The phenomenon of the emission of a g-ray photon without loss of energy due to recoil of the nucleus and without thermal broadening is known as Mössbauer effect. Since the gamma emission is recoil-free, it can be resonantly absorbed by stationary atoms. It was discovered by Rudolph Mössbauer in 1957 and it has tremendous application in materials research. The direct application of the Mössbauer effect to materials research arises from its ability to detect the slight variations in the energy of interaction between the nucleus and the extra nuclear electrons, variations which had previously been considered negligible.
The Mössbauer effect has been detected in a total of 88 g-ray transitions in 72 isotopes of 42 different elements. Although in theory it is present for all excited state-ground state g-ray transitions, its magnitude can be so low as to preclude detection with current techniques. The useful Mössbauer isotopes so far found out are iron, tin, antimony, tellurium, iodine, xenon, europium, gold and neptunium and to a less extent nickel, ruthenium, tungsten and iridium. Among all these elements, the largest recoil free resonant cross section occurs for the isotope Iron 57. The resonant energies are extremely narrow which allows the observation of the hyperfine interactions between the nucleus and the surrounding electrons. The link between the Mössbauer spectrum and the electron structure of the sample can be exploited in the study of many types of materials.In the decay of 26Fe57 from its parent nuclei 27Co57, by an electron capture 27Co57 whose half-life is 270 days gets decayed to 26Fe57. Approximately 90% of the 26Fe57 nuclear excited state decays through the intermediate level to produce 14.4 keV gamma radiation. These gamma photons can then be absorbed by Fe57 in a sample.
The Mössbauer effect has been detected in a total of 88 g-ray transitions in 72 isotopes of 42 different elements. Although in theory it is present for all excited state-ground state g-ray transitions, its magnitude can be so low as to preclude detection with current techniques. The useful Mössbauer isotopes so far found out are iron, tin, antimony, tellurium, iodine, xenon, europium, gold and neptunium and to a less extent nickel, ruthenium, tungsten and iridium. Among all these elements, the largest recoil free resonant cross section occurs for the isotope Iron 57. The resonant energies are extremely narrow which allows the observation of the hyperfine interactions between the nucleus and the surrounding electrons. The link between the Mössbauer spectrum and the electron structure of the sample can be exploited in the study of many types of materials.In the decay of 26Fe57 from its parent nuclei 27Co57, by an electron capture 27Co57 whose half-life is 270 days gets decayed to 26Fe57. Approximately 90% of the 26Fe57 nuclear excited state decays through the intermediate level to produce 14.4 keV gamma radiation. These gamma photons can then be absorbed by Fe57 in a sample.
To use the Mössbauer source as a spectroscopic tool, we must be able to vary its energy over a significant range. For this the gamma ray source is mechanically vibrated back and forth to Doppler shift the energy of the emitted gamma radiation. The diagram Figure 2.5 below shows a transmission Mössbauer experiment. As the energy of the gamma radiation is scanned by Doppler shifting, the detector records the frequencies of gamma radiation that are absorbed by the sample. Moving the source at a velocity of 1 mm/sec toward the sample will increase the energy of the photons by 14.4 keV (v/c), which is equal to 4.8 x 10-8 eV. The ‘mm/sec’ is a convenient Mössbauer unit and is equal to 4.8 x 10-8 eV for Fe57.
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