Effective Use of Collimating Apertures in Small-Angle X-Ray Diffraction Cameras

Abstract

The use of simple, aperture-collimated cameras in x-ray diffraction studies at small angles is of value in investigating the molecularly large structural features of colloidal systems, particularly when pinhole collimation is required. To make effective use of available x-radiation one must (1) consider the initial limitations imposed on a camera by the purpose for which it is to be employed, and (2) choose remaining camera dimensions so that the registering film may receive as much radiation as possible. The initial conditions are three in number, involving the desired pattern magnification, the necessary angular or large-spacing resolution, and the requisite guarding against beam spill-over, which together essentially limit the specimen-to-film distance, the beam diameter at the film, and the diffraction area permitted to be unguarded but covered by a beam stop. Other camera dimensions are chosen so that the film center is as close to, while viewing as much of, the radiation source as possible. In accomplishing these aims it is useful to align the camera at near grazing incidence to the x-ray tube target, though there are limitations to this procedure. Rules for systematically accomplishing the above purposes are derived herein for the extremes of symmetrical (pinhole) and unsymmetrical (slit) aperture systems, as well as for ones employing a simple class of rectangular (unsymmetrical pinhole) aperture. Criteria are also given for evaluating the relative speeds of cameras which may depart from optimum because of mechanical limitations. Finally is described the construction of a series of cameras, found useful in studying the fibrous proteins, particularly collagen, and typical diffraction patterns obtained therefrom are reproduced.

Optimum geometric conditions in the design and use of X-ray diffraction tubes and cameras

“…If higher resolutions at the centre of the photograph are required, then a collimator with suitable guarding arrangements should be used; this case has not been considered here, as the introduction of the additional variables necessary renders the problem unmanageable. The paper by Bolduan & Bear (1949) should be consulted for certain types of solution.…”

“…A comprehensive treatment of part of this problem has already been published by Bolduan & Bear (1949) who considered the special slit and pinhole collimation systems suitable for use in low-angle diffraction work, where the specimen size can be selected as required and the film distance is controlled principally by the need to view conveniently very close-spaced patterns.…”

“…In biological studies, however, specimens are likely to be both small and weakly diffracting, and exposure time becomes a limiting factor in the application of the technique. It is of great importance, therefore, to make sure that the dimensions of the X-ray tube and camera are chosen so as to produce a satisfactory photograph in the shortest possible time.A comprehensive treatment of part of this problem has already been published by Bolduan & Bear (1949) who considered the special slit and pinhole collimation systems suitable for use in low-angle diffraction work, where the specimen size can be selected as required and the film distance is controlled principally by the need to view conveniently very close-spaced patterns.The present paper deals with wider-angle techniques, where a simpler collimator can be used but where specimen size is a controlling factor in the design of the camera and where the film distance may be altered within fairly wide limits. …”

“…It should also be pointed out that in many eases when only small foci are required it is desirable to modify the present technique of viewing the anode with a foreshortening ratio of 10:1. It is possible to obtain an increase in apparent brightness of focus down to viewing angles of 1 ° (Bolduan & Bear, 1949), and so a considerable gain would be introduced by using a foreshortening ratio of 40:1 or 50:1 and a focal line of appropriate dimensions.It is also desirable, when working with very small specimens, to be able to approach close to the focal spot. If a resolution of about 100 J~ is required, a 1 cm.…”

“…If higher resolutions at the centre of the photograph are required, then a collimator with suitable guarding arrangements should be used; this case has not been considered here, as the introduction of the additional variables necessary renders the problem unmanageable. The paper by Bolduan & Bear (1949) should be consulted for certain types of solution.It is necessary to specify the resolution required of the camera. In the case of the Buerger precession 458 DESIGN OF X-RAY TUBES AND CAMERAS camera, or others where a moderately undistorted picture of a single plane in reciprocal space is produced, the separation of the centres of the spots on the film is determined by the unit-cell dimensions and the film distance; the size of the individual spots will be determined by the geometry of the camera, which must be such that neighbouring spots do not overlap.…”