X-Ray Evidence on the Nature of the Surface Layers of Thin Ground Quartz Crystals Secured with the Cauchois Spectrograph

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Abstract
Spuriously doubled lines observed in spectra formed by the Cauchois transmission-type curved-crystal focusing spectrograph have been reported by several investigators. B. B. Watson has shown the varying separation of these doublets in several orders to be consistent with the assumption that they are caused by enhanced intensity of reflection in the front and back surface layers of the crystal lamina where its structure has been disturbed by grinding. The enhancement of x-ray reflection by grinding, polishing or other mechanical disturbance is already well known. The present authors note that the more roughly ground of two surfaces on the curved crystal gives rise to the more intense member of the spurious doublet and that heavy etching of the ground quartz surface with hydrofluoric acid suppresses the surface reflection. Cauchois has pointed out that an initially flat crystalline lamina subjected to elastic bending to a circular cylindrical curved form should (approximately) focus the selectively reflected radiation not only from various parts of the arc but also for all regions of the finite thickness of the slab; the second type of focusing is a result of the elastic change in grating constant at points off the neutral axis of the curved slab. The single lines we obtain with curved slabs that have been etched confirm the reality of this type of focusing. We draw the conclusion that the surface layer responsible for the enhanced x-ray reflection must have three interesting properties: (1) The mechanical stress and consequent strain, incident to bending, existing in the main body of the crystal is not shared by this surface layer which thus seems to be differentiated from the rest; (2) the surface layer is crystalline, however, and has the grating constant of unstrained quartz; (3) in spite of their disturbance the atomic planes in this surface layer must have with surprising accuracy the same average orientation as the planes in the main body of the supporting crystal. These properties differing from those to be expected for the amorphous glass-like layers first postulated by Sir George Beilby suggest a recrystallization of the displaced atoms on the underlying crystal foundation such as Finch has proposed.

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