Energy_Monitor affects final intensity?

[Kristian Nielsen]

> Hello Kristian,
> 
> I have just encountered a strange result with my last job:
> 
> The instrument looks like source, guide, Cu monochromator and several
> detectors at the sample position. To investigate more precisely what
> happens to the energy distribution I added another E_monitor just in
> front of the monochromator. But then the final intensity at the sample 
> position drops by a factor of about 2/3!!! This difference is a
> factor of 20 larger then the error. This happens also with fixed
> seed. Running with -s 2 gives

> It seems as if the E_monitor is absorbing neutrons??? Isn't it 
> supposed not to do so!? Am I wrong somewhere?

The problem is that in your instrument, the E_monitor is placed in the
middle of the third guide section:

> COMPONENT part3 = Guide2(
> 	w1 = 0.038644,
> 	h1 = 0.072392,
> 	w2 = 0.020,
> 	h2 = 0.060,
> 	l = 9.500,
...
>   AT (0,0,4.202) RELATIVE a1

> COMPONENT enmon = E_monitor(
...
>   AT (0,0,9.88) RELATIVE a1
                          ^^^

I suspect that you meant to say "RELATIVE part3" here, which would place
the monitor after the end of the guide, but then it would overlap the
monochromator! You need to put the E_monitor "AT(0,0,9.7) RELATIVE part3"
or something similar to put it between the guide section and the
monochromator.

This is a common "gotcha" of McStas, I am afraid, and I will look into
finding a way for McStas to help catch these kinds of errors (I have
a few ideas already).

I will look into your profile problem later. I also investigated the
PgPerl problem by logging in to the account you supplied me with. On my
system (Debian/GNU Linux), PgPerl is preinstalled, so I had no
problems. I tried installing it manually first on your machine and then
on my own, and both fail in the same way! So yes, there is a problem,
and I will sort it out as soon as I can (should be early next week at
the latest). I will also investigate the strange beam profile you
mentioned.

 - Kristian.