Electron spectrum on silicon detector with and without photonic contribution

I was trying to simulate the electron spectrum generated by a silicon detector.

  • In my geometry, I have simulated a silicon wafer with 2mm thickness and I am trying to study the spectrum generated by a high-energy electron source. A simplified visualisation of geometry looks something as follows.

There is an aluminium absorber of thickness ~50 um placed in between the source and the silicon wafer.

  • The electrons follow a subbotin(alpha=10) kind of energy distribution with maximum energy ~25 MeV

  • As expected the electrons will not deposit the full energy in the thin silicon detector and only the transmission peak will be seen. But I wanted to study the contribution in the spectrum from photons and from electrons only. So in my physics list I defined the cut for gamma as a large distance (like 200 km) and for the electrons and positrons as 1um. My main purpose was to separate the photon contribution from the spectrum, (which I expect to be there at very lower energies), but what I got was quite surprising.

  • Here the peak is the electron transmission peak due to the partial energy deposition of electrons in the detector. But I am not sure why this transmission peak position is shifting when the gamma cuts are defined very high. The position of the transmission peak is around 600-700 keV and I think the silicon detector is insensitive to photons of such high energies. (I have already calibrated the detector with the cobalt source and it is insensitive to the 1.17 MeV and 1.33 MeV gamma lines). I am guessing, the shift in the position of the peak can be due to the change in the stopping power value, but I cannot understand why this happens. Is this way of defining very high cuts for gammas the right way to eliminate the photons from the spectrum?

I am attaching my physics list also with this thread
PhysicsList.cc (12.0 KB)

Thanks a lot in advance for your time and interest.

So you are measuring both electron and photon depositions in your Si. You are using an electron source, and the electron source is producing photons via interaction with the aluminum? Then you are looking at the spectrum of energy deposited depending on particle (e vs p). And you expect that the electron spectrum should be independent of whether you are cutting off gammas or not.

My first guess is that the photons are producing electrons, and you see then your electron spectrum. It depends on how you are defining your electron and photon energy depositions. In theory, every photon absorbed will produce electrons in the si (via photoelectric, Compton scattering, or even pp)

you can send the whole project, is it to have more information?

Hi Matthew,
Thanks a lot for your response. Yes, these photons can generate electrons in the detector. I agree with that. But as you can see, when I removed the photon contribution (When I defined the gamma cuts to large values), it cause a shift in the peak position of the transmission peak to higher energy. I am confused why this happened. If you just check the stopping power value of electrons in Silicon or Aluminium, you can see that it is more or less independent of the energy of electrons in the energy region of our concern. The transmission peak should be at the same position for any high energy electron above 5 MeV for the silicon detector of thickness 2mm as used in the simulation. The shift in the peak position is resulting from the change in the stopping power value I guess. Even I the photon-induced electrons are present, I don’t think it can change the position of the electron transmission peak. I am attaching the stopping power of the electron here.

Based on that chart, I would not say that sthe stopping power is “more or less independent” of energy", and especially not in the 5-20 MeV range.

I am highly skeptical that merely changing the gamma cutoff length would in anyway change the physics of stopping power in your detector. If you want to compare the spectra from electrons interacting with your detector vs photons interacting with your detector, I would instead focus on looking at the primary particle interacting with your detector and separating the data depending on if it is an electron or photon->electron

Hi Matthew,
Thanks a lot for your suggestion. But I am unsure how to separate the electron and photon events interacting with my detector material. Is it through the stepping action?

You can use stepping action to determine what particle is being stepped and how much energy is deposited.

Are you trying to determine energy deposited from primary photons vs electrons? Or any photons/electrons?

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