A previous communication has described a beam intensity modulator for detecting noise intensity of the x-ray beam at the user end station and a noise compensating circuit which has resulted in significant improvements of signal to noise. Intensity and position noise modulations of synchrotron orbital radiation were not distinguished in that study. This paper describes the use of phase information from the modulator to detect positional changes of the x-ray beam. A circuit has been developed for continuously following the phase angle of the modulation. This position detector is sensitive to 3 nm with a 20-Hz bandwidth. Experimental studies at an end station at DESY indicate very low-frequency (<1 Hz) noise of the beam position of several hundred microns and a higher-frequency noise centered near 10 Hz with a peak amplitude of 24 nm. The use of this position tracking signal to correct the sample position is described.
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