Positron annihilation in G4Air?

Setup: A world volume box, Na 22 source
While using G4 air, I see position annihilation happening, the emission of two 180-degree photons. I am confused about whether this is realistic or if I need to change my material definition.
While using a vacuum-like material, I am not observing the positron annihilation.

Can someone please point out any inadequacies in the material and/or physics definitions that I am missing here ?

Material: G4 Air


Material: Galalactic

G4Material("Galactic", 1., 1.01*g/mole, CLHEP::universe_mean_density, kStateGas, 2.73*kelvin, 3.e-18*pascal);

Thanks a lot!

Think about the “reality” of what you’ve set up. Na-22 decays emitting a positron, right? If that positron hits material (G4_Air) with actual atoms, it should annihilate, giving you a couple of 511 keV gammas.

Your “Galactic” or “G4_Galactic” is defined with a density of 1e-25 g/cm3, or 0.06 atoms/cm3. That’s about 16 cm between atoms. How big is your box? 1 m? Then that positron might only encounter ~6 atoms or less before it leaves the world, so positron annihilation is quite unlikely.

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If you run TestEm0 with attached macro, you will see that mean free path for annihilation of positron is ~84 m in Air and ~1.6e+7 parsec in Galactic.

kitu.mac.txt (242 Bytes)

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