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X-Ray Instrument Design
X-ray astronomy is a relatively new science; it originated in the
1960s with a rocket flight which revealed the existence of X-ray
emitters in the cosmos. X-ray instruments on satellites since then have
discovered that X-ray emission is found from a wide variety of objects
in the sky: single stars, binary star systems, supernova remnants,
galaxies, clusters of galaxies, and active galactic nuclei.
Though new on the scene, there's a lot to be learned from X-rays.
From the earliest missions, it was clear that X-ray spectra of
celestial objects are complex and diverse, and carry a tremendous
amount of information. X-rays come from matter that is highly
energized: usually from gas that is very hot (millions of degrees
Kelvin) or very fast electrons losing their kinetic energy. Both the
shape of X-ray spectra and individual features, can tell much about a
source, making X-ray spectra useful scientific tools in X-ray astrophysics.
What can an X-ray
spectrum tell me?
What is resolution, and what effect does it have on
what we can learn from X-ray data?
What Makes Observing X-rays Hard?
The first problem with observing X-rays from Earth is that they are
absorbed by Earth's atmosphere. Because of this, X-ray detectors have
to be above all or most of the atmosphere, which has only been
possible since the invention of rockets.
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Another problem is caused by
the fact that there are much much fewer X-ray photons than, say, optical
photons. Optical photons are thousands of times less energetic than
X-ray photons, so if two sources emit the same amount of energy, one in
X-rays and one in optical light, there will be about a thousand times
more optical photons than X-ray photons (This is like comparing the
number of $100 bills that make up a million dollars compared to the
number of $1 bills it would take to make a million dollars). With so few
X-ray photons, every single one can show up on an image, and it is
crucial to capture as many as possible (see the image
of the Moon at right, for example).
Even if X-ray photons were as plentiful as optical photons, however,
their very nature makes them more difficult to observe. You already
know, for example, that X-rays tend to pass through many things rather
than being absorbed or reflected as optical light is. Special techniques
have to be developed to observe the Universe in the X-rays and learn its
X-ray secrets. |
ROSAT image of the Moon |
Designing X-ray Instruments
X-rays are like visible light, but their high energy means that they
behave more like particles than like optical light
(which behaves like a wave). They are few in number, they do not
penetrate Earth's atmosphere, and they cannot be focused by a lens or
single mirror.
With these characteristics in mind, scientists have developed a number of
effective ways to conduct X-ray observations. Until very recently, the
primary X-ray detector instruments were proportional counters and
semiconductor or sold-state detectors. Proportional counters rely
on the conversion of an X-ray energy to charge pairs, and their energy
resolution is fundamentally limited to no better than ~15%. With this
technique, only the overall spectral shape of the source of X-rays can
be obtained. That is enough to make general statements about the
temperature of the gas radiating in clusters of galaxies, or to
distinguish between a thermal and non-thermal process in supernova
remnants. It is not, however, enough to determine what elements and how
much are present in such systems.
Solid state X-ray detectors use a specially prepared volume of
semiconducting material that absorbs X-rays. The absorption creates
electron-hole pairs that can be counted, and the number of pairs created
is proportional to the energy of the incoming X-ray.
Tell me more about proportional
counters
Tell me more about solid state detectors
The quantum X-ray microcalorimeter, the primary instrument for
Astro-E, is a new approach to the problems of X-ray detection that
seems best able to maximize both the energy resolution of the
instrument, and the number of photons per energy resolution element.
This detector was invented and developed at the Laboratory for
High-Energy Astrophysics at NASA/Goddard jointly with the University of
Wisconsin (other groups, at Lawrence Livermore National Labs, for
example, are developing similar instruments). This detector can measure
the energies of incoming X-ray photons over a broad range of energies
all at once, and with an unprecedented spectral resolution (~0.4 to 10
keV with a 12 eV energy resolution).
Tell me more about microcalorimeters
New X-ray Science
With the next generation of X-ray detectors aboard X-ray telescopes,
scientists will be better able to determine what elements are present in
supernova remnants and galaxy clusters, measurements that may help us to
complete our understanding of how stars evolve and how galaxies form.
Another example where the improvements of the new instruments is
essential to further our understanding is the structure of Active
Galactic Nuclei, and of the black holes that lurk within. Definite proof
that black holes are present in AGNs requires a more direct measurement
of the conditions and kinematics in the AGN, measurements that are only
possible with greatly improved X-ray spectral resolution. The current
generation of X-ray telescopes has allowed us to infer much about the
physical conditions near black holes, AGN, neutron stars, and other
objects. The next generation will allow us to probe ever closer to the ultimateenvironmental limits of each of these objects.
Thanks to Greg Madejski for contributions to this article
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