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Re: GEANT3 energy loss [message #6556 is a reply to message #6277] Mon, 21 April 2008 18:12 Go to previous message
Susanna Costanza is currently offline  Susanna Costanza
Messages: 33
Registered: January 2008
Location: Pavia
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Sebastian Neubert wrote on Mon, 07 April 2008 16:32

We have the impression that the behaviour of GEANT is instable.
Has anything changed in the GEANT configuration?

I observed a different behaviour of electrons in GEANT3 and GEANT4 too while studying bremsstrahlung.
I generated electrons @ 1 GeV and I studied the energy and 1/p distibutions after passing through a plane either of aluminium or of argon, with a thickness of 1cm.
For the simulation, I set BREMS = 1 and BCUTE = 1 MeV in SetCuts.C. In this case I observe correct mean and RMS values for energy and 1/p distributions, either with GEANT3 or GEANT4:
mean(E) RMS(E) mean(1/p) RMS(1/p)
0.8921 0.2007 1.244 0.7997 for GEANT3
0.8886 0.2051 1.246 0.794 for GEANT4.

Instead, if I simulate with BREM = 2 (correct energy loss but not
secondary generation) and high BCUTE values (1 GeV), which are the settings needed for a correct comparison between MC and GEANE, I obtain:

mean(E) RMS(E) mean(1/p) RMS(1/p)
0.892 0.03561 1.124 0.06386 for GEANT3
0.8911 0.1995 1.235 0.7389 for GEANT4.

As you can see, the mean values are correct and comparable, but it's not the case of the RMS values: the ones obtained using GEANT3 are wrong!

I obtain different RMS values even in the case of BREM = 1 and BCUTE = 1 GeV:
mean(E) RMS(E) mean(1/p) RMS(1/p)
0.892 0.03624 1.124 0.06707 for GEANT3
0.8911 0.1995 1.235 0.7389. for GEANT4.


It seems there's something wrong with GEANT3.
So GEANT3 must be used with low BCUTE values, both in the case of BREM =1 and BREM = 2.

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