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Forum: Physics List
Re: Question G4 vs FLUKA at MeV energies (Daniel Egger)
Date: 03 Mar, 2010
From: Paul Nicholas Colin Gloster <Paul Nicholas Colin Gloster>

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On Tue, 2 Mar 2010, Daniel Egger sent:

|"Dear Geant4 users,                                                   |
|                                                                      |
|I'm trying to compare a Geant4 simulation with a FLUKA one. In both   |
|codes, the geometry is a cube of either Graphite, Stainless Steel or  |
|Tungsten into which a monoenergetic pencil beam of electrons is sent. |
|The beam energy is either 5 MeV, 50 MeV or 200 MeV.                   |
|                                                                      |
|The results show that the higher the energy and the Z of the material |
|the more the two codes tend to disagree. The worst case is 200 MeV e- |
|into a Tungsten bloc.                                                 |
|                                                                      |
|I've attached a .pdf with the energy deposition profiles in the target|
|(G4 is the blue curve that tends to spread out more transversely) as  |
|well as the physics list. I can't figure out what is the source of the|
|problem. Any ideas?                                                   |
|                                                                      |
|Best regards, Daniel"                                                 |

Dear Daniel Egger,

Geant4 and FLUKA and all other Monte Carlo codes are make-believe
instead of reality. They might accurately reproduce reality in certain
circumstances but always be suspicious of any extrapolations to
anything even slightly different than what has already been
experimentally confirmed.

Changing the models or the data may drastically change simulated

I note that the shapes of the curves which you attached are similar,
though the numbers are dissimilar. So though you believe that they
"tend to disagree", they tend to agree qualitatively instead of

Though these were not actually re electrons, I quote them anyway...

J. Ranft had in "Hadronic Collisions: Physics, Models and Event
Generators" in "Advanced Monte Carlo for radiation physics, particle
transport simulation, and applications: proceedings of the Monte Carlo
2000 Conference, Lisbon, 23--26 October 2000" edited by A. Kling
et al., Springer, 2001:

    Most hadronic event generators which can be used for simulating hadronic
and nuclear collisions up to the highest energies are quite similar in their con-
struction and in the underlying theoretical concepts. At energies, where data
from accelerator and collider experiments are available the models agree rather
well with each other and with the most important features of the data. As soon
as we compare the extrapolations of the models at higher energy we find in spite
of the similarities in the underlying theoretical concepts quite often striking dif-
ferences between the predictions of the models. [..]

F. Goldenbaum, M. Enke, D. Filges, J. Galin, C.-M. Herbach, D. Hilscher,
U. Jahnke, A. Letourneau, B. Lott, R.-D. Neef, K. Nünighoff, N. Paul,
A. Péghaire, L. Pienkowski, H. Schaal, U. Schröder, G. Sterzenbach,
A. Tietze, V. Tishchenko, J. Toke, and M. Wohlmuther had in
"Validation of MC Models of Spallation Reactions in Thin and Thick
Targets in the GeV Range" in the same book:

[..] While the predictive power of inter- and intra-nuclear cascade models
coupled to evaporation codes and transport systems is excellent as far as neutron pro-
duction in thick targets is concerned, there are considerable discrepancies not only
between experiments and models, but also among the different codes themselves when
regarding charged particle production in thin targets. [..]

Best regards,
Colin Paul Gloster

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