Physics 2000 Einstein's Legacy X-Rays

K-Shell Emission

In the K-shell process, what's with that electron that drifts in from nowhere after the electron gets knocked-out?

A heavy atom has lots of electrons surrounding the nucleus in various shells. To keep it simple I didn't show them all. Really that electron that drifts in comes from one of the other shells of the atom. The K-shell knock-out affects the innermost electron, so its like having a hole in the bottom. This "hole" causes a domino effect where the electrons above it cascade down to fill the hole.

But why the innermost electron? I would have guessed the outermost electron would be the easiest to knock out.

An x-ray photon has a lot of energy in it, and only transitions of the inner electrons release that much energy. Transitions of the outer electrons, which can happen, might be in the infrared or visible part of the spectrum. For the electron energies used in x-ray tubes, it turns out the inner electrons are the most likely to be knocked out.

You said earlier that the K-shell spectrum depends on the target element. Why?

Remember that we said each elements has its team colors? We're looking at the transition between two states, so we are looking at the team colors in the x-ray part of the spectrum. These fingerprints were used by Moseley to help understand atoms with lots of electrons.

What did Moseley actually do?

Back in 1913 when he was a graduate student he looked at the K-shell radiation coming out from various elements from aluminum to gold. He found a connection between the wavelength of the emitted K-shell x-ray and the element (atomic number) that helped us understand atoms better.

How can you find the wavelength of an x-ray? That seems like a hard thing to measure.

That's another story in itself. He used the (then) recently developed technique called Bragg scattering, where you scatter x-rays off a crystal. He was able to predict the existence of elements not yet observed, such as technetium, promethium and rhenium. And even today, because of the uniqueness of the fingerprints of each element, x-rays are used for chemical analysis as it is very sensitive to impurities.

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