Biomedical subjects

Alwyn Eades

Publications and source records attributed to Alwyn Eades.

3 recordsLinked to original sources

PubMed2006

Obtaining TEM images with a uniform deviation parameter.

In transmission electron microscopes made during the last quarter of a century, it has been impossible to take images in which the diffraction contrast conditions are uniform across the field of view. This is inconvenient when, for example, imaging dislocations at a relatively low magnification. The problem arises because modern microscopes use immersion lenses in which the sample sits in a high magnetic field. The resulting helical trajectories of the electrons at the sample plane mean that it is not possible to make a beam that is parallel at the sample. Results of a method to overcome this problem are presented. It is shown that a simple modification to the microscope (which, on a computer-controlled microscope, could be implemented in software) can be used to produce images in which the deviation parameter is essentially constant across many microns of image. By a happy accident, this method can be used, not only to correct for the helicity imparted by immersion lenses, but also to correct for buckling of the sample (up to a point).

Journal Article↗

PubMed2005

Errors, artifacts, and improvements in EBSD processing and mapping.

Electron back scatter diffraction (EBSD) mapping and indexing has rapidly come into widespread use. However, inadequate attention has been paid to the details of the method. Many of the algorithms in current use were chosen because they were the first ones that were found to work, rather than because they were optimum. Results of systematic study are presented. It is shown that more than one method can successfully correct a sampling artifact, that there is an optimum binning ratio, that Gaussian filtering provides an alternative to "butterfly convolution," that better alternatives for mapping image quality than those in current use are available, and that saving all the original patterns is practical and advantageous.

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PubMed2005

Measurement and mapping of small changes of crystal orientation by electron backscattering diffraction.

We have explored the possibility of measuring small changes of orientation within grains by electron backscattering diffraction (EBSD), in the scanning electron microscope. Conventional orientation maps (using EBSD) index the orientation of each position on the sample separately. This does not give accurate results for small differences of orientation. We have studied methods of measuring small changes in orientation by measuring the change from one EBSD pattern to the next directly, without indexing either. Previous workers have measured the change of position of a zone axis in the EBSD pattern. We have compared this with an alternative method, which we show to be superior, of measuring the shift of the peaks in the Hough transform from one diffraction pattern to the next. This means that we are measuring the change of orientation of sets of crystal planes within the grain, rather than measuring the change of orientation of zone axes. We show that it is possible, with a standard EBSD configuration, to measure the shift of the Kikuchi bands to a precision of about a 10th of a pixel, which corresponds to a change of orientation in the sample of about 0.1 mrad (0.006 degrees ).

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