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Measuring Photon Energies
by European Space Agency
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!
Gas detectors mentioned above are fine although both their energy
resolution and range of detection is limited. There are several other
methods to measure the energy of a photons and each has its advantages and
the problems. By far the most common X-ray detectors are the Charge
Coupled Devices (CCDs). CCDs are silicon chips in which the incoming
photons excite the electrons in the conduction band. Each CCD consists of
an array of small pixels which act essentially like small photomultiplier.
The photoelectrons produced are stored in the pixels and then read at
once. These solid-state detectors are perfectly suited for the cold
environment of space (as they require a minimum of cooling) but are very
susceptible to radiation damages. Virtually all new missions have had CCD
detectors on-board: ASCA,
Chandra, and now XMM-Newton.
One of the new technologies being tried on XMM-Newton is the EPIC p-n
which consists of one single silicon wafer instead of the multiple usually
used.
CCDs based instruments do not have the best spectral resolution possible.
To observe line emission with a high accuracy, scientists have
investigates several methods. The XRS which was developed to
be flown on ASTRO-E is a microcalorimeter,
the first time such instrument has been part of an X-ray astronomy
satellite. XMM-Newton uses a different technique based on gratings. In
these detectors, the incoming X-ray are dispersed (which the
microcalorimeter does not do) according to their wavelength (or energy).
The RGS on-board XMM-Newton are
the first Reflection Grating Spectrometers ever flown.
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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