The application of interference fits for overcoming limitations in clamping methodologies for cryo-cooling first crystal configurations in x-ray monochromators

Abstract

The work detailed here describes how a novel approach has been applied to overcome the challenging task of cryo-cooling the first monochromator crystals of many of the world's synchrotrons' more challenging beam lines. The beam line configuration investigated in this work requires the crystal to diffract 15 Watts of 4-34 keV X-ray wavelength and dissipate the additional 485 watts of redundant X-ray power without significant deformation of the crystal surface. In this case the beam foot print is 25 mm by 25 mm on a crystal surface measuring 38 mm by 25 mm and maintain a radius of curvature of more than 50 km. Currently the crystal is clamped between two copper heat exchangers which have LN2 flowing through them. There are two conditions that must be met simultaneously in this scenario: the crystal needs to be clamped strongly enough to prevent the thermal deformation developing whilst being loose enough not to mechanically deform the diffracting surface. An additional source of error also occurs as the configuration is assembled by hand, leading to human error in the assembly procedure. This new approach explores making the first crystal cylindrical with a sleeve heat exchanger. By manufacturing the copper sleeve to be slightly larger than the silicon crystal at room temperature the sleeve can be slid over the silicon and when cooled will form an interference fit. This has the additional advantage that the crystal and its heat exchanger become a single entity and will always perform the same way each time it is used, eliminating error due to assembly. Various fits have been explored to investigate the associated crystal surface deformations under such a regime.