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PowderCell as teaching tool
(1998)
PowderCell represents a user friendly program which supports the solution of scientific problems as well as teaching and education. Especially for the last one the program offers a lot of information regarding the space-group type as well as crystal structure used. Therefore, on some universities the program is used successfully to make students familiar with x-ray crystallography. The quasi-simultaneous diffraction pattern simulation visualized the changes caused by the respective crystal structure. However, it is also possible to vary different diffraction parameters and investigate the resulting changes in the interference intensity or the reflection position. In principle, the aim of the program is the intuitive generation of structure models. Therefore, special tools have been implemented to move (rotate or shift) or transform the crystal structure.
An algorithm has been developed to remove reliably dotted interferences impairing the perceptibility of objects within a radiographic image. This particularly is a major challenge encountered with neutron radiographs collected at the NECTAR facility, Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II): the resulting images are dominated by features resembling a snow flurry. These artefacts are caused by scattered neutrons, gamma radiation, cosmic radiation, etc. all hitting the detector CCD directly in spite of a sophisticated shielding. This makes such images rather useless for further direct evaluations.
One approach to resolve this problem of these random effects would be to collect a vast number of single images, to combine them appropriately and to process them with common image filtering procedures. However, it has been shown that, e.g. median filtering, depending on the kernel size in the plane and/or the number of single shots to be combined, is either insufficient or tends to blur sharp lined structures. This inevitably makes a visually controlled processing image by image unavoidable. Particularly in tomographic studies, it would be by far too tedious to treat each single projection by this way. Alternatively, it would be not only more comfortable but also in many cases the only reasonable approach to filter a stack of images in a batch procedure to get rid of the disturbing interferences.
The algorithm presented here meets all these requirements. It reliably frees the images from the snowy pattern described above without the loss of fine structures and without a general blurring of the image. It consists of an iterative, within a batch procedure parameter free filtering algorithm aiming to eliminate the often complex interfering artefacts while leaving the original information untouched as far as possible.