Proportional counters and microchannel plates

Developed right at the beginning of the space age in the 1940s, the proportional counter was the first detector used in X-ray astronomy and stayed its workhorse for almost four decades. Although the principle of such a detector seems to be rather simple, over time it underwent considerable performance improvements and the lifetime under orbital conditions was extended tremendously. Particularly the invention of position-sensitive proportional counters provided new and sophisticated methods to discriminate background and thus enabled observations of much weaker sources. A leap forward in position resolution was achieved with the advent of microchannel plate (MCP) detectors in the 1970s. In contrary to gas filled detectors, they provide no considerable energy resolution but feature spatial resolutions reaching down to a few tens of micrometers, fitting ideally the angular resolution of the novel grazing incidence imaging X-ray telescopes upcoming at that time. Even today, both types of detectors are still relevant in space-based astronomy. However, in case of MCPs new developments focus on the far and extreme ultraviolet wavelength range, while the Chandra X-ray observatory is most likely the last mission applying this technology for X-rays. In contrast, compact detectors with gas electron multiplier (GEM) foils and micropattern readout are currently under heavy development for the soft X-ray range, since they allow for the first time to measure polarization in X-rays over a broad energy range. This chapter presents the principles of proportional counters and MCP detectors, highlights the respective performance characteristics, and summarizes their most important applications in X-ray astronomy.