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.

  • 3
    $\begingroup$ 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

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).

  • 1
    $\begingroup$ 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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.