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Reading about differences between Silicon detectors and Germanium detectors, I decided to learn a bit more about the latter, since I've always used Silicon detectors in all the experiments I worked for. While reading about them, I found that Germanium detectors are usually calibrated using $^{60}$Co sources. These sources emit gammas in the ~MeV energy range, but I was wondering what is the process taking place in this measurement, is it photoelectric effect, Compton scattering or pair production? I ask this question because I do not have in mind the energy range of these processes and I know that, for example for gamma-ray satellites like Fermi, that uses a silicon tracker, the dominant effect is pair-production. On the other hand, I also know that gamma-ray satellites of lower energy like Comptel use the Compton effect. There are other gamma-ray satellites proposed as eAstrogam that use both effects. So I was wondering, at MeV energies, of those that I have mention, which one (or which ones) is negligible to measure the total energy of the $^{60}$Co emission?

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    $\begingroup$ Welcome to Physics! Your initial questions are very interesting and I hope you'll stick around. $\endgroup$ – rob May 3 '18 at 19:27
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It is not pair production. Although the energy of the Cobalt gamma rays is in principle greater than the 1.022 MeV needed for to create an electron and a positron, the cross section is negligible below ~4 MeV

Both photoelectric and Compton effect processes are important. In photoelectric absorption the electron gets all the energy of the photon, and as the electron energy is what's actually recorded these events appear as a sharp peak on the energy spectrum produced. For Compton processes the electron gets some of the energy (the recoil photon gets the rest) so these events appear as a continuum.

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  • $\begingroup$ You did not explain at all the physical mechanisms by which gamma rays are detected by a Ge detector. $\endgroup$ – freecharly May 6 '18 at 22:21
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The gamma photons produce highly energetic electron in Ge which by ionization generate a large number of electron-hole pairs proportional to the energy of the electron in the depletion or intrinsic zone of the Ge pn-junction. This is similar to a ionization gas chamber detector. This charge generation leads to a corresponding current current pulse in the outer circuit for the detection of the radiation. All three named processes can produce energetic electrons in Ge. For gamma energy spectroscopy, the photoelectric effect is preferred because it generates electrons with the same energy as the absorbed photon.

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