Free-electron lasers (FEL), synchrotron radiation sources or other soft X-ray sources are of great signifi cance to materials research and materials analysis as well as to micro-lithography. To detect the corresponding radiation, PTB has, in cooperation with the Deutsches Elektronen- Synchrotron (DESY) in Hamburg and the Ioffe Institute in St Petersburg, developed a low-pressure gas detector. It is directly connected to the ultra-high vacuum of the radiation source by differential pumping. The rare gas, e. g. xenon, which is typically used in the detector at a pressure of 10–4 hPa is ionized by means of the photon radiation. Using electric fi elds that are perpendicular to the direction of the radiation, photoions and photoelectrons are then extracted and detected. By calibration against radiometric primary standards, and from the known cross section for photoionization, it is possible to deduce the photon intensity. Due to the low pressure of the gas, the detector is almost transparent and can, thus, be used as a monitor. Besides measuring the intensity, it is also possible to determine the position of the EUV or VUV beam accurately to a few micrometres.
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