Monochromator for Advanced Photon Source (APS)

A high-precision water-cooled, small-offset, double crystal monochromator has been developed for the APS UNICAT (Sector 34) beamline. The design incorporates support and gravity-feed water cooling of two diamond or silicon crystals correctly positioned about a common rotation axis so that the incident and diffracted beam do not walk off the edges of the crystals within the energy range 6-15KeV (30° <q<11. 6°). In the 34ID implementation, the first crystal sees unfocussed pink beam from an undulator. The second crystal has a ±5° motorized tilt motion around an axis parallel to its face and around the beam direction.

Monochromator Design

The design was created using parametric solid models. Finite Element Analysis was conducted on several critical parts to determine the possible deflection under different load and heat constraints. Key design features include:

First crystal holder. This is a copper block excavated under the crystal and cooled by a continuous water tube starting and ending outside the vacuum. This tube features sections with wave-like bends to accommodate rotation of the crystal holder relative to the tubing entrance/exit flange. This was especially difficult to produce since it was made from OFHC copper which is available only in a relatively hard temper.

Second crystal holder. This tilts ±5° by being attached to a small vacuum-compatible tilt stage driven by an arrangement of miniature gears, shafts and couplings connected to a motorized feedthrough. A short manual horizontal adjustment is also incorporated. Both holders incorporate a matrix of very small tapped holes for attachment of the crystals via small wedges.

Compton shield. This is attached to the second crystal holder to absorb parasitic radiation emitted from the first crystal holder. It is cooled by a tube virtually identical to that on the first crystal holder. Because of the very small area through which these tubes must pass, and their proximity to each other, the four wave-like areas mentioned above are staggered so they can nest together without interference.

Publication: 8th International Conference on Synchrotron Radiation Instrumentation (SRI) meeting in San Francisco, California, August 25th 2003, Development of a Double Crystal Monochromator