[mcstas-users] adjusted dist, xw and yh

[Peter Willendrup]

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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