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

The high capacitance results in a high contribution

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denser with the capacitance proportional to the area. The high capacitance results in a high contribution of the serial noise component that requires longer shaping time. For long shaping time, however, the shot-noise contribution from the leakage current increases. In the SDD, the charge is drifted from a large area into a small read-out node with low capacitance. Thus, the serial noise decreases and shorter shaping time can be used. This offers two advantages: first faster counting is enabled and at the same time, higher leakage current can be accepted, drastically reducing the need for cooling. Presently, SDD at room temperature achieves an energy resolution of the order of 180 eV [4], comparable to the early liquid nitrogen cooled systems, with shaping times of 250 ns. A detailed discussion of these detectors is given in the first section of the chapter. In the second section, the perspective of a combination of energy and spatial resolution using silicon detectors is discussed. A consequent next step on the path to higher energy resolution was the search for “materials” with even lower “ionization” energy. The electrons
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4 X-Ray Detectors and XRF Detection Channels 201 of superconductors bound in Cooper pairs are such a material. The bind- ing energy of the Cooper pairs is of the order of a few meVs. Thus about 1,000 times more charge is generated per absorbed energy. This charge can be detected with a superconducting tunnel junction (STJ), a Josephson con- tact where the super conducting Josephson current is suppressed by a mag- netic field. Potentially, the resolution limit should be about 30 times lower as compared to semiconductor detectors. An energy resolution in the range of 12 eV for manganese X-rays (6 keV photon energy) has, indeed, been demon- strated [5]. Even more impressive, the energy dispersive spectra of optical photons recorded with these detectors which look very much like the spectra of few-keVs X-ray photons measured with a Si(Li) detector [6]. The price to be paid is to go to an even lower temperature. Si(Li) detectors are operated at about 100 K and STJs below a few 100 mK. If detectors are cooled to about 100 mK or below, the heat capacitance becomes low enough so that single photons create a measurable increase of the detector temperature. The best energy resolution demonstrated so far, 4.5 eV for Mn K α , was performed with the so-called micro-bolometer [7]. A micro-bolometer consists of an absorber at low temperature coupled to a heat sink and a thermometer to measure the temperature change. A large variety of possibilities [8] exists for the real- ization of either the absorber or the thermometer, and further subgroups of detectors can be defined in terms of whether the signal detected corresponds to thermal equilibrium (classical bolometer) or nonequilibrium excitations, e.g. hot electrons or phonons which are detected. Based on such indirect de- tection schemes, spatially resolving devices have also been demonstrated for low-temperature detectors [9]. Although mK-operation sounds difficult, as liq-
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  • Spring '14
  • MichaelDudley

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