absolute flux and M2
artus at obiwan.kri.physik.uni-muenchen.de
artus at obiwan.kri.physik.uni-muenchen.de
Mon Nov 23 16:43:18 CET 1998
Hello Kristian,
> This looks fine, but I think I understand your confusion. A common
> mistake (at least I made it many times while working on McStas) is to
> think of the weights as probabilities, ie. numbers smaller than
> one. Weights are NOT probabilities and they may well be greater than
> one. For example, for the Source_flux example above, N=384 and I=127625,
> so the average neutron weight is a bit more than 300.
>
> In fact, we can see that between the two sets of numbers, I increses by
> a factor of about 10**8, while M2 increses with a factor of about
> 10**16 = (10**8)**2. This fits well since M2 is the sum of the squares
> of the weights (I = sum(p_i) and M2 = sum(p_i**2)). You still get a good
> estimate of the statistical error from the square root of M2.
>
> Does that clarify matters for you?
Yes, thanks. My error was to ignore that sigma**2 is calculated. I somehow
supressed that ...
> When you say 'transform using solid angle', I am not quite sure what you
> mean; the Source_flux component itself makes a correction for the solid
> angle in which neutrons are emitted ... ?
>
The solid angle in my example
COMPONENT source = Source_flux(
radius = 0.050,
dist = 2.000,
xw = 0.040, yh = 0.075,
E0 = 81.804,
dE = 1.636,
flux = 3.63E+12)
is 2.387E-4. 0.04 * 0.075 are exactly width an height of my guide. If
according to my example I(flux)=3.66E+6 and I(flat)=0.0496 then
I(flux) = I(flat) * flux * solid angle / 4 / pi
(The numbers don't fit exactly because I didn't use the same seed for
the two calculations.)
Georg
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