How Do You Detect an X-ray?
A telescope is, no matter what its shape, a device to collect
radiation from astronomical sources (stars, galaxies, etc.)
and "deliver" those photons to the detector. The detector then
determines as best it can the direction, brightness changes, and
spectral intensity (or color) of the incoming photons, and when they
Optical detectors measure the visible light from a source; in fact, the Universe has many very bright optical sources to look at. The light they emit is
made up of large numbers of photons all hitting the detector at the same
time. This is because most optical sources put out a lot of photons, and you
collect a large number of them in the large optical telescopes which exist.
A single optical photon is usually hard to see, but many photons make a
significant signal in your detector.
X-ray detectors, in contrast, usually detect individual X-ray photons
which react with the detector. So instead of measuring a spectrum of
light or a bright blob of light, they measure each individual photon
and then, over time, accumulate enough measurements to make an
accurate picture of the total source. Since X-ray photons have a
higher energy than optical photons, an individual X-ray photon is
easier to notice, so counting individual photons is not that
difficult. And, most sources (by the time their light arrives at
Earth) tend to have low overall count rates (number of photons hitting
the detector) in general.
It would be as if you looked at a white lightbulb and, instead of
seeing white, saw one red photon, then one blue one, then one yellow
one, then perhaps another red, then a green, and so on. After you had
seen enough photons, you could combine them and say "Ah, I see, it's a
In addition to getting energy information (so you can produce a
spectrum of the source), timing information and position information
are crucial. Position information is needed so you can distinguish
different regions of the source. For example, you would want good
spatial resolution if you are looking at a binary system and wanted to
distinguish the two stars, or if you were looking at an extended
source that had different processes at the edges than the core.
Timing information is crucial because many X-ray sources (such as
pulsars) undergo changes on time scales much less than one second. To
get a good measurement on short time scales, your detector actually
needs two abilities. It has to be able to accurately determine when
the X-ray hit the detector, and it needs to have a large enough
collecting area so that you get lots of X-rays in your short time
interval. This last bit, called sampling, is important so that you
can have confidence in your results. If you measure a single X-ray
from a source to a nearly perfect level of both timing and energy, it
is still not very useful because you don't know the whole picture.
But if you had 10 very accurate X-rays measured, you'd have a better
idea what the typical X-ray emission was like. And if (for that same
time instant) you have 1000 X-rays measured, well, you'd have a very
good idea of what the source is like during that time interval.
Making Sure You Get the Right X-rays
All of this assumes you're actually looking at the source you want,
and not just measuring random areas of the sky. Because X-rays are
high energy photons, they generally don't reflect well with ordinary
mirrors, and don't refract well with ordinary lenses. X-rays,
instead, go right through the material. This is why we use them for
medical work-- they go right past the skin and only interact (are
absorbed) by denser materials in the body. So if you put a regular
lens in front of an X-ray detector, the X-rays would happily just go
right through it without being affected.
In fact, X-ray telescopes often have non-focusing collimators to
restrict the field of view of the telescope. These are dense material
that blocks X-rays that are coming from directions other than directly
ahead of the detector. This way, you can be reasonably sure that what
you do detect is from the source, and not from (for example) something
to the side of you.
They can also have reflecting mirrors to try to focus X-rays from a
wider area of the sky. Such materials (covered in the "X-ray
telescope" section) use "grazing angle incidence" mirrors. Although
X-rays generally go straight through ordinary telescope mirrors, if
the right materials are chosen and the angles are right, you can
reflect X-rays at a grazing angle (sort of like skipping a rock over
water, instead of dropping it straight down from above).
These details become important when you consider the X-ray background.
In addition to what you are pointing at (and want to measure), there
are photons and high-energy particles hitting your telescope and
from all angles. These can be solar X-rays reflected from the
high-energy particles from the Sun that are reacting with your detector
and thus pretending like they're X-rays, X-rays from your power source,
and other problem cases. So it is important that your detector be
so that the overall background is minimized.
