From: Alberto Fasso' (fasso@SLAC.Stanford.EDU)
Date: Fri Mar 23 2007 - 02:18:51 CET
Lindley,
> > "Everything is clear now. Counting gammas or neutrons or events has no
> > meaning. You should always count the corresponding weights."
>
> This means that running 10,000 lowEnergy neutrons has no meaning. I have
> actually run the sum of the weights of those 10,000 neutrons? Is the number of
> neutrons made be n,2n reactions sum of parent weights - sum of daughter
> weights?
Your primary neutrons (those you start with the BEAM command) have all
weight 1, so you have run 10,000 neutrons with a total weight of 10,000.
So far the two things coincide.
But unless you make sure there is no biasing, the weights of the secondaries
produced may have weights different from 1. You are using the normal default
(NEW-DEFA), which does non-analog absorption for the thermal neutrons: at
each thermal neutron interaction the neutron is absorbed with a probability
of 5% and is scattered with a probability of 95%. Each time the weight
of the scattered neutron and that of any secondary particle (including
the gammas) is adjusted to take into account the ratio of the artificial
probability used and the actual physical probability. From now on, only the
weight has physical meaning: the "number" of neutrons, gammas etc. is only the
number of artificial Monte Carlo particles transported. Each of those particles
"represents" with its weight a different number of physical particles.
So, a gamma with weight = 2 is actually two gammas in the physical reality.
But FLUKA only transports one: the total weight is always conserved.
This is very useful to save computer time in many applications.
But if you don't want it, you can change the default using command LOW-BIAS
(WHAT(2)) or setting DEFAULTS to CALORIMEtry, NEUTRONS or PRECISIOn.
> In a previous post Alfredo told me compound works like this for atomic composition.
> C9H12 would be 9/21=+0.428... 12/21=+0.571... My liquid scintillator is actually a mixture of several things but for the previous discussion I thought talking about C9H12 would be easier to talk about.
>
> Alfredo's post:
> http://www.fluka.org/web_archive/earchive/new-fluka-discuss/0654.html
But Alfredo's post was based on the guess that you had 1 atom of C and 2 of H:
so indeed 33% and 66% with WHAT(1) > 0 and WHAT(2) > 0 was ok.
But you had said 9 C and 12 H, which would give +0.428 and +0.571 as you say
above (or simply +9. and +12. as I wrote: there is no need to normalize to 1,
the program does it anyway). So I was puzzled by the numbers +0.3381203 and
+0.6618797 I saw in your input.
Alberto
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