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

A particular motivation for new developments of x ray

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A particular motivation for new developments of X-ray sources was pro- vided by the recent progress in X-ray optics such as glass capillary optics, Fresnel and Fresnel–Bragg optics as well as HOPG crystals. The efficiency of these optics depends critically on the brilliance of the X-ray source and the compact design of the source. The anode spot should be well focussed and the optics should be mounted as close as possible to the anode spot with the aim to catch as much of the diverging radiation as possible. This allows to re- duce the tube power well below 50 W and to achieve short measuring times at improved local and energy resolution. In consequence, different manufacturers are now offering first samples of miniaturized low power microfocus X-ray
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2 X-Ray Sources 35 tubes. At present the state of the art is not yet quite satisfying and the im- provement of various parameters is highly desirable, e.g., the focus size should be reduced at higher current density, end-window design is preferred and the use of field emission electron emitters would be very attractive. Recent developments in X-ray microscopy and extreme ultraviolet lithog- raphy demand bright table-top sources of extreme soft X-rays, in particular in the region of the water window. Most desirable are sources with a high degree of coherency and ultra short pulse duration. At present laser-based plasma X-ray sources meet these demands best but they are still bulky, expensive and in an experimental state [4–7]. Natural or artificial radioactive isotopes are in principle convenient X-ray sources. They are compact, low cost, continuously radiating at high constancy, independent on surrounding conditions and do not need any power supply. These advantages originated many engineering applications in XRF devices, in particular in portable instruments. The major problem is their radiation hazard potential leading to very stringent safety conditions making these sources quite unpopular now. But they are still irreplaceable in many industrial applications. In radioactive radiation sources the decay products are used for direct excitation of the studied samples. For direct excitation in XRF applications mainly γ -radiating isotopes are used for decaying by K-electron capture (EC) processes which are accompanied by emission of K- and L-lines of X-rays. The latter allows an effective excitation of K-lines and L-lines of numerous elements. The 55 Fe electron capture source which, as an exception, does not emit γ -radiation is widely used as a standard source for calibration of X-ray detectors. Other sources use the generation of mono-energetic X-rays in a special target material where secondary fluorescence is excited by the emitted γ - radiation. However, the photon output of such indirect excitation sources is rather low.
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