Post-monochromator

The post-monochromator is installed in OH2 at 55 m from the source point. His role is to further reduce the heat load so that there is no “ thermal” broadening of the energy resolution function. His operation is optional, and necessitates the use of the collimating CRL in order to provide the highest possible throughput: due to the higher reflection order, the spectral angular acceptance, the Darwin width wD, is significantly smaller than the natural vertical divergence of the x-ray beam. At present a silicon (3 3 1) symmetric channel-cut is installed, providing an intrinsic energy resolution of (DE/E)int = 1.44x10-5. The Table below summarises its main characteristics.

Si (n n n)	B[deg]	D [mrad]	h
8	18.336	4.77	5.7
9	16.237	4.19	5.76
11	13.226	3.38	5.84
13	11.161	2.84	5.89

_B denotes the Bragg angle and Dh is the beam offset induced by the post-monochromator.

Picture 1 : View of the Post-monochromator vacuum chamber.	Picture 2 : View of the Si(331) chunnel-cut crystal installed on the nrad goniometer.
Picture 3 : Inside view of the Post-monochromator chamber.