Again a question

[Kristian Nielsen]

> Date: Tue, 20 Jul 1999 16:15:32 +0200
> From: "Dr. Oliver Kirstein" <o.kirstein at fz-juelich.de>

> the following instrument definition I would like to use a vanadium
> sample (radius 1.5 cm, height 3 cm). Some of the input parameters are
> the focusing parameters (x,y,z and the radius of the target sphere).
>
> If i choose a target sphere with a radius of 2 m i think this must be
> the the parameter rt. Unfortunately I get the error message '...Did not
> hit cylinder from inside...'. I started the simulation with

Well, it looks like you forgot to attach the instrument to the mail?

I would suspect a problem with the focusing. A target radius of 2 meter
for the sample seems very large. Note that the sample cannot be placed
inside the target sphere; in that case you will get a cryptic error
message similar to the one you mentioned. So with a target sphere radius
of 2 meter, the distance (length of (x,y,z)) must be more than two meter
or things will not work.

Maybe you have misunderstood the meaning of the target sphere? It is not
explained very well in the manual. The idea is something like the
following:

                                     ____
				    /  	 \
         O                          |    |
                                    \____/
      sample
	        		 target sphere

Neutrons will be scattered from the sample only in the solid angle
corresponding to the target sphere as seen from the scattering point. So
the sphere should be chosen just large enough to completely surround the
object of interest (eg. an analyser). If you really need to have the
sample inside the target sphere, you should instead set the target
sphere radius to zero to make the sample scatter in all directions.

I hope this helps, otherwise ask again. I will be away for most of three
and a half weeks starting tomorrow (Thursday), so you might want to send
any later questions to the list (neutron-mc at risoe.dk) so that Kim
Lefmann or others may also help you.

 - Kristian.