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If an excited free atomic nucleus goes into the ground state with the emission of a gamma quantum, then the quantum does not receive the entire excitation energy because of the conservation of momentum and energy. The same applies to the case of the absorption of a gamma quantum by a free atomic nucleus in the ground state. Nuclear resonance absorption of gamma radiation is therefore not observed with free atomic nuclei at rest. However, it can take place if both the emitting and the absorbing cores are built into crystal lattices. In this case, extremely sharp emission or absorption lines are obtained. The recoil-free resonance absorption of gamma radiation by cores built into solid bodies is called the Mössbauer effect. Mössbauer spectroscopy makes use of the energy sharpness of these absorption resonances in order to measure very small shifts or splitting of the core levels through the surrounding electronic structure. The transition from the first excited state to the basic state of a nucleus is observed, whereby the gamma energy is specifically changed by using the Doppler effect by moving the source in order to pass through the resonance.

The Mössbauer effect can be used to investigate all physical phenomena that change the energy of gamma quanta by amounts of the order of magnitude of their line width. Mössbauer spectroscopy allows, for example, the determination of core properties of excited core states (mean life, magnetic dipole moment, electric quadrupole moment) or the measurement of local magnetic or electric fields generated by the (solid) environment at the core location. Using the example of the 14.4 keV level in Fe-57 in a non-ferromagnetic or ferromagnetic environment, the service life, the magnetic moment of the excited core state, the magnetic field at the core location and the isomer shift between source and absorber are to be determined.

learning goals

  • Dealing with measurement electronics in nuclear physics
  • Recording and evaluation of γ spectra
  • Determination of the energy splitting through hyperfine interaction
  • Measurement of the natural line width

Experimental technique

NaI (Tl) scintillation detector, amplifier, single-channel discriminator (SCA), analog-digital converter (ADC), multi-channel analyzer (VKA), frequency generator, Mössbauer drive with control unit, signal delay, oscilloscope

Experiment instructions

Moess [pdf, 211 kb]

Supervisor: Dr. U. rice wage earners