McStas and monochromators

[Kristian Nielsen]

> Date: Mon, 11 Oct 1999 09:49:29 +1000
> From: "JAMES, Michael" <mja at ansto.gov.au>

> A colleague mentioned that he thought that a new flat monochromator component had been developed to
> account for secondary extinction.  I have searched for this component and have turned-up a blank.
> Also have you heard of any similar components that account for variations in reflectivity with
> wavelength ?

We do have a new monochromator in development, and it is close to
finished, but not completely finished yet unfortunately. I could send
you some code, but given that there are still unresolved issues and no
documentation yet I am not sure that it would be useful, unfortunately.

The component is designed to handle multiple scattering events inside
the crystal, and should correctly handle secondary extinction as well as
wavelength dependency. I think the main error compared to a real
monochromator is in the modelling of the mosaicity. A gaussian mosaicity
is assumed with a large number of small microcrystals. This assumption
may not be valid in real mosaic crystals.

 - Kristian.