Questions

[Kristian Nielsen]

> I'm still working on the PST component but I have now another question
> concerning the handling of the neutron coordinates in McStas: I try to
> build a large focusing monochromator (horizontal & vertical) for a
> backscattering device (this is simply a three axis spectrometer where
> the third axis points in the opposite direction of the first defelector
> crystal. I attached two examples focus69.instr (somthing like a 3-axis
> spec) and focus90.instr (backscattering type) and you will se the
> difference with mcdisplay.

Ok, I tried your examples and did a little experimentation.

First of all, there is a bug in the Monochromator component. When the
neutron wavelength is longer than the maximum for which bragg scattering
is possible, an invalid operation (asin(x) with x > 1) is
performed. This seems to happen in your focus90 instrument. I have a
corrected component, and I just made it available from the "known bugs"
page:

    http://neutron.risoe.dk/mcstas/known-bugs.html

But I am not sure if this problem affects your results.

> What make me wonder is the fact that the number of neutrons which reach
> the final energy monitor reduces drastically in the case of focus90
> although the reflectivity due to the 90-deg-geometry should be better.

The number of neutrons in the detector generally carries no physical
significance in McStas. If you look instead at the intensities, they are
1.8e6 for focus69 and 1.3e7 (8 times bigger) for focus90, so this is as
you expected, I think?

The decrease in the number of neutrons is of course a problem for the
efficiency of the simulation. The reason is that you have a very broad
energy band emission from the source compared to the very small energy
band accepted by the large monochromator; most of the neutrons have an
energy that makes it impossible to ever reach the final detector. (I
actually have a new source component that automatically adapts itself to
situations like this and picks more neutrons with useful energies, but
this is still a bit experimental. Meanwhile, if you match the energy
range in the source to the range accepted by the large monochromator,
you should get better statistics.)

Hm, actually the accepted energy range in focus69 is much bigger than in
the backscattering focus90 mode, so I would actually have expected the
intensity in backscattering to be lower than at 69 degrees. But I guess
that there is a simple explanation for this?

> Is it possible that the neutron passes the first deflector crystal
> (called mono1) twice in contrast to the focus69-type and that the
> neutrons detected are those which have been transmitted?

No. A component can interact at most once with each neutron, regardless
of its physical position.

I hope this answers your questions; otherwise please ask again. Things
are currently a bit quiet with regards to McStas, so I am happy to know
that it is still being used.

 - Kristian.