Shedding a New Light on the Universe
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!
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.