Computing activity of a radioactive source

Hi everyone,
I am trying to simulate a 138La ion as a primary particle (one particle per event).
To get its activity say after N events I just have to divide, N by the “total time elapsed” till N events. I am planning to get the total time elapsed by summing over the GetGlobalTIme() after the end of each event (i am not sure if it is possible to get directly the total time of run from the first event).

Can someone please help,
1.) Verify if this approach will be right to get the source activity ?
2.) Suggest a more efficient/fancy method to get source activities.

Thanks a lot !

No, that won’t get you the activity at all. Geant4 simulates events, not activity. Each event starts over at time T=0, and for a radioactive decay, the global time assigned to the secondaries is a random throw using the decay half-life.

You can’t convert directly from number of events to activity without some knowledge of the real-life set up you’re trying to model. If you have an estimate for the density of La-138 in your device, then you can use that plus the volume to estimate the total number of La-138 atoms in real life. Known the half-life of La-138, you can translate N events back into clock time in real life.

Thanks for your reply, Michael.

The device is a LaBr3(Ce) crystal, and its volume is known. I know the natural abundance of 138La in a LaBr3 crystal and use it to find the number of atoms in real life.

I am planning to “implant” the radioactive 138La in this volume such that the decay happens randomly throughout the crystal. However, I am not sure how to incorporate the “number of atoms in real life information” in the simulation, specifically in the gps.

I think my query is not very clear at the moment, please let me know for any clarifications.

Can you please share how can I do the conversion between events and time ? Thanks