Stabilizing feedback system for synchrotron radiation monochromators

We have designed and constructed a feedback system that will maintain the angle between two x‐ray diffracting crystals. The system was designed for use in conjunction with a double crystal synchrotron radiation monochromator at CHESS and will preserve the relative angle between crystals during energy scans and as a function of time for a fixed wavelength. A voltage proportional to the slope of the rocking curve is used as the feedback signal. This signal is normalized in such a way that the time dependence of the incident beam intensity is removed. The feedback system allows one to sit not only at the peak of the rocking curve, but also on the flanks, an aspect crucial when utilizing the harmonic rejection properties of the double crystal monochromator.