[mcstas-users] adjusted dist, xw and yh

Aaron M. Percival percival at physics.queensu.ca
Thu Jul 31 05:45:27 CEST 2008

Hello again Tom,

In taking a closer look at your instrument, I would personally be focusing
to slightly in front of the M2Exit slit.  My preference would be the

xw=0.08m , yh= 0.13m, dist=3.9m

This will assure that the focusing parameters do not influence the results
of the simulation.  Looking at your project 3, these parameters are probably
fine.  However, in project 4 you are probably under illuminated the
collimator and definitely under illuminating the guide.

Now, to continue on with your simulation, I would suggest using the
virtual_input after collimator1 along with the same assortment of
detectors.  Run a simulation such that you have a good distribution of
divergence and histories on your detectors.  You can then continue the
simulation form the point after the collimator1 using the virtual_output.
You will find that the repeat parameter will come in handy.  Remember to use
the SPLIT 10 keyword on the powderN component.

Hope this helps, feel free to ask further questions.


On Wed, Jul 30, 2008 at 4:14 AM, Peter Willendrup <peter.willendrup at risoe.dk
> wrote:

> Hello Tom,
> On Jul 30, 2008, at 6:17 AM, wokaoyan1981 wrote:
> Thank you for your wonderful suggestions. I delete components behind
> collimator 1 and set a series of  monitors instead . Parameters dist, xw
> and yh are set as the following:
> 1: xw=0.06m、yh=0.11m、dist=2.000m(prototype);
> 2: xw=0.08m、yh=0.13m、dist=2.000m;
> 3: xw=0.08m、yh=0.13m、dist=4.049m (focusing to the M2Exit slit);
> 4: xw=0.08m、yh=0.13m、dist=10.649m (focusing to the end of the collimator1)
> The simulation suggests: N increases remarkably while ERR decreases.
> Yes, this is indeed the case. Consider a given McStas detector bin. N
> neutron rays (not actual neutrons but neutron "rays",   cf. chapter 4 in the
> manual) hit this bin.
> The N neutron rays each have a given weight p_j, from which we determine
> the intensity (in units of neutrons/second) by Sum(p_j). ERR is nothing but
> the RMS error in units of neutrons/second, given that N rays of individual
> weight p_j hit the bin. So yes, ERR will decrease with increased N.
> In summary, I and ERR is your intensity with errorbars while N describes
> your statistics (and hence defines the errorbars).
> I don't know which project to choose since the results differ in neutron
> intenstiy, divergence and distribution. See I, y_div, x_I in enclosure.
> I did not look at your instrument file in detail, so exuse me if I get
> something wrong. The table above suggests that you are experimenting with
> focusing from the source? Source focusing is only meant to reduce the solid
> angle of the produced neutrons, ie. it does not make sense to emit neutrons
> in full 4PI (as a real world neutron source) when we are only interested in
> neutrons that hit our guide entrance.
> Beware however; If you reduce the solid angle so much that eg. the first
> guide element is under illuminated you WILL get faulty results, in terms of
> underestimated divergence and wrong intensity.
> Will have a real look at your instrument file as soon as time allows.
> Hope this helps,
> Peter Willendrup
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Aaron M. Percival
M.Sc. Candidate
Dept. of Physics, Engineering Physics & Astronomy
Queen's University
Kingston, Ontario, Canada, K7L 3N6
Office: 613-533-6000 ext. 74789
Fax: 613-533-6463
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