Stop Them in Their Tracks
Using these special X-ray imaging techniques, you can then get the
individual X-rays herded on down to your
detector. You have to be careful in choosing the material of
the detector-- you don't want the X-rays passing through your
detector without being noticed, either! So X-ray detectors are
specifically made of materials that X-rays will interact with. This
can range from choosing a gas that X-rays will cause to "glow", to
using silicon "chips" that X-rays can only get halfway through before
The point is that you want to stop the X-ray in your detector. If the
X-ray passes entirely through the detector unstopped, it's as if (to
you) it was never there. If it interacts with the detector (perhaps
losing some energy) but still makes it out the other side, you haven't
done a very good job of measuring it, you've just cut it down a bit.
So you want two parts to your detector.
You want everything around the actual detector "core" to be as
transparent to X-rays as possible. This way, X-rays won't be absorbed
by the detector housing before they reach the measurement devices.
Then, you want your measurement device to stop the X-rays in their
tracks, so they can measure them. This means the detector size and
materials must be designed so X-rays that enter are completely
absorbed, producing some sort of signal in the process that you can
This signal can be of three forms. Some detectors, such as
proportional counters, CCD (semiconductor) devices, and microchannel
plates, measure the electric charge that occurs when the incoming
X-ray interacts with the detector's atoms and strips off electrons or
causes photo-electrons to be emitted. These electrons can be measured
as an electric current, and from this you figure out how much energy
the X-ray originally had to create that many electrons. Some
detectors, such as scintillators and phosphors, actually measure the
light produced when the X-rays interact with the atoms and are
absorbed, producing photons (light) in return. Again, measuring the
amount of light gives you an idea of how energetic the incoming X-ray
was. And some detectors, called calorimeters, do a direct measurement
of the heat produced in the material when the incoming X-ray is
Different Kinds of Detectors for Different Jobs
A principal question with selecting a detector for a given application is to
determine what you exactly want to measure. One can try to get an
image of the source, recording detailed position information of the incoming light; one can try to measure the spectrum of the source, which requires
getting a very accurate measurement of the energy of each incoming X-ray; and
one can try to get timing information, measuring the exact time of arrival
for each of the incoming X-ray photons. Finally, you want to try to capture as
many X-rays as possible, and thus have a large detector surface area.
An ideal detector produces excellent resolution for all three
quantities, but in practice, detectors are generally optimized for one
quantity and then have less accuracy in determining the remaining ones.
All detectors have to deal with background. In addition to the
ambient background described earlier, there is a background emission
of X-rays that "hides" the incoming signal. The overall X-ray
background is generally about the same strength as the source count you want to
measure, and your detector therefore has to be able to either not
notice this background, or be able to get direction and energy
information so that you can later (when doing your data analysis) be
able to figure out what were background events, and what were actual
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 it's hard to hear what everyone is saying. However, if someone across
the room says your name, you can generally pick it out from the
noise. This is because your name is a clear signal, with a specific
shape, while the overall noise is a somewhat homogeneous mess.
Detecting X-ray signals over the background noise is a subtle art that
is very important when doing analysis.
Specific Detector Types
Proportional counters are one of the most common X-ray detectors used by
recent missions, although CCD chips are rapidly gaining popularity as
the technology improves. Microchannel plates are also a workhorse of
satellite missions and continue to be flown today. Calorimeters
are a new technology for X-ray measurements, and will be flown on
upcoming missions such as Astro-E. Each uses a different approach to
detecting incoming X-rays.
A proportional counter is somewhat like a fluorescent light tube in
reverse. Instead of applying an electric charge to get light, you let
X-ray photons hit it and measure the resulting electric charge. The detector
consists of a gas that reacts well to X-rays, in a tube that has
electrodes and some applied voltages. The incoming X-ray reacts with
the gas, producing electrons through photoionization. These
electrons are propelled by the electrode voltage, travel down the
detector, and are measured by the electronics at the end.
You can then figure out what the energy of the X-ray was (from the
signal strength) and when it hit (from the arrival time and shape
of your electronic signal.) You also get some positional information,
based on the timing and signal shape. By dividing the proportional
counter into smaller cells, you can more accurately determine the
of the incident photon. The most accurate measurement is typically the
the energy resolution of proportional counters. An advantage is that
they also have large surface areas, which means they can capture more
incoming X-rays than a smaller detector might, without needing a mirror
arrangement to focus X-rays onto them.
