MECHANICAL DESIGN OF A HIGH-RESOLUTION TUNABLE CRYSTAL MONOCHROMATOR FOR THE MULTIPLE ENERGY COMPUTED-TOMOGRAPHY PROJECT

We describe the mechanical design of a medical-imaging monochromator for the Multiple Energy Computed Tomography project at the X17B superconducting wiggler beamline at the National Synchrotron Light Source. The monochromator consists of two independent two-crystal Laue-Laue units in tandem, with the option of using a Laue-Bragg/Bragg-Laue configuration. It provides two collinear exit beams of different energies that can be alternatively selected by a shutter. The heart of the design is a mechanically driven spindle with a double-gimbal mechanism for precise angular positioning of each of the four crystals about two orthogonal axes of rotation. An angular resolution of 0.041 arcsec is achieved by using a linear motion of 0.1 mum on a lever arm of 50 cm. The main features of the system include a fixed-exit beam having a 15 mm vertical offset, water-cooled first crystals in both units, a 12 cm beam width, a 3 mm maximum beam height, an energy range of almost-equal-to 25-50 and 75-100 keV, a 2.5 s beam-switching time, and high vacuum compatibility.