γ-ray (i.e., the speed of light = 2.998 × 10
11 mm/s). The variable is therefore
energy, and such measurements are said to be carried out in the energy domain.
Mössbauer spectra collected using a synchrotron source can be carried out in the
energy domain using SMS, but can also be carried out in the time domain using NFS.
In the latter method all resonant nuclear transitions are excited at time zero by a sharp
pulse, after which interference between the decaying energy levels in the quiet time
between pulses produces an intensity spectrum as a function of time. More details are
available in the literature, for example [17], but for the purpose of the present chapter,
Sect. 5.3 focuses specifically on aspects of energy and time domain measurements
that are relevant to small or heterogeneous samples in geoscience investigations,
namely spectral deconvolution and counting time.