Crystal monochromators can provide the exceptionally fine collimation requisite for the study of small‐angle diffraction phenomena. To obtain sufficient intensity and contrast it is necessary not only to bend the crystal but also to use two crystals in tandem with their focal circles mutually orthogonal.
The apparatus described uses the x‐ray ``weak window'' line source to illuminate the (200) planes of mica in transmission and the (1011) planes of a cylindrically ground, polished, and etched quartz crystal in reflection. Symmetrical couple mounts were used to minimize aberrations. The camera essentially isolates the Kα1 component and consequently reduces diffuse background and general radiation artifacts as shown by pertinent applications to collagen and feather keratin. Although operating in air and using exposure times comparable (50–250 hr) to those encountered with aperture‐collimated systems, the camera has resolved the central first order of the 640‐A macroperiod of collagen fibers and is capable of resolving consecutive orders of spacings as high as ca 3800 A.
The Johansson, Johann, and Cauchois geometries are illustrated in a new manner to show the importance of weak and large sources of x‐rays, to demonstrate the focal pattern characteristic of two‐crystal point‐focusing arrangements, and to show why mosaic crystals are incapable of the fine focus required.