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Question pi+ decay to mu+ 

Forum: Hadronic Processes
Date: 08 Jun, 2012
From: Tony Slaba <Tony Slaba>

Hello,

I've encountered a problem with pion decay. Initially, I had written an application to look
at transport of GCR protons through slab geometries. When looking at the flux at various depths
within the slab, I noticed the mu+ flux had a strange jump discontinuity near 4 MeV. I eventually
back-tracked the problem to low eneryg pi+ decay (< 20 MeV) and was able to reproduce the issue 
with the example application Hadr01. 

Using Hadr01, I added a line to StackingAction.cc to print out the energy of any mu+ produced
through pi+ decay. For some targets (G4_Galactic, G4_He), the energies of the produced mu+ seem 
reasonable, but for other targets (G4_WATER, G4_Al, G4_Si), almost all mu+ get generated with 
4.11981 MeV. This problem only seems to be obvious when the pion energy is small ( < 20 MeV)
The case of mu- production from pi- decay seems to work fine in general.

I've attached a copy of the macro file I used with Hadr01 to reproduce the problem. I'm using 
Geant4.9.4 (default build - no patches installed) on a Linux machine.

I also downloaded and ran a pre-built version of Geant4.9.5 with Hadr01 and got the same issue
to come up - although I was less confident about the installation/build/compilation process
since an issue with the EXPAT library had to be worked out. 

Thanks,
Tony

*****************************
/run/verbose 0
/event/verbose 0
/tracking/verbose 0
/testhadr/TargetMat G4_Li
/testhadr/TargetRadius 1 km
/testhadr/TargetLength 4 mm
/testhadr/NumberDivZ 1
/run/initialize
/gun/particle pi+
/gun/energy 7.5 MeV
/testhadr/PrintModulo 1000
/run/beamOn 1000
*****************************

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1 None: Re: pi+ decay to mu+   (Dennis H. Wright - 13 Jun, 2012)
(_ None: Re: pi+ decay to mu+   (Tony Slaba - 14 Jun, 2012)
(_ More: Re: pi+ decay to mu+   (Marcus Mendenhall - 15 Jun, 2012)
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