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Fe55 is one radioactive isotope.

It emits X ray photons : mainly k-alpha & k-beta lines.

Why does a single X-ray photon generate 1620 electrons when it hits CCD detector ?

While, in photo-electric effect, single photon generates single electron i.e. a photo-electron.

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    $\begingroup$ There are several different types of CCD detectors used with x-rays. You'll need to provide us with details about the type of detector you are referring to. Possibly you could link the article where that figure of 1620 electrons is given. $\endgroup$ – John Rennie Mar 27 at 12:02
  • $\begingroup$ The x-ray creates one highly energetic electron. Through electron-electron scattering, that energetic electron creates the other thousand-odd electrons as it relaxes down to the conduction band edge. $\endgroup$ – Jon Custer Mar 27 at 12:51
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The iron K-alpha edge is at around 7,000 eV and that bandgap of Silicon is about 1.1 eV.

Let's assume that all that happens when the x-ray hits the ccd is that the x-ray is completely absorbed to create excited electrons. Then we can expect 7000/1.1 or about 6300 electrons from a single x-ray photon. This is a rough estimate, but you can see that x-rays should be easily able to create many electrons at a detector.

Edit: Just to be clear to OP, as Jon Custer mentioned the initial process is a single excited electron (i.e. leading order absorption $H_{int} \sim \mathbf{p}\cdot\mathbf{A}$ term), but the lifetime of this electron is so short that it creates more excited electrons almost immediately (within attoseconds).

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    $\begingroup$ It is well known that for either high energy photons or high energy electrons you never get anywhere close to the (absorbed energy)/(band gap energy) - the process of one highly energetic electron producing additional electron hole pairs has too many losses along the way. $\endgroup$ – Jon Custer Mar 27 at 14:47
  • $\begingroup$ @Jon Custer I know that. The question here is why you can get more than one excited electron from an x-ray, which I believe can be simply answered with an energy-in energy-out argument. Of course not all energy is deposited this way, there are way too other excitations in the energy landscape $\endgroup$ – KF Gauss Mar 27 at 17:18
  • $\begingroup$ I agree, although I think it is useful to note that the x-ray makes just one excited electron, which then cascades down to result in all the others. $\endgroup$ – Jon Custer Mar 27 at 17:20
  • $\begingroup$ In silicon the number is around 3.64 eV per electron. The number is larger than the bandgap because some energy goes into phonon creation rather than ionization. See [Phys. Rev. B. v. 22, p 5565 1980] for details. $\endgroup$ – Laurence Lurio Mar 27 at 18:18

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