Introduction


The Mossbauereffect was discovered in 1956 by R.L.Mossbauer. He showed that nuclear radiation can be emitted and absorbed recoilless if the atoms are placed in a solid state. For this experiment, also called nuclear resonant absorbtion, one neads a radioactive source which decays via an excited state into the so-called Mossbauer isotope. Depending on the lifetime of the excited state, the energy of the radiation can be extremly sharp. In the case of the iron 57 isotope the energy uncertaincy, called natural linewidth, is 5 *10^-9 eV compared to the energy of 14.4 *10^3 eV of the radiation. Because the energy of the nuclear state is so well defined, the difference between the hyperfine interaction of the isotopes in the source and the sample can be studied . The influence of the hyperfine interaction in the case of iron 57 is shown in the picture. The spectra resulting from these interactions are also shown. They can be measured by moving the source. Via the Doppler-effect the energy increases if the source moves towards the sample and vice versa. A velocity of 1 mm/s corresponds to an energy of 50 *10^-9 eV. Absorbtion can only occure, if the spectrum of source overlapps with energy levels of the sample. So the Mossbauer spectrum is a picture of the hyperfine interaction of the sample.
Several parameters can be extracted from the spectrum. These parameters can determine the chemical and magnetical phases of the sample like a fingerprint.
 

HyperFineInteraction



 

Methods of Measuring

There are different methods to measure the Mossbauereffect. Next picture shows the main principle of the experiment.
A radioactive source is mounted onto a drive system. One measures as a function of the velocity either the countrate of the transmitted radiation through the sample or the countrate of the backscattered electrons or gamma quanta.
 

PRINCIP



Transmission Geometry is the standard Method. The experimental set-up is very easy, but it is limited to powders or thin foils. The countrate decreases in resonance, because the radiation can be preferred absorbed. The whole sample contributes to the spectrum.
Backscattering Geometry measures either the emitted gamma or x-ray radiation or the emitted electrons. As this radiation has to leave the sample, only a layer at the surface contributes to the backscattered spectrum. The thickness of this layer depends on the range of the radiation. In the case of Fe 57 the gamma or x-rays have a range of approx. 10 e-6 m. If ones uses electrons ( CEMS = conversion electron Mossbauer spectroscopy ) the range is smaller. Furthermore the thickness of the contributing layer can be varied by detecting the electrons energy dispersive. DCEMS
 

Courtesy of University of Damstadt