A device for selecting and rejecting X-ray harmonics in synchrotron radiation beams

A practical device (`beam cleaner') is described that selects a specific harmonic (e.g. third order) from other nearby harmonics (e.g. fourth or fifth orders) that are transmitted by the beamline monochromator. The ability to isolate and use a clean harmonic beam effectively extends the energy range of existing beamlines. This device is a tunable medium-resolution X-ray bandpass filter, which consists of a double bent Laue crystal post-monochromator. It is a simple and inexpensive device that does not require precision goniometry and can be operated in air. The device can also be used to passively compensate for vertical beam motion as the primary beamline monochromator is scanned. Experimental results are presented that demonstrate the improvement in XAFS data quality under high harmonic conditions.