[neutron-mc] Single crystal peak in McStas1.6-ill

[Kristian Nielsen]

"Gutmann, MJ (Matthias) " <M.J.Gutmann at rl.ac.uk> writes:

> and observe a strange peak shape for some parameter values in the
> simulation. The peak shape changes if an additional TOF monitor is inserted
> at the sample position. Is this to be expected?

> 2) An m=0 guide to mimic a rectangular collimator of 1cm^2 cross section at.
> 3) A TOF monitor before the entrance of that collimator.

> L2 = 0.225. 10^8 neutrons were used. The shape changes if the TOF monitor
> at the sample position is uncommented.

Just a guess from my part, but your problem could be caused by
components that are overlapping and/or written in the wrong sequence.

McStas uses a simple (but fast) algorithm for propagating neutrons that
assumes that components are passed by neutrons in exactly the order in
which they are written in the instrument.

Your instrument has a monitor which is physically before the collimator,
but is written after the collimator in the instrument. Also your
(commented) monitor overlaps the sample.

Try to re-write the instrument so that no components overlap physically,
and so that components appear in the instrument in the order that a
neutron will encounter them. Remember to leave a small gap (say .1 mm)
between adjacent components. Perhaps this will solve your problem; if
not then perhaps someone else on the list can help.

 - Kristian.

P.S. Good luck to all in the McStas community; it is nice to see the
project I started still alive.