X-ray Detectors

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 arrived.

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 white light."

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 detector 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 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 housed 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 being 'stopped'.

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 measure.

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 absorbed.

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 source events.

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. 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 position 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 the 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!