Assigning Nuclide Specific Activity for Radioactive Decay

Hello! I am stuck on this problem for quite a while, so I would really appreciate if anyone give me some suggestions. I am trying to simulate the radioactive decay of materials inside the glass pressure vessel of PMTs. The paper I am currently following has listed some specific activity for specific radioactive nucleus inside the glass material and I want to consider those activities while simulating the decay inside pressure vessel. Therefore, I was just wondering, is there any way to assign nucleus specific activity to each radioactive nucleus? Thank you in advance.

Since you have to assign the radioisotope as a primary particle (with a G4Ions as particle definition), probably the best way to do this is generate each isotope as a separate run. From the number of events generated, you can compute the corresponding specific activity.