Microchannel plates are essentially large X-ray photomultipliers.
Made of layers of reactive material divided into narrow channels, these
detectors can be made with a good sized surface area, and therefore
are good when you want to collect a lot of X-ray photons (without
requiring focusing.) Incoming X-rays react within one of the plate
glass or metal layers via the photoelectric effect, as with a
proportional counter. By measuring the induced signal, and noting the
channel location and time of the event, you can get a good measurement
of the energy and location of the incoming X-ray. Because they can be
made quite large and the technology is relatively immune to distortion
by magnetic fields, these large-area detectors have been used on
many space missions.
In contrast, a newer technology has become more widespread since the
late 1990's. Solid-state detectors like silicon CCDs (Charge-Coupled
Devices, similar to the CCDs in video cameras) consist of silicon (the
standard computer chip material) doped with impurities to create sites
where the conductivity is different. Other solid state devices exist,
using similar principles as for CCDs. Unlike optical CCDs, which
measure light impacting the surface of the chip, X-ray CCDs measure
X-rays that penetrate into the middle of the CCD. There, the incoming
X-ray creates a cloud of electrons when it reacts with the
silicon/impurities, and this cloud is moved (by voltages applied to
the chip) in bucket-brigade fashion across the chip and measured at
the end as an electric charge. The charge measurement gives you a
very accurate estimate of the energy of the original X-ray. Timing
measurements are decent, since you have regular clock-like readouts of
your CCD. One issue with CCDs is that they are typically small, and
thus have a small collecting area. In other words, you can get very accurate
energy measurements, but not as many measurements as a larger detector
(like a proportional counter) might. Thus, CCDs work best in situations
where you have telescope mirrors to focus X-rays onto them, such as
Chandra X-ray Observatory and XMM-Newton.
Calorimeters are devices that take a completely different approach. By cooling
a small amount of X-ray reactive material to nearly zero Kelvin, one can
detect individual X-ray events by measuring the heat increase that
results when the X-ray is absorbed. From this, you get a very accurate
measure of the X-ray's energy. However, because you have to cool down the
material so much (and because the detectors are typically very small), you get
low count rates relative to other detectors.
Which is Best?
The different types of detectors all have different strengths. You want to get
a large number of accurate energy measurements for individual regions
of the source with exact timing and a good ability to ignore or reject
background counts. This means you want a large area detector (so you
can capture lots of photons) that has excellent intrinsic energy
resolution. You want excellent timing resolution (meaning you can tag
each photon with a highly accurate arrival time, which generally means
you're reading out data very quickly). And you want to be able to
perfectly distinguish the source from the overall sky. Lastly, you want
any electronics or read-out devices not to add any noise themselves, but
perfectly transmit all the signals received.
With a real device, all of the objectives listed above cannot be
simultaneously achieved. You must pick one quantity (or perhaps pair of quantities) which is the most important to your experiment and maximize the detector for that particular kind of observation. If you are doing spectral work over long time
scales, energy resolution is more important. If you are more interested
in rapid source changes, a detector with good timing is more critical.
If you are looking at very faint sources, you should choose a larger
surface area or use a telescope with mirrors to increase your area.
If you want something that is hardy, will last a long time, and be
relatively unaffected by orbital changes, that also becomes a factor.
Detector performance degrades in orbit, and useful detector lifetime should
be considered. Microchannel plates generally last a long time. Proportional
counters in a vacuum will ever-so-slowly leak gas and thus degrade over time.
CCDs are damaged by the particle flux around the Earth (kept from us
by the Earth's magnetic field), and thus decrease in effectiveness
over time. Calorimeters are limited by the amount of coolant they can
carry on board. Each detector has its strengths and weaknesses, which
is why many missions now fly a variety of detectors. Even as detector
technology continues to improve, there will always be trade offs. However,
it's a guaranteed bet that today's satellites are able to see further and
more accurately than yesterday's, and tomorrow's will be better still!