Design and technique of operation of a double crystal spectrometer

Abstract

The double crystal x-ray spectrometer is distinctly different in principle from the single crystal spectrometer and calls therefore for a completely new design rather than an adaptation of single spectrometer design. One such new design is described in this paper. Emphasis is laid on the fact that the wave-length selected by the process of two successive crystal reflections in any prescribed orders depends solely on the dihedral angle between the crystals. The advantages of rotating both crystals in equal and opposite directions with respect to a plane through their axes of rotation are pointed out and a spectrometer designed to accomplish this is described. In this spectrometer the turning of a single shaft drives the two crystals, the spectrometer as a whole and the x-ray tube, at the proper angular rates about the proper centers to insure that the x-ray beam will at all times remain centered on the crystal faces and the window of the stationary ion chamber. It is pointed out that spurious fluctuations may be introduced in spectral curves by the x-ray beam migrating across small steps in the cleavage surface reflecting the radiation. Description of design covers design of spectrometer and turning mechanisms, the tube housing, lead shields, the detecting system consisting of an ion chamber with internal grid of special design to minimize natural leak and a Hoffmann vacuum electrometer connected to it through a short evacuated shield. Description of operating technique covers optical method of accurately orienting the crystal faces, methods of aligning the tube, precautions to eliminate background.