FOCUSING MONOCHROMATORS FOR HIGH-ENERGY SYNCHROTRON RADIATION

Bent crystals are introduced as monochromators for high energy synchrotron radiation. The reflectivity of the crystal can be calculated reliably from a model where the bent crystal is approximated by a stack of lamellas, which have a gradually changing angle of reflection. The reflectivity curves of a 4 mm thick, asymmetrically cut (chi = 9.5-degrees) Si(220) crystal are measured using 150 keV radiation and varying the bending radius from 25 to 140 m. The width of the reflectivity curve is up to 50 times the Darwin width of the reflection, and the maximum reflectivity exceeds 80%. The crystal is used as a monochromator in Compton scattering measurements. The source is on the focusing circle, so that the resolution is limited essentially by the detector/analyzer. A wide bandpass, sharply focused beam is attained when the source is outside the focusing circle in the transmission geometry. In a test experiment. 10(12) photons on an area of 2 mm2 was observed. The energy band was about 4 keV centered at 40 keV. A powder diffraction pattern of a few reflections of interest was recorded by an intrinsic Ge detector, and this demonstrated that a structural transition can be followed at intervals of a few milliseconds.