Calulating intensities

[Kristian Nielsen]

> Date: Fri, 04 Feb 2000 16:58:40 +0100
> From: stuart <stuart at studsvik.uu.se>

> I was trying to compare the intensities given from a simulation when
> using different energy/wavelength neutrons, and have been trying to
> understand the way in which the monochromator mosaicity would affect the
> number reflected, for those different incident energies/wavelengths.
> 
> Should the energy spread of the neutrons, from the source be different
> for each energy simulated in order to give an identical wavelength
> spread, incident upon the monochromator?  Or maybe I'm thinking about it
> the wrong way!

I am not quite sure here about what you have in mind, but it sounds like
your problem is that the neutron source you use specifies the spectrum
in terms of energy, while what you need is to specify it in terms of
wavelength. In that case you should try to use the Source_flux_lambda
component that is included in the just-released McStas version 1.2 (I
assume you use McStas though you did not mention it explicitly). That
will give you easily the same wavelength spread at all wavelengths.

More generally, the prediction of absolute reflectivity at different
wavelengths of mosaic crystals is (as far as I know) a very hard
problem. Complications arise because both primary and secondary
extinction becomes important, and these depend strongly on the precise
characteristics on the mosaic (a simple Gaussian model may not be
sufficient). Samuel Werner discusses the problem in his 1965 PhD thesis
(I think the title is "The multiple Bragg scattering of neutrons from
mosaic crystals").

Hope this helps,

 - Kristian.