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

Goal can be achieved by means of the integration of

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goal can be achieved by means of the integration of the front-end transistor of the amplifying electronics directly on the detector wafer [40–43]. This solu- tion allows to minimize the stray capacitance of the connections because the bond wire connecting the detector and an external amplifier is substituted by a short metal strip on the chip. By a proper design of the input transistor, also the capacitative matching condition, C detector = C FET , can be achieved. Moreover, the detector–preamplifier sensitivity to microphonic noise (mechan- ical vibrations) and electrical pickups is highly reduced. To fully exploit this solution, in addition to the first transistor of the preamplifying electronics,
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228 A. Longoni and C. Fiorini 500 1000 O(K α ) Na(K α ) Mg(K α ) Al(K α ) Si(K α ) F(K α ) P(K α ) S(K α ) D(K α ) K(K α ) K(K β ) D(K β ) Counts 1000 1000 1500 2000 Energy [eV] 2500 3000 3500 4000 Measurement performed at the MPI PANTER Facility, Munich SDD low energy response function Fig. 4.21. Low energy spectrum of a Macrolon target acquired by means of a SDD. The energy resolution measured on the oxygen K α line (524.9 eV) is 92 eV (figure from [39]) all devices required for the discharge of the signal charge and leakage current have to be integrated on the detector chip [44]. An example of integration of the input transistor in an SDD is shown in Fig. 4.18. The transistor is a nonconventional n-channel JFET, designed to be operated on completely depleted high resistivity silicon, placed inside the ring shaped anode. A narrow metal strip connects the JFET gate to the anode. A circular deep p implantation, biased through a guard ring, divides the tran- sistor region from the collecting region of the detector. The transistor works in a source–follower configuration with an external current supply connected to the source. The discharge of the leakage current from the detector and the reset of the signal charge accumulated on the anode are done continuously by means of the gate-to-drain current of the FET, which is provided by a weak avalanche mechanism occurring in the high-field region of the transistor channel [44]. The mechanism is self-adapting to new values of leakage current avoiding the need of external circuitry. SDDs with on-chip JFET have allowed to reach state-of-the art energy res- olutions in X-ray spectroscopy at room temperature or with moderate cooling by a single-stage Peltier cooler. In Fig. 4.22, two 55 Fe spectra measured by using a selected SDD of 5 mm 2 of active area produced at MPI are shown [39].
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X-Ray Detectors and XRF Detection Channels 229 5 0 200 400 600 800 Counts Counts 1000 1200 1400 1600 1800 2000 5.25 5.5 5.75 Energy [keV] 6 6.25 6.5 Mn K b FWHM = 176eV MnKa (5.895keV) (6.490keV) 6.75 7 5 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 5.25 5.5 5.75 Energy [keV] MnK a (5.895keV) FWHM = 142eV Mn K b (6.490keV) 6 6.25 6.5 6.75 7 Fig. 4.22. 55 Fe spectra measured with an SDD (5 mm 2 ), respectively, at 25 C with 0.25 µ s shaping time (left) and at 10 C with 0.5 µ s shaping time (right) (figure from [39]) The energy resolution at the Mn K α line (5.898 keV) is 176 eV FWHM at room temperature and 142 eV FWHM at 10 C, measured with 0.5 µ s shaping time.
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