Mössbauer Spectroscopy

The technique of Mössbauer spectroscopy involves using the gamma rays emitted from the nuclei of a radioactive source to probe those in the sample to be studied.^2.2 The source contains the parent nucleus of the Mössbauer isotope, embedded in a rigid matrix to ensure a high $f$ factor. The gamma rays emitted from this are passed through the material being investigated and those transmitted through the absorber are detected and counted.

If the nuclei in the source and absorber are in the exact same environment (ie the energy of the nuclear transition is equal in both nuclei) the gamma rays will be resonantly absorbed and an absorption peak will be observed.

In order to probe the energy levels in nuclei in different environments we must scan the energy of the Mössbauer gamma ray. This is achieved by moving the source relative to the absorber. The Doppler effect produces an energy shift in the gamma ray energy allowing us to match the resonant energy level(s) in the absorber.

The simplest case is shown in Fig 2.3. The spectrum recorded is a plot of transmission intensity versus source velocity in mms. The x-axis, through the Doppler effect on the gamma ray energy, is effectively an energy scale. The lineshape of the recorded peak in a thin sample is theoretically a Lorentzian, with a FWHM of twice the uncertainty in the energy of the excited state, $\Gamma$.

Figure 2.3: Example Mössbauer spectrum showing the simplest case of emitter and absorber nuclei in the same environment. The uncertainty in the energy of the excited state, $\Gamma$, is shown exaggerated.