**From:** Vasilis Vlachoudis (*Vasilis.Vlachoudis@cern.ch*)

**Date:** Mon Dec 10 2007 - 18:12:16 CET

**Previous message:**Jamshid Soltani: "photon interaction"**In reply to:**Jamshid Soltani: "photon interaction"**Next in thread:**Jamshid Soltani: "Re: photon interaction"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ] [ attachment ]

Hi jamshid,

welcome to the wonderful world of monte carlo.

This zig-zag has a name (and a family-name :) and is called statistical

fluctuations and you will have to learn to live with it. Actually is a

very useful information that tells you how well you should trust your

result.

However a better quantity to plot, is the statistical mean of each bin

together with the error on the mean. In principle MC is a counting

experiment and should follow a Poisson distribution, therefore the error

is the square root of the counts in each bin, which is no longer true if

you are using biasing or other techniques. Therefore FLUKA exploits the

Central Limit Theorem to get the correct mean and error on the mean. For

this reason you need to perform a few cycles and based on the CLT the

final distribution on the average will tend to be Normal (for infinite

cycles) no matter what was the original distribution. Actually it

converges very quickly and 5 cycles are enough.

Please give a look on the FLUKA course material and especially the

presentations about the monte carlo and estimators.

Regards

Vasilis

Jamshid Soltani wrote:

*> Dear fluka Users,
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*>
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*> I made a graph for interaction of Photon with 1.3 Mev with water as
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*> attach input file.there is my graph in attach file too.there is more
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*> widspreding(Zig-zag) in the graph. I changed DEFAULTS to PRECISIOn and
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*> EM-CASCAde but there is zig-zag yet.Can I omit it or not? what is the
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*> reasons of these event?
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*>
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*> thanks
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*> jamshid Soltani
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*>
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*>
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*>
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*>
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- text/x-vcard attachment: Vasilis_Vlachoudis.vcf

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