In order to measure the energy of a photon, we first need to be able to detect
it. If an X-ray passes through a detector unstopped, it might as well not
have been there. The solution is to direct an X-ray coming into our detector
to where it can interact with an electron. One way this is done is by
filling a detector with a gas like xenon. When an incoming X-ray hits the
xenon gas, it will transfer its energy to the xenon atom, causing an
electron to be knocked off. Because of the strong electric field set up
in the detector, the electron accelerates, causing it to knock the outer
electron out of another xenon atom. This continues to happen until this
cascade creates a small cloud of electrons. This cloud cascades onto one of
the wires in the detector, which causes an electrical charge on it. The size
of the electrical charge is proportional to the energy of the initial photon.
So this method helps us not only to detect X-rays but measure their energies
as well!
There are several kinds of detectors that work this way, among them
proportional counters. Detectors like
scintillators and phosphors actually measure the visible
light produced when the X-rays interact with and are absorbed by the atoms
contained the gas-filled detector.
Measuring the amount of light gives you an idea of how energetic the incoming
X-ray was. Another kind of detector, called a calorimeter, directly
measures the heat produced in the material when an incoming X-ray is absorbed.
One obstacle to X-ray observations is background X-ray interference. In
addition to X-rays coming from the source you are pointing at (and want to
measure), there are photons and high-energy particles hitting your telescope
and detector from other sources and from all angles. These can be solar X-rays
reflected from the atmosphere, high-energy particles from the Sun that are
reacting with your detector and pretending they're X-rays. This
extraneous stuff is known as "noise".
A reasonable analogy of the "source" versus " noise" problem can be found in
the school cafeteria at
lunchtime. Usually, there is a hubbub of noise and conversation, and it's
hard to hear what everyone is saying. Imagine trying to pick up the one
conversation you want to hear amongst all of the other conversations going on
around you. Being able to isolate and detect X-ray signals from a source
over the background noise is a subtle art that is very important.
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