Nuclear Forward Scattering

Experimental Setup / Basic Principle

Setup for the NFS measurement. The pulsed SR (left side, pulses separated by Dt) penetrates the sample and reaches the detector. The decay of the nuclear excited states, which takes place in the time window Dt (right side), reflects the hyperfine interactions of the resonant nuclei.

When a synchrotron radiation pulse penetrates the sample containing resonant nuclei, then

  • electric charge scattering
    happens almost instantaneously (t £10-15 s)
  • while the processes involving
    nuclear excitations are delayed:
    the mean life time of the excited state of 57Fe is 141ns.

Therefore, it is possible to separate the events originating from nuclear and electronic scattering in the sample.

Since the incident beam is broad in energy compared to the nuclear level width (4.7neV for 57Fe) or the hyperfine splitting of these levels (less than 1 meV),

all nuclear levels are excited.

  • The collective and coherent part of the radiative decay
    is peaked in forward direction and shows
    interference caused by the energy difference
    of the allowed nuclear hyperfine transitions.

    This leads to quantum beats in the time spectrum.
    From this beat structure
    • the hyperfine splitting (magnetic or quadrupole)
      of the excited and ground nuclear states and
    • the energy shifts for different chemical environments
      at different nuclear sites (isomer shift) can be derived.

  • Another important feature of coherent scattering by a large ensemble of nuclei is the speed-up of the nuclear decay and the modulation of the time evolution due to multiple scattering [Phys.Rev.B46(1992)6207].

    This results in
    dynamical beats with increasing distance of the beat minima and maxima with time. The minima can be used to determine directly
    • the effective thickness of the sample and from there
    • the Lamb-Mössbauer factor.

  • As a random process diffusive motion of the nuclei will dephase the radiation-field components form different sample places and will also result also in a speed-up decay of the forward scattered intensity.

    From this
    "diffusional acceleration"
    • the jump rate of iron atoms
    • the mechanism of diffusion
    • the diffusitivity
    can be derived.

Based on the PhD thesis
of Alessandro Barla, Herdecke 2001
and Hanne Grünsteudel, Lübeck 1998

Last modified 11/06/02 06:31 PM by Ernst Schreier