5 Mössbauer Spectroscopy with High Spatial Resolution …
237
using synchrotron radiation do not require such considerations due to the minimal
divergence of the X-ray beam compared to a radioactive source.
The other distance to be considered when setting up a Mössbauer measurement,
namely the sample to detector distance (D 2 ), does not influence the solid angle
of radiation since the detector window is usually much larger than the beam size
(Fig. 5.10). Here the distance D 2 should be sufficiently large to minimise the amount
of radiation scattered from the sample that reaches the detector, which contributes
only to the background, not the signal.
5.4.2 Sample Preparation
Mössbauer spectroscopy is extremely flexible with regard to sample form and
measurements with small beam size are no exception. Beam diameters for Mössbauer
spectroscopy using a radioactive source typically start at 100 μm and go upwards
for unenriched samples [14], and
57 Fe enrichment allows even smaller samples to be
measured (for example the spectrum in Fig. 5.4a was collected on a 30 μm diameter
sample). Focussing optics at synchrotron sources enable beam diameters as small as
10 μm to be used [31, 32].
Many samples can be used with minimal preparation, although there may be
advantages to investing some effort into cutting and polishing, particularly in the
case of inhomogeneous samples. Ultimately the choice of sample form depends on
the nature and amount of sample available, as well as the desired sample thickness (Sect. 5.4.4). A final step for all sample types before collecting spectra with a
radioactive source is to ensure that the detector records only γ-rays that have passed
through the sample, which is usually accomplished by masking the sample with a
thick absorbing foil (for example Pb or Ta).
A common sample form, especially for large samples measured using a conventional source, is powder, meaning that the grain size is much smaller than the beam
size. Conventionally a standard sample holder is used, into which an appropriate
weight of sample is placed (Fig. 5.11a). For smaller quantities of sample, a holder
can be made by drilling a hole in lead foil and securing the contents with cellophane
tape (Fig. 5.11b), or by mixing the powder with glue and affixing it to a plastic sheet
(Fig. 5.11c). Sample diameters of 500 μm or less can be easily achieved using the
latter two approaches, and are suitable for use both with a radioactive source and for
synchrotron measurements.
Samples in the form of grains or shards can be used without alteration if their
thickness is appropriate for their composition. The grain can be simply affixed onto
a plastic sheet (Fig. 5.11d) or multiple grains can be glued in the form of a mosaic if a
larger diameter is required (Fig. 5.11e). In this case empty space should be minimised
and thickness should not vary substantially across the mosaic (Sect. 5.4.4). It is also
possible to measure rare or precious samples such as polished gemstones if at least
one part of the stone has a thickness appropriate for its composition.
237
using synchrotron radiation do not require such considerations due to the minimal
divergence of the X-ray beam compared to a radioactive source.
The other distance to be considered when setting up a Mössbauer measurement,
namely the sample to detector distance (D 2 ), does not influence the solid angle
of radiation since the detector window is usually much larger than the beam size
(Fig. 5.10). Here the distance D 2 should be sufficiently large to minimise the amount
of radiation scattered from the sample that reaches the detector, which contributes
only to the background, not the signal.
5.4.2 Sample Preparation
Mössbauer spectroscopy is extremely flexible with regard to sample form and
measurements with small beam size are no exception. Beam diameters for Mössbauer
spectroscopy using a radioactive source typically start at 100 μm and go upwards
for unenriched samples [14], and
57 Fe enrichment allows even smaller samples to be
measured (for example the spectrum in Fig. 5.4a was collected on a 30 μm diameter
sample). Focussing optics at synchrotron sources enable beam diameters as small as
10 μm to be used [31, 32].
Many samples can be used with minimal preparation, although there may be
advantages to investing some effort into cutting and polishing, particularly in the
case of inhomogeneous samples. Ultimately the choice of sample form depends on
the nature and amount of sample available, as well as the desired sample thickness (Sect. 5.4.4). A final step for all sample types before collecting spectra with a
radioactive source is to ensure that the detector records only γ-rays that have passed
through the sample, which is usually accomplished by masking the sample with a
thick absorbing foil (for example Pb or Ta).
A common sample form, especially for large samples measured using a conventional source, is powder, meaning that the grain size is much smaller than the beam
size. Conventionally a standard sample holder is used, into which an appropriate
weight of sample is placed (Fig. 5.11a). For smaller quantities of sample, a holder
can be made by drilling a hole in lead foil and securing the contents with cellophane
tape (Fig. 5.11b), or by mixing the powder with glue and affixing it to a plastic sheet
(Fig. 5.11c). Sample diameters of 500 μm or less can be easily achieved using the
latter two approaches, and are suitable for use both with a radioactive source and for
synchrotron measurements.
Samples in the form of grains or shards can be used without alteration if their
thickness is appropriate for their composition. The grain can be simply affixed onto
a plastic sheet (Fig. 5.11d) or multiple grains can be glued in the form of a mosaic if a
larger diameter is required (Fig. 5.11e). In this case empty space should be minimised
and thickness should not vary substantially across the mosaic (Sect. 5.4.4). It is also
possible to measure rare or precious samples such as polished gemstones if at least
one part of the stone has a thickness appropriate for its composition.