Physics 2000 Einstein's Legacy X-Rays


Why is tungsten used in x-ray tubes? Can't other elements produce x-rays?

Most elements emit x-rays when properly bombarded with electrons. Heavier elements (like tungsten) are best because they emit a higher intensity through bremsstrahlung, but there are plenty of heavy elements to choose from. The real issue is engineering: Most electrons that hit the tungsten don't do anything special at all -- no bremsstrahlung, no K-shell emission. All of the energy from the electrons' impact then goes into heating the tungsten. Tungsten is used because it can withstand this bombardment, as it has a high melting point and can conduct heat away very well.

What would happen if you replaced the tungsten with something else?

The bremsstrahlung pattern looks very similar no matter what element you use. The K-shell emission spectrum is unique and different for each element.

In bremsstrahlung, why is a range of photons emitted instead of just one wavelength?

The incoming electron is accelerated and strikes the tungsten at a high speed and has a lot of energy. Recall that we called it "braking radiation." The electron might be slowed a little or a lot.

So the amount of "braking" determines which wavelength of photons are emitted.

Yes, but there is more. If we represent all of the energy of the electron as a pie, there are bazillions of different ways of cutting up this pie.

But one thing's for sure, you can never end up with more pie than you started with. If all of the energy goes into producing only one photon, there is no way you could have a photon with more energy than that!

So there should be a sharp cut-off in the spectrum. It looked like there was a cut-off on each end.

No, there is only one cut-off that corresponds to a minimum wavelength. There is no limit to a maximum wavelength emitted. Go back to the bremsstrahlung spectrum and see how it fades gradually to zero for long wavelengths.

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