The Astrophysics Spectator



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Observational Astronomy

Detecting X-rays

X-ray astronomy is a window on the hot universe, the universe of stellar corona, hot interstellar gases, compact stars, and black holes. It is one of several disciplines in astronomy that can only be conducted in space, outside of our protective atmosphere. Advances in x-ray astronomy are therefore bound to advances in space flight.

Interactions with Matter

Our ability to image the interior of our bodies with x-rays is so familiar that the inability of x-rays to travel long distances through Earth's atmosphere is startling. In fact, x-rays travel through the atmosphere over relatively short distances, less than ten kilometers, with the lower-energy x-rays being absorbed by air before the higher-energy x-rays. The atmosphere absorbs the x-rays of out space before they can reach any point on Earth's surface; in fact, short of going to space, the only craft that can reach an altitude high enough to observe cosmic x-rays is the high-altitude balloon. Much early work on x-ray astronomy was done by balloon. Observing from balloons, however, has some shortcomings, such as the short duration and limited season for ballooning—wind condition must be right so that a balloon doesn't drift over heavily populated areas—and the interaction of x-rays and gamma-rays with the thin atmosphere above the balloon. Today balloons are primarily used to test new x-ray detector designs. Almost all x-ray telescopes are now mounted on satellites.

X-rays are readily absorbed by the inner, lowest-energy electrons of an atom. When an electron in an atom absorbs an x-ray, it either jumps to a higher energy level in the atom or it is freed from the atom. The energy absorbed when an electron moves to a higher atomic level is releases as the electron decays to lower levels, which involved either the emission of optical and ultraviolet photons or the exchange of energy to another atom during a collision. A free electron loses energy by scattering with atoms and other electrons until it is captured by an atom; these processes distributes the original energy of the x-ray into many optical and ultraviolet photons and into thermal energy that is shared among many atoms.

While the low energy x-rays are absorbed by bound electrons, high-energy x-rays are scattered by electrons This process is called Compton scattering. In these interactions, the x-ray loses some of its energy to the electron. The electron is usually freed from its binding atom in these interactions. After scattering several times, the x-ray will have lost enough energy to be absorbed by a bound electron. Again the energy that was contained in a single x-ray is distributed among many atoms and many optical and ultraviolet photons.

This complex behavior points to a big problem in both x-ray and gamma-ray astronomy: a high energy photon can be turned into a low-energy photon before it reaches a detector. For instance, a gamma-ray striking a spacecraft carrying an x-ray detector can scatter several times before entering the instrument, where it appears as an x-ray. This problem plagues all high-energy observatories, and presents part of the challenge in interpreting x-ray data.

Imaging with X-rays

From the ways that x-rays interact with atoms, and from our experience with x-ray machines, we would not think that x-rays could be focused to a point by a telescope; in fact, the lowest energy x-rays can be focused quite precisely by a mirror if the path of each x-ray is nearly in the plane of the mirror. These grazing-incidence mirrors are at the heart of x-ray telescopes such as NASA's Chandra X-ray Observatory.

When optical light is reflected by the metal coating of a mirror, the electromagnetic field of the light interacts with many electrons within the mirror, causing those electrons to accelerate. These accelerating electrons take energy from the electromagnetic wave of the incoming light and create a new wave traveling away from the mirror's surface, a mirror image of the original wave.

Normally this interaction does not occur with x-rays, because the wavelength of the x-ray is too short to allow it to interact with more than the electrons bound tightly to an atom. But if the x-ray travels almost parallel to the surface of a metal, its electromagnetic field will interact with many electrons, and as with optical light, the x-ray will be reflected.

For an x-ray to be reflected by a metal surface, it must approach the surface at the angle of 85° or more from the perpendicular. For a mirror with a width of one meter in the direction of travel of an x-ray, the collecting area, the area that the x-ray “sees” has a width of only 8.7 cm; To create an x-ray telescope of a given collecting area requires a mirror with a surface area that is 11 times the collecting area.

In practice, the mirrors in an x-ray telescope are hollow cylinders that are wider in the front than at the back. The x-rays reflect off of the inner surface to the focal point along the axis. Because of the large incidence angle, the focal length of a single-mirror telescope is extremely long For instance, if the mirror has an opening radius of one meter, the focal point must be about 5.8 meters from the opening of the mirror.

These properties of an x-ray mirror present difficulties for developing a space-born instrument, which must be compact and light if it is to be placed in orbit on a rocket. The area of a telescope can be increased by nesting mirrors of different radii. The focal length can be shortened by adding a second set of mirrors behind the first; the costs is a loss of x-rays, since a mirror does not reflect 100% of the x-rays that strike it. While all modern x-ray telescopes employ these strategies to increase the collecting area and shorten the focal length, an x-ray telescope has a smaller collecting area and a longer physical length than an equivalent radius optical telescope.

Coded Aperture Mask Telescopes

Above about 20 keV x-rays cannot be imaged by mirrors. For these energies, a less precise method method is used to image the sky: the coded aperture mask. The idea is to let an object cast an x-ray shadow onto a detector. By knowing where the object is relative to the detector, we know the direction to the source from the position of the shadow on the detector. If more than one source exists, one can find the direction to each source by finding the multiple shadows on the detector. As a practical matter, this method only works for locating point sources; a broad area of x-ray emission cannot be imaged through this method.

In practice, an instrument designed to determine direction from a cast shadow has a mask with holes over the instrument's aperture. The pattern of holes in the aperture is carefully chosen to simply the problem of determining the direction to every source in the field of view. This mask is called a coded aperture mask. Telescopes designed on this principle can locate a source with an accuracy of several arc minutes.

Detecting X-rays

Today's x-ray observatories use CCDs to observe x-rays, devices that are similar to the devices found in digital cameras. The CCD is a solid state device that converts the energy of a photon into an electric charge.

In a single atom, the electrons orbit the nucleus at a fixed and precise set of energy levels; an electron cannot change its orbit unless it absorbs or releases enough energy to place it into one of these other energy levels. The property of atoms has its origin in quantum mechanics. When groups of atoms are bound in a material, as happens in a metal or crystal, many of the electrons are no longer bound to a single electron, but are bound to the material as a whole. The energy levels for these electrons cease to be precise value; instead one finds that the electrons are confined to energy bands, with any energy within a band available to an electron, and the energies between the bands forbidden to the electron. The density of electrons in an energy band is limited by the Pauli exclusion principle. Once a band is filled, electrons can only be placed into the next-higher band.

In a metal, the highest energy band is only partially filled. As a consequence, when an electrical potential is placed across a conductor, the electrons can move to higher energies within the energy band, allowing the electrons to flow down the potential. In contrast, all of the energy bands containing electrons in an insulator are filled, so when an electric potential is placed across the insulator, the electrons are unable to change their energy and move down the potential. The only way an electron can move down the potential is to acquire enough energy to jump to a higher energy band containing no electrons.

The designers of CCDs exploit this solid state behavior to create a photon detector. In the case of an x-ray detector, an x-ray is absorbed by several electrons in a filled band, and the electrons jump to an energy band free of electrons. By measuring the amount of charge that collects into the free band over an interval of time, we can measure the amount of x-ray energy absorbed by the detector. The number of electrons in the band is a measure of the energy carried by a single x-ray.

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