Astroparticle Physics

128 6 Primary Cosmic RaysFig. 6.53Principle of operation of aproportional counterphotomultiplier, channeltronangular resolutioncharge-coupled deviceThe incident photon transfers its total energy to the photoelectron.This energy is now amplified during avalanche formationin a proportional fashion. Therefore, this techniquenot only allows to determine the direction of the incidenceof the X-ray photon but also its energy.With photomultipliers or channeltrons the incident pho-ton is also converted via the photoelectric effect into an electron.This electron is then amplified by ionizing collisions inthe discrete or continuous electrode system. The amplifiedsignal can be picked up at the anode and further processedby electronic amplifiers.The energy measurement of X-ray detectors is based onthe number of charge carriers which are produced by thephotoelectron. In gas proportional chambers typically 30 eVare required to produce an electron–ion pair. Semiconductorcounters possess the attractive feature that only approximately3 eV are needed to produce an electron–hole pair.Therefore, the energy resolution of semiconductor countersis better by a factor of approximately √ 10 compared to proportionalchambers. As solid-state materials, silicon, germanium,or gallium arsenide can be considered. Because of theeasy availability and the favourable noise properties mostlysilicon semiconductor counters are used.If a silicon counter is subdivided in a matrix-like fashioninto many quadratic elements (pixels), which are shieldedagainst each other by potential wells, the produced energydepositions can be read out line by line. Because of thecharge coupling of the pixels this type of silicon image sensoris also called charge-coupled device. Commercial CCDswith areas of 1 cm × 1 cm at a thickness of 300 µm haveabout 10 5 pixels. Even though the shifting of the charge inthe CCD is a serial process, these counters have a relativelyhigh rate capability. Presently, time resolutions of 1 ms up to100 µs have been obtained. This allows rate measurementsin the kHz range which is extremely interesting for the observationof X-ray sources with high variability.6.4.4 Observation of X-Ray SourcesThe Sun was the first star of which X rays were recorded(Friedmann et al. 1951). In the range of X rays the Sunis characterized by a strong variability. In strong flares itsintensity can exceed the X-ray brightness of the quiet Sunby a factor of 10 000.