So...what would a perfect X-ray detector be like?
What would be the ideal detector for satellite-borne X-ray astronomy?
It would possess high
resolution with a large useful area,
excellent temporal resolution with the ability to handle large count rates,
resolution with unit quantum efficiency over a large bandwidth.
Its output would be stable on timescales of years and its internal
background of spurious signals would be negligibly low. It would be immune
to damage by the in-orbit
environment and would require no
consumables. It would be simple, rugged, and cheap to construct, light in
weight and have a minimal power consumption. It would have no moving parts
and a low output data rate. Such a detector does not exist.
- taken from X-ray Detectors in Astronomy by G. W. Fraser.
Introduction to X-ray Detectors
The quest of X-ray astronomy is to be able to detect a weak source against
a fairly strong background. Principally because of the relative weakness
of the sources, integrating detectors (such as film) have not found a place
in cosmic X-ray astronomy. Source detection is done on a
photon-by-photon basis. A flux of one photon per square centimeter per
second (in the 1-10 keV range) observed at Earth constitutes a bright cosmic
X-rays, just like any other kind of light, can be thought of as either
electromagnetic waves or as massless particles. Both ways are
right -- this is known as duality. The way we use to think about
X-rays in a given context is the way that makes the problem the
easiest. Thus, when we talk about optics or dispersive spectrometers
we will talk about waves, because in these cases we need so see how
the waves will interact with surfaces and interfaces, and how they
will combine and interfere with each other. When we speak of
non-dispersive spectrometers, however, it makes sense to talk
about particles of light, called photons.
So how do we measure the energy of an X-ray photon? We need
the X-ray to give all its energy to some kind of detector. That
energy will change something in the detector and, by measuring that
change, we can determine the energy of the incident X-ray. X-rays interact
strongly with electrons. You may know that lead is good at blocking X-rays.
Well, that's because each lead atom has 82 electrons and lead is a metal,
so the atoms are packed together pretty close. Lead stops X-rays
because it has a large electron density.
An X-ray can give all of
its energy to an electron (a process called photo-electric absorption),
or it can give some of its energy (a process called Compton scattering),
or it can scatter without losing any energy (a process called Rayleigh
scattering). The probability associated with each
of these kinds of scattering depends on the energy of the X-ray photon,
the bound state the electron is in, and the scattering angle. For the
X-ray energies of most interest to X-ray astronomy, photo-electric
absorption is much more likely than either kind of scattering.
So, when an X-ray stops in a detector, it has given all of its energy
to one electron. That electron can rattle around in the detector and
give energy to other electrons. In some materials, these
electrons will have enough energy to be free of their host atoms. If an
electric field is applied, these electrons can be collected and counted. The
number of electrons collected tells you the energy that was deposited.
Such detectors are called ionization detectors.
Another approach is to measure heat. All those excited electrons
would rather go back to their original energy. They want to return
to what is called the ground state. Through scattering with other
electrons or with vibrations in the solid itself, they can lose that
extra energy. But that energy has to go somewhere. What is does
is heat the solid and increase its temperature. If you measure the change
in temperature, you can measure how much energy the X-ray originally
No matter which observational technique is used to view the X-ray sky, the
final outcome of a measurement is determined by the properties of the
electronic detector with which the collimated, focused, or diffracted X-ray
photons eventually interact. There are several classes of these detectors,
each with their strengths and weaknesses. They include: