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Measuring Photon Energies
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!
Other detectors actually measure the visible light produced when the X-rays
interact with the atoms in a 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: the
XRS on-board Suzaku is the first such instrument
on an X-ray astronomy satellite.
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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