Re: Low energy muons - how to limit their step length

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On Wed, 14 Oct 2009 09:18:24 GMT, Kamil Sedlak wrote:

> Dear Michel, Vladimir and Peter,
> 
> thanks a lot for all your suggestions and advices to my muon-range
> problem.
> 
> The difference in the range plotted by Michel and by me is due to
> the introduction of the momentum smearing of the initial muons.  While
> you, Michel, use monoenergetic beam of 28 MeV/c, I used muons with
> the Gaussian momentum spread of +/- 3MeV (=sigma).
> 
> When I simulate the monoenergetic beam of 28 MeV/c as you did, I do
> get a similar picture as you:
> http://www-hep2.fzu.cz/~ksedlak/penetrationDepthInCuPlate_monoenergeticII.png
> (even though there is some difference in the RMS).
> 
> One can see that even in this monoenergetic case, there is a difference
> between the case when I limit the step length of the muons to 0.01 mm, or 
> imply no limit at all.
> 
> I also attach once again the plot with the muon beam with the Gaussian
> momentum spred of sigma=3MeV/c, this time including also a smooth profile
> for the case when the step length is limited to 0.01 mm:
> http://www-hep2.fzu.cz/~ksedlak/penetrationDepthInCuPlate_comparison.png
> http://www-hep2.fzu.cz/~ksedlak/incoming_muon_momentum.png

 Yes, this is true. When the particle energy is too small, the constraints on tracking vanish. the tracking is done in only 1 step, and we loose energy loss fluctuations, hence straggling. There are several ways to force more than 1 step:
make more severe physics constraints (max fractional energy loss and/or multiple scattering), or simply, define a step limit by hand.

  You can clearly realize this if you run TestEm11 interactively and visualise few events.

 I attach an old presentation, not directly connected to this question, but slide 18 is relevant.

> 
> The following processes are switched on in my simulation:
> G4MultipleScattering, G4MuIonisation, G4MuBremsstrahlung, G4MuPairProduction,
> and G4StepLimiter,
> however the penetration depth looks strange (is not smooth) also when only
> the G4MuIonisation is switched on and all other processes are off.

  This is normal. Multiple scattering is somewhat responsible for dispersion.
  If you desactivate him, you see purely this effect of fluctuations lost.

> The muons pass only through the G4_Galactic material before they enter 
> the sample, and there are therefore no energy losses in front of the sample.
> I checked that the muon momentum just in front of the sample is identical 
> to the initial muon momentum when they are generated.
> 
> Best regards,
> Kamil
> 
> PS for Michel: I tried to run your test example, but even though the job
>    seemed to finish OK, no histograms were plotted, and no file was stored
>    in the directory where I run the jobs (TestEm11).  Perhaps because I do
>    not have any "AIDA" installed on my computer?
>

  You have already root installed. You need only an aida interface. I recommend the one of OpenScientist : see InstallAida.txt

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2009/10/14/11.12-93216-testem_series.pdf