X-Ray Polarization Optics and Coherent Nuclear Resonance Scattering Using Synchrotron Radiation.
Abstract
Two projects, both involving X-ray scattering with synchrotron radiation, are presented in this dissertation. (1) A system of diffracting perfect crystals for the generation of variable, elliptically polarized X-rays was tested at the Cornell High Energy Synchrotron Source under the conditions of a standard undulator source. The phase retarding optical component was a 4-bounce, Ge(220) Bragg reflection channel -cut crystal. The full polarization state (density matrix) of the output beam, including the circular polarization purity P_3, was determined using the multiple-beam Bragg diffraction technique with a GaAs crystal polarimeter and was found to agree with calculations. In addition to measuring the optics' efficiency, the ability to scan the system in energy, while frequently reversing the circular helicity, was demonstrated at the vicinity of the Fe K-edge at 7.1 keV. The setup was applied to a circular magnetic X-ray dichroism measurement. (2) The time distribution of delayed photons from resonant forward scattering of 14.4 keV synchrotron radiation pulses by ^{57}Fe nuclei was investigated over the temperature range from 9 K to just above the Curie point at 1043 K, with particular attention to precise measurements of the Lamb-Mossbauer factor f_{LM } ~ e^{- <({bf k }cdot{bf r})^2 >} , whose change was determined from its influence on the "speed-up" of coherent decay. Apart from its importance in Mossbauer effect studies, knowing the temperature dependence of f_{LM} can be valuable for studies of lattice dynamics and structural phase transitions. The change in the nuclear hyperfine splitting was also measured. The synchrotron technique has precision-enhancing advantages over conventional Mossbauer spectroscopy methods employing radioactive sources because dealing with source effects and absolute intensity measurements is eliminated. The results also straightforwardly illustrate an interesting principle concerning the temperature dependence of scattering --that for "slow" resonance scattering (i.e. where the photon-scatterer interaction time scale is long compared to the thermal vibration time scale), the temperature factor is not the well-known X-ray Debye-Waller factor f _{DW} = e^-{1over 2} <[({bf k}_ {f}-{bf k}_{i}) cdot{bf r}]^2> which has the temperature independent value of unity in the zero-momentum transfer geometry (k _{i} = k_{f }) of forward scattering.
- Publication:
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Ph.D. Thesis
- Pub Date:
- January 1995
- Bibcode:
- 1995PhDT........40S
- Keywords:
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- Physics: Optics; Physics: Radiation; Physics: Nuclear