[neutron-mc] Error in component Source_simple.comp ?

[Peter Willendrup]

Hello Frederik,

On Tue, 2006-02-14 at 16:12 +0100, Frederik Zilly wrote:
> Points near the center have greater probility than points with a higher 
> radius.

Actually, this is not the case. The square root actually gives zero
probability for points exactly at the origin; more events must be
distributed over a circular area element at the circle perimeter  than
at the center to achieve a uniform distribution.

I wrote up a few lines of matlab code to show this in graphics (you need
the attached hist2d code to reproduce the attached graphics):

chi=2*pi*rand(1e6,1);
r=sqrt(rand(1e6,1)); % Source radius assumed one metre
x=r.*cos(chi);
y=r.*sin(chi);
mXY=[x y];
vXEdge = linspace(-1,1,128);
vYEdge = linspace(-1,1,128);
img=hist2d(mXY,vXEdge,vYEdge);
imagesc(img);

As you can see in the jpg graphic, the result of the above code (which
is equvalent to the c-code in Source_simple) is completely uniform
across the source area.

Hope this helps?


Cheers,

Peter
-- 
Peter Willendrup <peter.willendrup at risoe.dk>
Risø National Laboratory
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