| Mössbauer Spectroscopy is a spectroscopic technique
that investigates the absorption and emission of gamma radiation in a
material, based on the occurrence of the Mössbauer Effect.
A typical experimental setup for Mössbauer Spectroscopy consists
of a source for γ-photons., an absorber that is the material
under investigation, and a detector. The energy of the
photon beam is adjusted by using the Doppler Effect. The source is
moved towards and away from the absorber.
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Source Absorber Detector
There are two conditions that need to be satisfied in order for this
technique to work. First, the experimental conditions need to satisfy
the occurrence of the Mössbauer Effect, that is the recoil
energy of the photon emissions and absorptions must be significantly
smaller than the energy of the lattice vibrations. The intensity of the
Mössbauer Effect is determined by the recoilfree fraction or
f-factor, which can be considered as a kind of efficiency. The second
condition that must be satisfied is that one needs nuclei in the excited
state as a source for the γ-photons. These nuclei are made using a
nuclear accelerator and consist of a specific atomic isotope that
decays to the excited state of the nucleus under investigation at a
specific half life time. A necessary condition for an observable
Mössbauer Effect is thus that one has a source which decays to
the excited state of the nucleus under investigation with a sufficiently
long life-time such that the experiments are practical.
An example of a spectroscopic measurement would be the analysis of
iron oxide (the absorber) with a 57Co source. The
57Co isotope decays to 57Fe with a half life of
270 days, while emitting photons with almost the correct
energy. The energy levels are not entirely matching, since the iron in
the iron oxide lattice is coupled trough hyperfine
interactions; the nuclear levels in the absorber have slightly
different energies than in the emitter. Therefore, the energy of the
photons from the source is varied using the Doppler Effect. If the
emitter is moved towards the absorber at a velocity v, the
energy of the photon (E(v) becomes:
E(v) =
E0(1+v/c)
Where E0 is the energy difference between the excited state
and ground state of the nucleus. and c is the velocity of
light. The Doppler velocities are usually in the range of -10 to 10
mm/s.
A typical Mössbauer spectrum will show the γ-ray intensity as a
function of sample velocity. This mode, Mössbauer Absorption
Spectroscopy (MAS) is the common mode of operation. It is also possible
to fix the source, and move the absorber. This technique is called
Mössbauer Emission Spectroscopy (MES).
The advantage of Mössbauer Spectroscopy is that it uses γ-
radiation of high penetrating power; this allows the technique to be
used in situ. Applications of Mössbauer Spectroscopy are:
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