I am not quite sure what you are asking. That may just be because I don't know what is "the saturation limit."
Do you mean the saturation current in the photoelectric effect? My understanding of that is as follows. If you have a photoelectric system with a voltage across a gap, then it can produce a current when impacted by photons. This current will increase with increasing voltage but only up to the saturation current. That is the current at which all electrons knocked loose by gammas are captured. There are no more to capture, so increaed voltage can't increase the current.
If that is it, then the basic steps you need are the following.
- Set up your MCNP problem and get the tally of electrons knocked loose per photon started.
- Get the number of photons started per second in your system. (More on this in the following.)
- Multiply your tally by this number to get electrons per second.
- Divide by one Coulomb (that's 6.241 x 10^18 electrons) to get Amps.
To get the number of photons started per second you need some information about the system. You might be given the number directly. Or you might be given the intensity of gammas in Watts. If so, you need to divide that intensity by the energy per photon. That would require knowing the wavelength of the impacting gammas. Possibly the average. Then the frequency is nu = c/lambda, where c is speed of light and lambda the wavelengthy. The energy per photon is then E = h nu, where h is Planck's constant.
I usually found doing these calcs on a spread sheet to be the easiest. That way I could put little labels at each step indicating what was going on.