Gamma to muon-pair cross-section values

Physics Processes, Models and Cross Sections
1

Dear Geant4 experts,

I am trying to produce muon pairs from gammas. My case involves gammas with relatively low energy (200 MeV to 1-2 GeV). I have plotted a cross section for my process and I found a huge discontinuity in the region below ~500 MeV. I am attaching a plot of the cross section for gamma energies starting above 500 MeV and the same plot from 200 MeV. The first one looks like the plot shown in the official physics reference manual. In the second one, one can see flat values with enormous cross section. I believe the cross section should be very small in this region, instead of very large. (The single point at 200 MeV with cross section = 0 is physically correct. This process has a threshold at ~211 MeV).

This is the code I am using to get the cross section values:

muPairProc = new G4GammaConversionToMuons();
muPairProc->ComputeCrossSectionPerAtom(gamma_energy, Z=82);

I have tried versions 11.1.1 and 11.3.2 (newest) on linux.

Is there something wrong with the way I am trying to get the cross-section values? Or is this some artifact of the parameterization that is being used when evaluating it?

Best,
Martin

2

Here are the plots:

Untitled 3
Untitled 32826×772 133 KB

4

Hello,

the 1st plot is correct. The code you have shown is also correct. How you get the second plot?

VI

5

Hello,

I get both plots with using the same code. The first plot starts at 500 MeV and the second plot starts at 200 MeV. (Please note that the vertical axes have different scales.)

I have researched the problem a little bit more and it seems that in the theory behind it, there is a hard limit for minimum energy of gammas: E_gamma > 4*m_muon. (Gamma Conversion into Lepton Pair — Physics Reference Manual 11.3 documentation).

However, the function to compute the cross section has a limit at 2*m_muon:

/* Geant4 source code */
G4double G4GammaConversionToMuons::ComputeCrossSectionPerAtom(
                         G4double Egam, G4int Z)
			 
{ 
  if(Egam <= LowestEnergyLimit) { return 0.0; }
   // LowestEnergyLimit = 2*m_muon. 
...
}