Imaging

Organic or inorganic heterogeneous matter attenuates and scatters X-rays in different manner, which is used to characterize the morphology of the specimen using a variety of contrast techniques.

A range of Imaging techniques are available at Diamond and the spectroscopy village supports a number of specific imaging modes.

Phase Contrast Imaging

Absorption imaging relies on a strong aborption of the x-rays to obtain a high contrast image but this can be difficult to achieve in many cases such as in the case of hard x-ray imaging of biological tissue.

In phase contrast imaging we use the property that different materials have different refractive indexes and so the x-rays are deflected or retarded differently by the different parts of a sample. There are a number of ways this can be implemented, but the most common is differential phase contrast and projection phase contrast.  

In scanning probe experiments we implement differential phase contrast using a segmented detector or CCD and use the difference in intensity from different sections as a indication of the deflections of the x-rays as they pass through a material. 

For projection based phase contrast, a CCD records projected images from a sample at different distances and a phase retrival algorithm is used to reconstruct the sample image.   

Ptychography

Coherent Diffractive Imaging (CDI) uses a coherent x-ray beam to illumunate a sample
and retrieve the phase information on the sample from phase-retrival analysis of the far-field diffraction pattern.

Of the many CDI variants developed ptychography has been shown to be the most robust and is a scanning technique where diffraction information is collected from overlapping regions and used to reconstruct the sample phase properties.

The advantage of ptychography is that the spatial resolution achieved can be higher than that achieved with conventional optics.

X-ray Fluorescence Mapping 

X-ray fluorescence (XRF) occurs when the inner shell electrons of atoms in the sample get excited by the incident X-ray photons (synchrotron beam) and subsequently release an X-ray photon when the system relaxes, that is when  electrons transition from the higher energy levels of the atom to the vacant inner shell.  The beauty of this process is that each secondary X-ray photon (sometimes called characteristic radiation) emitted from the sample has a specific energy which is characteristic of the atom from which it has originated.  By measuring the energy of the secondary photons it is possible to establish the elemental composition of the sample at the point where the X-ray beam hits the sample.

In µXRF mapping we use a focussed spot to illuminate only a small section of the sample and to determine the composition at that point. We then move the sample in a grid pattern so that, pixel by pixel, we build up a 2D image of the varying composition of a sample.