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) Theta-capB[deg] OmegaD [mrad] Delta-caph
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



Theta-cap B denotes the Bragg angle and Dh is the beam offset induced by the post-monochromator.


Post mono vacuum chamb

Picture 1 : View of the Post-monochromator vacuum chamber.

chunnel cut

Picture 2 : View of the Si(331) chunnel-cut crystal installed on the nrad goniometer.


Post mono chamb

Picture 3 : Inside view of the Post-monochromator chamber.