Introduction to X-ray Detectors(2)

    An X-ray can give all of its energy to an electron (a process called photo-electric absorption), or it can give some of its energy (a process called Compton scattering), or it can scatter without losing any energy (a process called Rayleigh scattering). The probability associated with each of these kinds of scattering depends on the energy of the X-ray photon, the bound state the electron is in, and the scattering angle. For the X-ray energies of most interest to X-ray astronomy, photo-electric absorption is much more likely than either kind of scattering.


      So, when an X-ray stops in a detector, it has given all of its energy to one electron. That electron can rattle around in the detector and give energy to other electrons. In some materials, these electrons will have enough energy to be free of their host atoms. If an electric field is applied, these electrons can be collected and counted. The number of electrons collected tells you the energy that was deposited. Such detectors are called ionization detectors.


      Another approach is to measure heat. All those excited electrons would rather go back to their original energy. They want to return to what is called the ground state. Through scattering with other electrons or with vibrations in the solid itself, they can lose that extra energy. But that energy has to go somewhere. What is does is heat the solid and increase its temperature. If you measure the change in temperature, you can measure how much energy the X-ray originally had.