Image of CCD assembly made for the ASTRO-E mission from ASTRO-E learning center.
There are many ways to detect X-rays, all of which detect incoming photons by their interaction with the detector material. That interaction produces a signal, which can be in the form of an electric current, a low-energy photon (typically visible light) or heat. Several types of detectors will be described below.
Proportional counters are large area detectors. They are filled with gas that produces an electrical charge when an X-ray passes through. One of the detectors on the Rossi X-ray Timing Explorer was a proportional counter detector filled with Xenon gas. The photon's energy is determined from the strength of the electrical signal; its time from the arrival time and shape of the electrical signal. In addition, the position in the sky from which the X-ray came is determined by the timing and shape of the electrical signal.
Microchannel plate detectors are also large-area detectors. They are basically X-ray photomultipliers. They are composed of layers of reactive material divided into narrow channels. The energy and location of incoming X-ray photons are determined by the strength, channel location and time of the electrical signal produced by the photon's interaction with the detector.
In contrast to proportional counters and microchannel plates, CCDs are small-area detectors and require that the photons be focussed onto the detector plane. CCDs are made of silicon doped with impurities to create sites with different conductivities. Incoming X-rays then interact with the silicon and impurities to create a "cloud" of electrons. A voltage is applied across the CCD, and this cloud of electrons follows that voltage to the end of the CCD chip. From the charge of the electron cloud, the photon's energy is determined. Since regular readouts are performed, the timing can also be determined.
Image of CCD assembly made for the ASTRO-E mission from ASTRO-E learning center.
Microcalorimeters are an upcoming technology for X-ray detection. Like CCD detectors, microcalorimeters are small-area detectors and require that the X-rays be focussed. The way a microcalorimeter works is to directly measure the heat of an incoming X-ray photon by measuring the change in temperature of the detector material after it absorbs an X-ray. Since X-rays are very energetic, this may not sound too difficult, but consider this: if a 6 keV photon were to be absorbed by a typical US penny, it's temperature would only change by 3 parts in 1018 (that's 3 parts in a billion-billion)!! (This example is from the GSFC X-ray microcalorimeter group webpages.) To overcome this problem, a microcalorimeter uses materials which have a much higher heat capacity than copper and runs at a very low temperature (50 milli-Kelvin, or -273.1° Celsius, or -459.58° Fahrenheit). By doing these two things, we can get a detectable change in temperature.
This image is the a microcalorimeter detector array from the GSFC X-ray microcalorimeter group webpages.
