I think x-rays have had their dramatic potential shortchanged by the way they’re actually useful. You hear “gamma rays” and your mind is drawn to the Incredible Hulk and how he gained his bright purple shorts. Cosmic Rays? Space madness! But when your mind turns to x-rays you start thinking “dentistry.” Much less exciting.
Right. Computers. Today we’re going to spend one more post on Electron Microscopy, and another way these things are useful. This one is actually pretty straightforward from topics we’ve already covered. I’m sure y’all have been taking notes, and know immediately that I’m referring to Computers 5: Fundamental Chemistry, where I described the process of prodding electrons into giving up photons. I’ll save you the reread, even though jokes about New Jersey never get old. Here are the useful bits:
- Electrons hang around atoms in energy levels called ‘orbitals’
- If you jab an electron with a powerful enough photon it’ll jump from one orbital to a higher energy one, or out of the atom altogether.
- That leaves an open parking space near the door, which quickly gets re-filled by that or another electron.
- When an electron drops from a higher energy level to a lower energy level like that it gives off the extra energy in the form of a photon.
Having mastered those ideas you know enough to follow the clever bit. Orbitals have definite energy levels. When you drop an electron from a higher energy level to a lower energy level it gives off a photon. Since those energy levels are well defined, then you get a very specific energy for that photon. And since those energy quantities depend on the number of protons in the nucleus you can tell what kind of atom it is. A jump from the 3d orbital to the 1p orbital in a gold atom will give you a different x-ray than a jump from the 3d to the 1p orbital in a, I don’t know, molybdenum atom.
Let me try that a bit more specifically. I’ve got an unknown atom. I throw an electron at it, much like I’m throwing electrons at it in a SEM. I knock an electron from a lower orbital (say, 1p) out entirely. One of the higher orbital electrons (say, 3d) drops out of its energy level and fills up the empty 1p slot. That’s going to give out a specific amount of energy. How much? It depends on how strong the electric field it’s dropping through is, which is another way of saying how many protons are in the nucleus. If you measure that number exactly you can tell the difference between an iron atom and one that’s made of gold, precious gold!
Let’s cut to the chase. You set up a detector to detect x-rays, and you program a table of known x-ray energies into your computer. You let it detect x-rays for a while on a specific bit of your sample, and your program spits out one of these:
You’re looking at a space about ten microns wide between two bond pads on a part. The bond pads are made of copper and the space is on a plastic surface. The plastic is about ten microns thick itself, and there’s stainless steel below that. The knobby bits in the middle, the parts that look like popcorn scattered on the floor between the couch and the love seat? We’re trying to figure out what they are.
Carbon, Oxygen, and Florine are parts of the plastic. It’s possible that the popcorn stuff is also an organic contaminant and is also showing up as carbon but we can’t actually tell that just from this. Oxygen might also be part of a metal oxide (rusted something.) You’ll note the yellow slice of pie indicates 1.9% iron. That may be an echo from the stainless steel underneath the plastic bits. You’ll also find chrome mixed into the stainless steel, but if you’re seeing 1.9% Iron and 5.5% Chrome then chances are pretty good they aren’t both coming from the stainless. Give that and the copper it looks like we’ve got metallic contamination from… somewhere. I’ve got no explanation for the existence of chlorine.
One quick wrinkle; your photon energy relates to the charge in the nucleus. A lighter atom has less charge and hence gives off weaker photons. To the extent that you can’t detect the lighter elements at all, and I’m now suspicious about the presence of carbon and oxygen in that chart after all. To detect the lighter stuff you deal in Auger spectroscopy, which I don’t, so we won’t go into it.
Okay, but I skipped a step. “Just detect the x-ray” he says, like that’s the sort of thing I do all the time. Oh sure, I’ll detect your x-rays. Would you like that with or without a tachyon pulse? Okay, how do you go about detecting x-rays? Photographic film? Eh, probably not. You need to be able to tell which x-ray is which for this to work out. How do we detect x-rays?
I’m going to level with you; I don’t remember. I delayed this article a week looking through my various textbooks for a better explanation, but it seems to be missing. Even from my materials characterization text, which ought to have one. Wikipedia is uncharacteristically laconic. Two things I can tell you; you wall off your detector with beryllium. That’s the lightest element we’ve got that’s still a solid. Light means you won’t see any x-rays coming off of it. Solid means it’ll block out anything lower energy than an x-ray. Once you’re past the beryllium you collect the x-rays themselves with a photo cell, similar in principle to the things we discussed in Computers #5.
Note that this technique is largely useful for determining what kind of elements you have, not what kind of compounds. You can see if you’ve got Phosphorus in your sample, but you can’t tell the difference between sucrose (a mix of carbon, oxygen, and hydrogen) from high-fructose corn syrup (a different mix of carbon, oxygen, and hydrogen.)
To get compounds you use a different device. Rather than shoot the sample with electrons and measure the x-rays that come off, you shoot the sample with x-rays and measure the electrons that come off. It’s a process called x-ray photoelectric spectroscopy, which we’ll get to next week in… no, wait. I was actually really bad with that machine. We’ll skip that one too.
Instead, we’ll spend a bit of time discussing how you can tell what really small things are like by touching them in “The Many Uses of Tiny, Tiny Needles” or “The Atomic Microscopy Force.” That last one might actually be a thing.
This is part 100000 (in binary) of my ongoing series on building a computer, Pomp and Circumstance way. You may find previous parts under the tag How to Build a Computer. This week’s post has been brought to you by Macho Man Randy Savage!