Physics 2000 Einstein's Legacy X-Rays

Making X-rays

Where do x-rays come from?

An x-ray machine, like that used in a doctor's or a dentist's office, is really very simple. Inside the machine is an x-ray tube. An electron gun inside the tube shoots high energy electrons at a target made of heavy atoms, such as tungsten. X-rays come out because of atomic processes induced by the energetic electrons shot at the target.

X-rays are just like any other kind of electromagnetic radiation. They can be produced in parcels of energy called photons, just like light. There are two different atomic processes that can produce x-ray photons. One is called Bremsstrahlung, which is a fancy German name meaning "braking radiation." The other is called K-shell emission. They can both occur in heavy atoms like tungsten.

So do both ways of making x-rays involve a change in the state of electrons?

That's right. But Bremsstrahlung is easier to understand using the classical idea that radiation is emitted when the velocity of the electron shot at the tungsten changes. This electron slows down after swinging around the nucleus of a tungsten atom and loses energy by radiating x-rays. In the quantum picture, a lot of photons of different wavelengths are produced, but none of the photons has more energy than the electron had to begin with. After emitting the spectrum of x-ray radiation the original electron is slowed down or stopped.

What is the "K-shell" in the other way of making x-rays?

Do you remember that atoms have their electrons arranged in closed "shells" of different energies? Well, the K-shell is the lowest energy state of an atom.

What can the incoming electron from the electron gun do to a K-shell electron in a tungsten target atom?

It can give it enough energy to knock it out of its energy state. Then, a tungsten electron of higher energy (from an outer shell) can fall into the K-shell. The energy lost by the falling electron shows up in an emitted x-ray photon. Meanwhile, higher energy electrons fall into the vacated energy state in the outer shell, and so on. K-shell emission produces higher-intensity x-rays than Bremsstrahlung, and the x-ray photon comes out at a single wavelength. Have a look at both mechanisms in the experiment below.

How X-Rays are Emitted

To see x-rays produced by Bremsstrahlung, click on the picture and press "B".
K-Shell Knockout
To see an x-ray photon produced by K-shell knockout, click on the picture and press "K".