Multiple vertex generation with General Particle Source

Particles, Track, Event, Run and Biasing
1

_Geant4 Version: 10.4.3
_Operating System: macOS Sequoia 15.5
_Compiler/Version: Apple clang 16.0
_CMake Version: 3.25.1

I’m currently simulating the radioactive background due to the decay of K-40.
I’m quite familiar with the use of multiple sources in GPS for generating different spectra, but I would like to use the ‘/gps/source/multiplevertex true’, where, if I understood well, it’s possible to generate more than one particle per run.
However, the beahoviour seems quite strange because, either using ‘/gps/number’, either adding the same source with ‘/gps/source/add’, the number of products of the decay doesn’t seems to grow linearly with the number of sources: for example if I set number 2 or add 1 source the number of products is not doubled, and it gets worse when increasing it.

Is there a proper way to setup this thing?

I know I could just increase the number of run but I have limitation due to the max size of an integer, and I need to simulate roughly 2e12 particles with limited parallelization.

I leave you here the gps code:

#Source: Potassium-40

/gps/source/multiplevertex true
/gps/number 10
#/gps/source/intensity 1.
/gps/particle ion
/gps/ion 19 40 0 0
/gps/energy 0 eV
/gps/pos/type Volume
/gps/pos/shape Para

/gps/pos/centre 0. 0. -1250. mm
/gps/pos/halfx 750. mm
/gps/pos/halfy 750. mm 
/gps/pos/halfz 50. mm
/gps/pos/confine physConcreteFloor
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From the documentation:

/gps/source/multiplevertex: Specify true for simultaneous generation of multiple vertices, one from each specified source. False [default] generates a single vertex, choosing one source randomly.

That command only applies if you’ve defined multiple sources within GPS.

For what you want (each of N particles in the same distributed source starting at a different position), you might have to write your own primary generator action. You can still use GPS (as a data member of your PrimaryAction), but you’d call it N times, each time for a single particle to make a single vertex). Several of the extended and advanced examples use GPS.