A new water-cooled and doubly bent crystal monochromator for Compton scattering experiments

The design and performance of a new water-cooled and doubly bent crystal monochromator for Compton scattering experiments are described. Its mechanical system is similar to that of a usual bent cylindrical X-ray mirror. A monolithic Si (111) crystal is mounted on a water-cooled and cylindrically polished copper crystal holder with liquid Ga-In alloy between them in order to obtain good thermal and mechanical contact. The sagittal bending radius of the holder, R_s, is 508 mm. The holder is mounted on a mechanical bender making a meridian radius, R_m, of 596 m. The performance of the monochromator, which has been installed at the Photon Factory Accumulator Ring beamline NE1, is as follows. The focused beam size, flux and energy resolution for 60 keV X-rays are 0.5 mm in height and 2.0 mm in width, 5 × 10¹² photons s^-1 and about 60 eV, respectively. The new monochromator gives one order higher brightness at the sample position and also better energy resolution than the previous monochromator. The overall momentum resolution for high-resolution Compton scattering experiments becomes 0.08 atomic units, and the background can be reduced by a third, because it is possible to insert a fine slit after the analyser crystal without losing any Compton scattering signal.