X-ray Diffraction- Synchrotron Radiation Flashcards
General set up of a synchrotron
Linac injects electrons into synchrotron. Synchrotron accelerates them into storage ring. This follows various straight lines between bending magnets which alter the path of the electrons around a large circle. There is an insertion device just before one bending magnet. The beamline that doesn’t change direction goes through focussing mirror either side of a monochromator to a diffractometer. A RF cavity near end of one full loop
What happens before electrons go into storage ring?
Synchrotron light starts with an electron gun. Heated element or cathode produces free electrons which are pulled out of end of gun by powerful electric field. This electron stream fed into linear accelerator (linac). High energy microwaves and radio waves chop stream into pulses. Pick up speed by catching these waves. Leave linac nearly at speed of light. Fed into booster ring which uses magnetic fields to force electrons to travel in circle and radio waves add even more speed. Then fed into storage ring
Storage ring and onwards
It is a many-sided donut-shaped tube. Is maintained under vacuum as atoms can deflect e beam. Synchrotron light produced when bending magnets deflect the beam. Each set of bending magnets connected to a beamline. Machines filter, intensify the light at each beamline to get right characteristics for experiment.
Features of RF cavity, insertion device and beamline
RF cavity restores energy of electrons by coupling to radiofrequency field. Insertion device is a periodic array of magnets which cause electron trajectory to wiggle or undulate, results in emission of intense synchrotron radiation over narrow range of wavelengths. Beamline is guide tube for synchrotron radiation, monochromator selects wavelength, focussing mirrors maximise intensity and resolution, diffractometer is sample and detector
Properties of synchrotron radiation
High intensity so can study small samples and get rapid data collection. Tuneable wavelength, anomalous dispersion and access to short wavelengths. Low divergence and highly monochromatic means very high resolution
Intensity of synchrotron radiation
Defined as brilliance. Is photons per unit solid angle per unit of bandwidth per unit source area
Why is synchrotron x-ray powder diffraction good for small/weakly scattering samples?
Data collection time is acceptable due to high brilliance of source. Can do high pressure studies, micro-crystal diffraction, protein crystallography
Why is synchrotron x-ray powder diffraction good for special environment diffraction studies
Transmission through windows of environment cell due to high brilliance and tuneable wavelength. Can do Diffraction experiments at extreme conditions (high pressure/variable temperature)
Why is synchrotron x-ray powder diffraction good for time resolved diffraction studies?
Rapid acquisition of diffraction data due to high brilliance and high resolution (low divergence). Can do study of solid state reactions like enzyme-substrate reactions
Why is synchrotron x-ray powder diffraction good for high resolution diffraction
Can resolve overlapping/weak reflections due to low divergence and high resolution. Can determine small deviations from ideal symmetry, incommensurate modulations
How does high pressure synchrotron radiation x-ray powder diffraction work?
Sample is compressed between two gem quality diamonds. Gasket between them is sample chamber (steel disk with central hole). Pressure medium maintains hydrostatic conditions. Sample and ruby chip in gasket with pressure medium. Pressure determined by shift in Cr fluorescence line from ruby, excited by Ar ion laser. Can get pressures over 200GPa. Detect diffracted beams transmitted through rear of diamond
Useful things about synchrotron radiation for high pressure x-ray powder diffraction
High intensity means can have small sample size (few mg). Select short wavelength to penetrate diamonds. Low divergence means can focus beam through gasket aperture (because of low wavelength
Applications of high pressure diffraction
Crystallographic phase transitions. Electronic phase transitions (metal/insulator transition). Pressure induced intercalation reactions
Simultaneous SAXS/WAXS
Two detectors can be used at same time, one for wide angle region (checking crystallinity) and the other for small angle (checking long range ordering)
