Some questions concerning a vanadium scan

[Kristian Nielsen]

> Date: Wed, 14 Jul 1999 11:27:50 +0200
> From: "Dr. Oliver Kirstein" <o.kirstein at fz-juelich.de>

> I hope that you can help me with the following effect: I tried to make a

> The effect is the following: If I don't use the 'Detector System 2' I
> get distributions which are shown in mcstas2.ps whereas the use of
> 'Detector System 2' gives the results shown in mcstas1.ps. 
> It makes me wonder because both detector parts don't use the same arm
> definition.

> Do you know what happens?

Yes, I think I can explain the behaviour.

Without the second detector group, the third detector group receives the
full blast of the directly scattered beam from the vanadium sample, and
this completely hides the faint signal from the scattering off the
monochromator.

With the second detector group, all neutrons scattered away from this
second group (ie. the direct scattering onto the third group) is
absorbed. This is a special "feature" of the monitor components in
McStas that neutrons that move in the wrong direction are silently
absorbed. Thus in this case only the scattering off the monochromator is
observed.

I think you will want to put in some shielding (just a simple slit
perhaps) on arm2. You will also want to focus the scattering off the
sample on the monochromator to get reasonable statistics. And you should
eliminate the overlap between detectors Analyser_Div and Analyser_Energy.

The silent absorbtion of backwards-moving neutrons is really a problem,
and I will find some solution for it in the next McStas release.

Hope this helps (otherwise ask again),

 - Kristian.