Positron Annihilation Energy mismatch

Dear Geant4 Users,

I am studying the annihilation of low energy positrons into a pair of back to back photons.

Sometimes I get weird events in which the energy is not symmetric and doesn’t match the energy in the center of mass (I expect 1.022 MeV) and looks more like the log below where
from a positron which has 0 keV kinetic energy at hist last step we get 2 gammas with 0.354 keV and 1.17 keV.

* G4Track Information:   Particle = e+,   Track ID = 1,   Parent ID = 0
Step#    X(mm)    Y(mm)    Z(mm) KinE(MeV)  dE(MeV) StepLeng TrackLeng  NextVolume ProcName
    0        0        0        0     0.597        0        0         0 water_block_phys initStep
    1     0.33    -0.24   -0.345         0   0.0944    0.596     0.596 water_block_phys annihil
    :----- List of 2ndaries - #SpawnInStep=  2(Rest= 0,Along= 0,Post= 2), #SpawnTotal=  2 ---------------
    :      0.33     -0.24    -0.345     0.354              gamma
    :      0.33     -0.24    -0.345      1.17              gamma
    :----------------------------------------------------------------- EndOf2ndaries Info ---------------

I tried looking on the literature for some rare events that would give such a combination but didn’t find anything helpful.
I’m simulating right away positrons with a kinetic energy distribution and let them annihilate inside a body of water.
From my understanding the medium electrons are expect to always be at rest according to the physics list that I am using for the simulation (in this case FTFP_BERT_ATL).

Any help would be very helpful.



annihilation may happens at rest (higher probability) or in fly (less probable). If energy of a positron is larger, the probability of on-fly annihilation also increases.