Nuclear Lighthouse Effect


Like in the case of Nuclear Forward Scattering, the synchrotron radiation creates a collectively excited state (nuclear exciton) in the Mössbauer isotope. In a rotating sample, these excited states aquire a phaseshift while evolving in time. The radiative decay proceeds therefore in a deflected direction:

The time spectrum is mapped onto an angular scale. This allows one to record time spectra with a position sensitive detector without timing electronics, independently of the time structure of the exciting radiation.

Thus, it enables time resolutions that are not limited by the bunch length and exceed the performance of present detector systems.


advantages:
  • no timing mode,
  • no fast electronics,
  • monochromatisation less critical
  • high time resolution possible

disadvantages:

  • small angle scattering background,
  • tiny sample environment

Nuclear Lighthouse Effect
at high energies

  • Small Angle Scattering is much reduced and leads to low background. Early decay times become accessible.
  • Absorption in the rotor material becomes negligible
  • Undulator flux is lower
  • Low Lamb-Mössbauer factor of isotopes in that range, thus sample cooling is a must.

Perspectives on new samples

Plot of all known Mössbauer isotopes as a function of resonance energy and lifetime. In green: Isotopes that have been applied in nuclear scattering experiments with synchrotron radiation. In red: Isotopes that become accessible with the technique of the nuclear lighthouse effect.


Summary

At high energies it is impossible to build efficient high-resolution monochromators for Nuclear Forward Scattering.

Presently, the Nuclear Lighthouse Effect seems to be the only method to access Mössbauer isotopes with high transition energies and short lifetimes.

Rotational frequencies up to 70 kHz seem to be feasible in the near future. This leads to time resolutions in the range of a few ps, that are beyond the limit of existing x-ray detectors.

Further, Small Angle Scattering is much reduced at these high x-ray energies. Problems might result from a limited sample environment and the feasibility of a correct cooling of the sample by a cryogenic gas-flow.


Last modified 6/24/02 24/06/02 03:46 PM by Ernst Schreier