display | more...
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.

  <- ->           _          ___
   ___           | |        |   |
  |___|- - - - - | | - - - -|-  |
                 |_|        |___|
  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:

Log in or register to write something here or to contact authors.