Radiography in Modern Industry - Kodak

intimate contact. In particular, the pressure supplied by a conventional spring-backed cassettemay be insufficient to produce adequate contact if the specimen--for instance, a leaf or a wrinkledpiece of stiff paper--is not itself flat.A method related to electron radiography is electron-emission radiography--sometimes referred toin Europe as "reflection microradiography." This method is based on the fact that electronemission from a substance exposed to x-rays depends on the atomic number of the material,among other factors. When a photographic or radiographic film is placed in intimate contact with aspecimen and the whole irradiated from the film side with hard x-rays such as are used inelectron radiography, differences in electron emission resulting from differences in atomic numberand variations in concentration of components are recorded on the developed film, as is shown inFigure 107.Figure 107: Schematic diagram of the technique for making electron-emissionradiographs. More electrons are emitted from the areas of the specimen that containmateials of high atomic number. (For illustrative clarity, the electron paths have beenshown as straight and parallel; actually, the electrons are emitted diffusely.)Hard x-rays are required in this technique for the same reason as in electron radiography: It isnecessary to maximize the ratio between the electron exposure, which produces the image, andthe uniform overall exposure produced by the direct x-rays.Specimens to be examined by electron-emission radiography need to be smooth and plane onone side only and can be quite massive--significant factors when the specimen cannot be cut.An electron-emission radiograph resembles a photomicrograph in that it shows details only of thesurface of a specimen whereas microradiography shows the distribution of a componentthroughout the thickness of the specimen. For this reason, and because electron emission is lesssensitive to differences in atomic number than is soft x-ray absorption, an electron-emissionradiograph of a specimen contains markedly less information than a microradiograph of a thinsection of the same specimen.Image definition is also poorer than in a microradiograph because of the diffuse emission ofelectrons from the surface and because of the increased graininess of the film associated with thehigh-energy x-ray and electron exposures. Nevertheless, electron-emission radiography is aRadiography in Modern Industry 170