Resolution beyond the limitations given by X-ray optics and scintillator-coupled detectors can be achieved with coherent diffraction imaging (CDI) methods. X-rays are diffracted by a sample, and the diffraction patterns are used to reconstruct an image via an iterative feedback algorithm which aims to retrieve the lost phase information. In effect, the objective lens of a typical microscope is replaced by software.
Such methods require highly coherent X-rays and can therefore not be conducted with conventional laboratory sources. The Coherence Branchline uses X-rays in the 6-20 keV range.
Methods used at the Coherence Branchline include:
In order to retrieve phase information, some known constraints must be placed on samples. In the early days of CDI, the common constraint placed upon samples was isolation from their environment. This approach is still sometimes used on the Coherence Branchline.
Ptychography (Scanning Diffraction Microscopy)
Ptychography does not require sample isolation, and so represented an advance in CDI methods. In a ptychography experiment, the X-ray beam is focused onto a sample so that a small area is illuminated. The sample is then moved with respect to the beam to create a sequential array of overlapping illuminated areas. For each area, the light scattered by the sample is recorded as a diffraction pattern. The diffraction patterns are then processed with an iterative algorithm which retrieves the phase information.
The output is a pair of images. One is a measure of the extent to which light has been absorbed by the sample. The other is a measure of the phase delay introduced to the beam as it passed through the sample.
Classical X-ray crystallography relies upon crystals which are large enough that their asymmetric units can be considered to repeat indefinitely; such crystals therefore produce Bragg diffraction spots when illuminated with X-rays. Nanocrystals produce diffraction patterns similar to their larger brothers, but with distributions of intensity where one would expect to find each Bragg spot. The shape of the nanocrystal may be deduced from shape of these distributions. Phase information arises from strains within the crystals, and these strains may also be imaged.
Advanced Microscopy Methods
As a result of its outstanding coherence properties, I13 hosts numerous experiments for advanced microscopies. New records have been achieved in two-dimensional focusing using multi-layer Laue lenses, in collaboration with the National Synchrotron Light Source (Brookhaven National Laboratory, USA) and the Advanced Photon Source (Argonne National Laboratory, USA). X-ray photo-correlation spectroscopy (XPCS) is a type of microscopy being developed for high temporal resolution, and we intend to further exploit this method.
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