Astroparticle Physics

6.4 X-Ray Astronomy 127In 1952 Wolter had already proposed how to build X-ray telescopes based on total reflection. To get reflection atgrazing incidence rather than absorption or scattering, theimaging surfaces have to be polished to better than a fractionof 10 −3 of the optical wavelength. Typically, systemsof stacked assemblies of paraboloids or combinations ofparabolic and hyperbolic mirrors are used (see Fig. 6.51).Wolter telescopefromsourcediaphragmparaboloidhyperboloidreadoutX-raydetectortelemetryFig. 6.51Cross section through an X-raytelescope with parabolic andhyperbolic mirrorsTo be able to image X rays in the range between 0.5 nmand 10 nm with this technique, the angles of incidence mustbe smaller than 1.5 ◦ (Fig. 6.52). The wavelength λ[nm] isobtained from the relationλ = c ν = hchν = 1240 nm . (6.86)E[eV]The mirror system images the incident X rays onto the commonfocal point. In X-ray satellites commonly several X-raydevices are installed as focal-point detectors. They are usuallymounted on a remotely controllable device. Dependingon the particular application an appropriate detector can bemoved into the focal point. In multimirror systems angularresolutions of one arc second are obtained. The requirementof grazing incidence, however, considerably limits the acceptanceof X-ray telescopes.As detectors for X rays crystal spectrometers (Braggreflection), proportional counters, photomultipliers, singlechannelelectron multipliers (channeltrons), semiconductorcounters, or X-ray CCDs (charge-coupled devices) are inuse.In proportional counters the incident photon first createsan electron via the photoelectric effect which then producesan avalanche in the strong electrical field (Fig. 6.53).In the proportional domain gas amplifications of 10 3 upto 10 5 are achieved. Since the absorption cross section forthe photoelectric effect varies proportional to Z 5 ,aheavynoble gas (Xe, Z = 54) with a quencher should be usedas counting gas. Thin foils (≈ 1 µm) made from beryllium(Z = 4) or carbon (Z = 6) are used as entrance windows.reflection coefficient [%]1008060402000 1 2 3grazing angle [degrees]Fig. 6.52Angular-dependent reflectionpower of metal mirrorsdetector for X rays