X-ray telescope

(engineering) An instrument designed to detect x-rays emanating from a source outside the earth's atmosphere and to resolve the x-rays into an image; they are carried to high altitudes by balloons, rockets, or space vehicles; although several types of x-ray detector, involving gas counters, scintillation counters, and collimators, have been used, only one, making use of the phenomenon of total external reflection of x-rays from a surface at grazing incidence, is strictly an x-ray telescope.

An instrument designed to collect and detect x-rays emitted from a source outside the Earth's atmosphere and to resolve the x-rays into an image. Absorption by the atmosphere requires that x-ray telescopes be carried to high altitudes. Balloons are used for detection systems designed for higher-energy (hard) x-ray observations, whereas rockets and satellites are required for softer x-ray detectors. See also X-ray astronomy; X-rays.

Image formation

An image-forming telescope lens for x-ray wavelengths can be based either on the phenomenon of total external reflection at a surface where the index of refraction changes (grazing-incidence telescope) or on the principles of constructive interference (multilayer telescope).

In the case of x-rays, the index of refraction in matter is slightly less than unity. By application of Snell's law, the condition for total external reflection is that the radiation be incident at small grazing angles, less than a critical angle of about 1°, to the reflecting surface. Based on these properties, x-ray mirrors have been constructed which focus an image in two dimensions. Various configurations of surfaces are possible. See also Reflection of electromagnetic radiation; Refraction of waves.

A second type of x-ray telescope is based on the principles of constructive interference in extremely thin layers of material deposited on a mirror surface. Unlike the grazing-incidence telescope, multilayer telescopes do not require the x-rays to strike at shallow angles in order to be reflected. Instead these mirrors are similar to normal optical telescope mirrors where the incoming radiation strikes the mirror at nearly normal incidence to be reflected and focused. See also Interference of waves.

The advantage of a multilayer mirror is that all of the mirror area is used in collecting the x-ray radiation, whereas the grazing-incidence telescopes have only a small projected area of the actual mirror surface collecting radiation. There is an offsetting disadvantage, as the multilayer mirror reflects x-rays only within a very narrow range of energy while the grazing-incidence mirror reflects over a broad range of energies. The effect is similar to using a narrow-band filter with an optical telescope, and in some cases this can be very useful. The first astronomical use of multilayer, normal-incidence x-ray telescopes was in photographing the Sun.

Image detection

The high angular resolution of the grazing-incidence telescope requires a camera that has correspondingly good spatial resolution. One type of detector uses microchannel plates and yields about 20 micrometers resolution for x-rays in the soft energy band (about 100–10,000 eV). The microchannel plate is an array of small hollow tubes or channels (about 10– 15 μm in diameter) which are processed to have high secondary electron yield from their inner walls.

The charge-coupled device (CCD) has been developed for x-ray imaging applications. This solid-state detector consists of microscopic silicon picture elements (pixels) in which electronic charges produced by the passage of an x-ray photon are collected. Typical charge-coupled devices have pixels that are 15–25 μm on a side, and there are up to 4096 × 4096 pixels on one such device. The charge-coupled device not only records the position of an event but can also yield information on the energy of the photon. See also Charge-coupled devices.

A position-sensitive proportional counter is a gas-filled counter in which x-rays are photoelectrically absorbed, yielding an electron which is detected by the ionization it produces in the gas. By operation of the counter in the proportional mode and use of planes of wires to localize the electrical signals, the position and amplitude of each event can be recorded. These detectors generally have lower spatial resolution (about 200 μm) than do microchannel plate or charge-coupled-device detectors, but they can be made larger in size and provide better energy resolution than microchannel plate detectors. See also Ionization chamber.

Another class of detectors consists of very low-temperature devices that detect the heat deposited in an absorber when an x-ray is stopped. These devices promise improved energy resolution and efficiency over a broad range of energies.