[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

These photons are then collected and converted in

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verted into a shower of optical photons. These photons are then collected and converted in charges by means of a photodetector (a photomultiplier tube or silicon photodetector). According to this “two-steps” conversion mechanism, 1 10 100 Argon Total Photoelectric Rayleigh Compton 1 10 100 Energy [keV] Energy [keV] Silicon Total Photoelectric Rayleigh Compton 1 10 100 Energy [keV] Germanium Total Photoelectric Rayleigh Compton 10 - 3 10 - 2 10 - 1 10 0 10 +1 10 +2 10 +3 10 +4 Mass attenuation coefficient [cm 2 /g] Fig. 4.1. Mass attenuation coefficients for Argon, Silicon, and Germanium. The photoelectric, Compton, and Rayleigh components of the total attenuation are also indicated in the graphs
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X-Ray Detectors and XRF Detection Channels 205 scintillator-based detectors are called indirect detectors. In cryogenic detec- tors, the energy of the radiation is converted into (a) heat (phonons) and measured by coupling a suitable thermometer (thermistor, superconducting strip) to the absorbing material, (b) quasiparticles in a superconductor mate- rial and measured with a superconducting tunnel junction (STJ). In most X-ray detectors, the charge collected at the output electrode of the device represents the basic electrical signal available for further processing. The amount of charge Q is proportional to the photon energy E by: Q = E/ε. (4.1) The conversion factor ε differs considerably among different detectors, according to the specific physics mechanisms involved in the charge gener- ation. Typical values for ε are 26 eV per each ion–electron pair generated in argon-filled detectors, 3.6 eV per electron–hole pair in silicon, of the order of 25 eV per collected electron in a CsI(Tl) scintillator coupled to a silicon photodetector and 1 meV per quasiparticle produced in a superconduct- ing detector. The statistical deviations from the average value expressed by (4.1) depend also on the specific interaction process, as discussed later in this chapter. The generation time of the carriers ranges from a few ps in semiconductors up to a few µ s for the photon emission of some inorganic scintillators. In most detectors, like gas-filled or semiconductor detectors, suitable electric fields are applied in order to separate electrons from positive charges (ions or holes) and to collect the carriers at the output of the device. The collection time depends on the mobility of the carriers in the detector material, on the travelling distances and on the applied fields and could range from a few nanoseconds in conventional silicon detectors up to several microseconds in gas-filled detectors. The signal shape at the output of the detector could be therefore represented as a current pulse, whose integral is equal to charge Q and width equal to the collection time, supposing negligible, as in most cases, generation time. The pulse width is smaller than the charge collection time in those detectors, like the SDDs (described later in this Section 4.2), where the signal is induced at the output electrode only when the charge, in its path, is in the proximity of this electrode.
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