As well as secondary electrons, X-rays are also emitted when a beam of high-energy electrons is focused onto a sample. Often these X-rays are used to identify the chemical composition of a sample using a detector that measures their energy. There is now, however, a way of using these X-rays to form an image using a technique called X-ray ultramicroscopy (XuM). Pioneered at the CSIRO in Australia, X-ray ultramicroscopy utilizes the X-rays generated in an SEM when the electron beam is focused onto a metal target. This essentially provides a point source of X-rays which then pass through a nearby sample and generate a projection image on an X-ray sensitive CCD camera. The contrast in these images is governed by two phenomena: absorption and diffraction. The more dense or thicker a region of the sample, the greater the X-ray absorption, causing that region to appear darker in the image – this is absorption contrast. Diffraction of the x-ray also occurs at every edge and boundary within the sample, and because of the relatively large sample to camera distance, interference between the diffracted X-rays and the primary beam occurs. This leads to characteristic dark and bright fringes appearing at edges and boundaries in the image which enhances the visibility of any micro-pores or cracks within the particle. This type of contrast is known as phase contrast.

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