THE CONSTRUCTION AND ANALYSIS OF A RATIO REFLECTOMETER
HARVARD UNIV CAMBRIDGE MA DIV OF ENGINEERING AND APPLIED PHYSICS
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An optical-electronic system was designed and built for reflectivity studies which has the following characteristics. Amplitude changes as small as .05 can be detected, permitting the experimenter to measure fine structure and the effects of perturbations. The readout of reflectivity data is direct and rapid, and the sample volume is very large, enabling one to modify the sample crystal environment with a variety of stimuli. The discussion of the ratio reflectometer contains a full description of the design problems and the final form of the optical, mechanical, and electronic components. We deal with the various sources of noise and their reduction, the factors affecting linearity and its optimization, and scattered light and electronic drift as sources of false reflectivity structure. Normal operating conditions are described and a guide for locating system malfunctions is included. False reflectivity structure of small magnitude, arising from the polarizing characteristics of our monochromator grating, was exactly correlated with peaks in the I0 curve occurring under the same polarization conditions. The analysis shows the effect is not unique to our system it may account for fine structure occasionally noted in the work of other investigators.