Coincidence Mössbauer effect
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Abstract
This dissertation is concerned with the incorporation of
coincidence methods into standard Mössbauer techniques and the application
of the Delayed Coincidence Mössbauer Effect (DCME) to the study of
certain solid state phenomena. This technique involves the accumulation
of a Mössbauer spectrum which reflects the environment of the decaying
nuclei during some preset time interval following the formation of
the Mössbauer state. Changing the preset time permits display of the
time evolution of the nuclear environment. The theoretical analysis
developed in this study involves a numerical integration of the exact
expression for time dependent Mössbauer absorption over the experimental
window in time and is shown here to be successful in matching
experimental data for single and multi-lined spectra while previous
analytical techniques have been unsuccessful. In systems where no time
dependent solid state effects are present we have demonstrated experimentally
and fit theoretically the expected line narrowing at long delay
times and have applied this effect to enhance energy resolution in
absorber experiments. Studies of time dependent solid state effects
reported here involve aftereffects and various relaxation processes.
The aftereffects problem pertains to the formation of highly charged
ions following K-capture and an Auger shower, and also the local
heating of the lattice associated with the relatively energetic events
in the formation of the Mössbauer state. We investigated the time
dependence of the ionic charge and the local heating, reflected in the
recoilless fraction, as these excited configurations relaxed to
equilibrium. Previous estimates of the characteristic time for the charge
state decay to equilibrium in an insulator such as CoO were about 10^-8 sec. Our experiments in MgC show that this time must be greater
than 2 x 10- 6 or that high charge states are stabilized in the lattice.
There are no theoretical models or predictions concerning this
characteristic time or that of the local heating decay. We show in this
study that the decay to equilibrium in the latter case, using a single
line source and absorber, takes place with a characteristic time of
less than 10^-9 sec. The DCME is also applied to the study of ionic
spin and electric-field-gradient (Jahn-Teller effect) relaxations. No
relaxation effects were observed in sources of Co^57 in Ti0_2 whille the
parallel absorber experiment has shown such an effect. Calculations
done here show that the applicability of the DCHE method to study of
dynamical-Jahn-Teller effect in MgO is doubtful due to the extremely
small quadrupole splitting observed.
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