Dear Andrea,
I would suggest setting to air also the space between the source and the scoring volume. You could also verify that your AIR material description matches the one used for the conversion factor computation you are referencing.
As usual, I would then work on transport thresholds, normalisation, and statistics of the run.
As you have already understood, in your current setup, the photon spectrum is essentially a delta function at e+/e- annihilation energy.
Best regards,
Tommaso
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