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

The spectrometer can be employed for an in situ iden

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no longer required. The spectrometer can be employed for an in situ iden- tification of chemical elements in pigments, stones, metal alloys and other materials of artistic and archaeological interest. Single SDDs and monolithic arrays of SDDs have also been used as pho- todetectors for the readout of scintillators in hard X-ray and γ -ray detection. With respect to the conventional photomultiplier tubes, the SDDs are more compact and are characterized by a higher quantum efficiency for the scintilla- tion light typical of silicon photodetectors. With respect to conventional PIN diodes, the SDDs offer a lower electronics noise. When coupled to CsI(Tl) scintillators, SDDs have shown to achieve both good energy resolution and low energy threshold in hard X-ray and γ -ray spectroscopy [52] as well as a sub-mm position resolution in γ -ray imaging [53]. 4.2.6 Basics of Signal Electronics The purpose of the electronic chain is to measure “at the best” the value of the charge Q delivered by the detector when an ionizing particle interacts in its active volume. “At the best” means with the maximum accuracy and precision compatible with many physical and practical constraints [54–57]. The charge Q is supposed to be proportional to the energy released by the ionizing particle in the detector. We do not consider in this section the statistical fluctuations in the amount of charge Q delivered by the detector for a fixed energy E released by the particle in its active volume. The presence of several unavoidable noise sources in the detector and in the electronic circuits affects the precision of the measurement of the charge Q . The precision of the measurement is usually defined in terms of the S / N (signal-to-noise) ratio. An alternative concept, very often used in the field of radiation detection, is the equivalent noise charge (ENC). The ENC is, simply,
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236 A. Longoni and C. Fiorini the charge delivered by the detector which makes the S / N equal to one. We will base our presentation on the ENC concept. Basic Electronics Blocks of an XRF Detection System An XRF detection system is basically composed of a detector, of a signal processor and of a unit for storing and analysing the acquired spectra. The detector can be usually modelled as capacitance in parallel with a current generator, which delivers a current pulse carrying a charge Q proportional to the energy E released by the ionizing particle in the detector. The charge Q is affected by statistical fluctuations considered above in this chapter. In order to introduce in the simplest way the basic concepts of signal processing, we will here consider “time-invariant analog signal processors”. By definition, a time-invariant analog signal processor is a circuit which produces at its output, when a signal pulse s in ( t t 0 ) is applied at its input, a signal s out ( t t 0 ) whose shape and amplitude is independent of the value of the time t 0 .
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