I usually simulate a complex source by selecting energies and particles randomly from given distributions. E.g. for Co-60 I generate photons of 1.17 and 1.33 MeV randomly at equal ratio (ignoring the betas).

"Activity" adds time which - at least in my field - does not influence the outcome of a simulation, since you assume the single events to be independent from each other. So if you create 10^9 particles you can interprete these as a GBq or a kBq - the absorbed dose is the same. This changes of course if your target has some sort of a time dependent memory.

Why do you think activity (1/time) could be important?

Actually I am trying to simulate an experimental setup for my project in which I have to expose a  wireless sensor node to high radiation and check what are the defect and effect are there. By considering time i want estimate its lifetime.

Can you please suggest on example from where i can learn to simulate the example (for Co-60 I generate photons of 1.17 and 1.33 MeV randomly at equal ratio (ignoring the betas)).

Well .. this is the easy part of your problem. In pseudo-code I do something like the following fragment in the GeneratePrimaries method:

=== Code ===

r = random(); // equally distributed between 0 and 1

if (r > 0.5){ energy = 1.17*MeV;} else{energy = 1.33*MeV;}

particleGun->SetParticleEnergy(energy);

particleGun->GeneratePrimaryVertex(anEvent);

======

But: This is not the essential part of you problem, you could as well work with a single average energy 1.25 MeV (the linear absorption coefficient does not change much). The essential part is: How do you want to model your detector and how do you want to decide if there is damage?

Thanks for such kind help, as i am beginner i am learning very slowly and dont have strong knowledge for C++

As you suggested that I used single average energy  1.25 MeV for Co-60.But when I run one lakh event the Dose scoring was 1.8 nanoGy in real time of 266.29 seconds. According to my calculation for this case Dose accumulation rate is very less.

Does I need to change the Physics list for Co-60 gamma source? If is it so which list i should use and in which example i will find  that?

For deciding damage of my device, i have performed few experiments by which i have some time period, i want to make the run for that duration as point of damage and calculate Dose.

For now i have kept silicon plate as PCB for dose scoring volume. I want to integrate CAD/SPICE simulation for online monitoring of damage.