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.