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

Uid nitrogen already poses a disadvantage for many

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uid nitrogen already poses a disadvantage for many applications, these sys- tems are no longer exotic. The first low-temperature detectors of the STJ and bolometer type are already being operated at scanning electron microscopes and there is an increasing number of commercial suppliers. A comprehensive overview of the recent achievements of low-temperature detectors for X-ray spectroscopy was presented in a paper by M. Frank et al. [10]. This is a short summary of the means to ever better energy resolu- tion. There are, however, other criteria that are even more important for many applications. A main issue for all detectors is the purity of the spec- tra recorded. X-ray fluorescence spectroscopy is based on the excitation with higher energy X-rays and the detection of the lower energy fluorescence X-rays. If monochromatic X-rays are used for excitation, low detection limits are prin- cipally possible because there is no background radiation at the fluorescence line energies. On the other hand, for some of the detected X-rays, energy loss processes in the detector can occur, leading to a shift in the measured energy toward a lower energy. These events create an artificial low energy background in the energy region of the fluorescence lines. Especially for total reflection X-ray fluorescence, this low-energy background from higher-energy photons incident at the detector deteriorates the detection limits of trace elements. Therefore, for many applications the so-called peak-to-background
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202 F. Scholze ratio, which defines the relative height of the full-energy peak in a spectrum relative to the low energy background, is very important. An unavoidable effect, which causes an energy loss, is that caused by the escape of photoelec- trons or Auger electrons from the active detector volume. X-rays are indirectly ionizing, i.e. after absorption of an X-ray the energy is either fully or partially transferred to a Compton electron or photoelectron, respectively. These elec- trons create the charge carriers that are subsequently detected by scattering processes during their slowdown. During this process, the electrons travel a certain distance depending on their initial energy. If such an electron is created close to the boundary of the active detector volume it might escape from the detector and a corresponding part of the initial energy would be lost. There is also the possibility for the opposite process, that is, energy from photons absorbed close to the active detector volume is partially transferred into the detector. It is obvious that the probability of these processes scales with the ratio of the active detector volume to the surface. This effect strongly com- promises its practical use, especially for the small low-energy detectors. This is especially true for the pure STJ detectors, which are made of thin layers of superconductors serving also as the absorber. For photon energies above about 1 keV, the majority of pulses detected with such a device are back-
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