question about the difference of F18-source and gamma-source

[SS99aaNN]

Dear Developer,

I am currently using GateV9.2 on Linux, and I have encountered an issue regarding the dose setting for the source.

I ran two voxelized phantom and source simulations, with the only difference being the type of source. One source was set as a gamma source, and the other as a positron F18 source. By setting the energy window to 0-999 keV, I aimed to capture as many counts as possible.

The results show that with the same dose settings, merely changing the type of source leads to a significant difference in event counts. The simulation with the gamma source resulted in 3.4x1e7 counts, whereas the simulation with the F18 source resulted in 1.6x1e8 counts.

Could this difference be due to the conversion of the gamma source dose being different from the F18 source, resulting in the final event counts not being on the same order of magnitude? If so, I would like to know how to convert the radiopharmaceutical dose used in actual applications to the dose used in simulations. For example, if I inject a total dose of 200 µCi of the drug into the phantom during an actual PET scan, what dose do I need for the gamma source and the F18 source respectively to achieve a similar count level to the actual PET scan?

Thank you for your assistance.

Best regards,
Sean