Hi Dr Zoomie! My neighbor works at a factory and was talking about learning to use an x-ray machine for his job. And it made me wonder what in the world a factory would use an x-ray machine for. Why does industry need to use x-rays? Can you shed some non-ionizing light on the matter?
Oh – so many things! Mostly in two different categories: To look inside of other things (I know that’s nebulous, but I’ll explain it shortly) and to see what things are made of.
Seeing inside of things
This is the classic use for x-rays; just as in a hospital, x-rays can be used by industry to see what’s inside of…well…just about anything a long as the energy of the x-rays is high enough. For example:
- One company I did some work for makes (among other things) rivets, bolts, and other fasteners to hold things together. Among the objects their fasteners hold together are airplanes. These fasteners are sprayed with an anti-corrosion coating that needs to have a specified thickness to be effective; the company uses x-rays to check to make sure the coating meets specifications.
- Another company makes brake shoes and rotors; they use x-rays to inspect each part they make for flaws that might cause them to fail, to not last long enough, or that put the drivers at risk.
- A food company puts everything they produce through an x-ray machine to check for bone fragments or any stray bits and pieces of…well…anything that might have fallen into the food.
- Yet another company I consulted for – one that makes high-end jewelry – x-rays visitors to their vault when they’re on their way out to check for theft.
- There are also prisons that x-ray inmate returning from work-release, in this case to check to see if they’re trying to smuggle anything in.
- Many manufacturers will take products off the production line at random and x-ray them to look for flawed welds, poorly fitting components, and so forth. This sometimes involves subjecting parts or products to pressure, stress and strain, and the like and then x-raying them to see if the stress, bending, twisting, and other strains cause anything to break, bend, or come out of alignment.
- Some companies make a point of x-raying incoming parts (especially metal fasteners) to make sure they’re high-quality. The reason for this is that too many suppliers send flawed parts – especially when they’re buying parts from the lowest bidder.
Analyzing composition
In addition to passing through materials to show what’s inside, x-rays can also be used to analyze materials to see what they’re made of. Lead paint analyzers do this, but there are other devices that use the same (or similar) technology to analyze all of the metals in an alloy, or to look for impurities in materials they purchase. What they use is a process called x-ray fluorescence.
What happens in x-ray fluorescence is a phenomenon we’ve talked about earlier ionization. When an x-ray with a high enough energy is absorbed by an electron it can eject the electron from an atom; if the electron was from an inner electron shell then outer electrons will drop down to a lower level, and every time an electron drops like this it emits an x-ray with a characteristic energy that’s equivalent to the energy difference between the higher and the lower energy levels. Because every element has a unique electron cloud structure with different energies for each level, the x-rays emitted for each of these transitions is unique, forming an “energy fingerprint” for every element.
So an x-ray fluorescence unit will have two parts – the x-ray emitter and an x-ray spectrometer that precisely measures the energy of the fluorescence x-rays and, with the help of an internal library of elements, identifies the element(s) present.
To make this more concrete, say a company is purchasing titanium ingots to make strong, lightweight parts for aircraft. Titanium’s a great metal, but it’s expensive and a vendor might be tempted to mix it with a lesser metal; on top of that, even an honest vendor can have impurities creep into a batch of metal if they work with multiple metals. To guard against that possibility, a company might test incoming ingots with an x-ray fluorescence device to check the purity of the titanium they’re receiving; they might test every single ingot, or perhaps only 10% of the ingots, chosen at random. No matter what strategy they choose, the x-ray fluorescence device is crucial.
X-ray diffraction
There’s another way of using x-rays to determine a material’s composition – x-ray diffraction. I used x-ray diffraction in a soil mineralogy course I took in grad school. Before taking the class I’d have sworn that the only thing less interesting than studying dirt would have been studying the clay minerals that make up dirt – turns out I was doubly wrong – soil is fascinating stuff (honest), as is clay mineralogy. And the clay mineralogy…we figured that out using x-ray diffraction. Oh – I should say, too, that using the x-ray diffractometer had me very cautious at first, mostly because I’ve met some three- and four-fingered mineralogists; they got that way by changing slides in the x-ray diffractometer without first turning off the beam (not possible with new devices, but all too frequently done in the older ones). Radiation dose rate can exceed 10,000 R/hr in the x-ray beam and scientists who put their fingers in the beam could easily receive enough radiation dose to those fingers to cause damage requiring the fingers to be amputated later. In any event, I avoided losing any fingers, I got some nice diffraction patterns from my soil sample which made it possible for me to identify the precise soil minerals that were present, and I ended up with a good grade for the class – and with a tremendous amount of respect for those who use these devices every day.
As far as how x-ray diffraction works – I’ve got to admit it’s fairly complicated and requires a fair amount of math to get to usable results; much of the calculation that has to be performed is beyond me. Luckily, virtually all of this can be automated to the point that even I (my math skills are not all that good) can use the devices and their results. Anyhow – here’s the basic principle.
Atoms in a crystal are arrayed in a distinctive structure – think of bricks that are arrayed make up a wall. This, in turn, can be part of a larger structure with its own pattern – think of the walls that make up a home or an office building. What an x-ray diffractometer does is to shoot a beam of x-rays into crystals – when the x-rays strike a plane of atoms (like the sides of a brick or the plane of a wall) they tend to bounce off in the same direction and by measuring the directions at which x-rays are diffracted we can reconstruct where these planes are and how they’re arranged within the crystal. By going all the way around a sample, mineralogists and materials scientists can get a good idea of the way that atoms and planes of atoms are arranged inside a crystal. That, in turn, can give us a better understanding of the material’s properties and what it can best be used for.
And a bonus use – curing paint!
And, as they say in some late-night television ads, “But wait – there’s more!” I ran into one more industrial use of x-rays a few decades back – using a linear accelerator to slam high-energy electrons into a tungsten target to produce x-rays that they used…to cure polymer-based paints just a little bit faster, letting the company print (among other things) milk cartons at a higher rate of speed. It turns out that, under x-ray radiation exposure, wet monomers will cross-link to form dry polymers. And, if this sounds a little abstract, here’s how I think of it.
Think of the arrangement of noodles in a box of uncooked spaghetti – they’re just sitting next to each other; pull out one piece and the rest just rearrange themselves slightly. But when spaghetti is cooked the individual strands curl around, each one touching perhaps a dozen other strands, and clinging to each because of the starch. Pull on a single strand of cooked spaghetti and the entire batch might well come out in a clump. Similarly, pre-irradiation the individual strands of molecules (called monomers) pretty much just ignore each other. But, under a radiation flux, these monomer strands begin to link to other strands (think of the starch that glues spaghetti strands together) to form a polymer. For the paint that was used by this particular company, the monomer product used to paint the individual containers could be irradiated with high doses of x-ray radiation to form a fast-drying polymer by linking the various strands together the same way that starch links together the strands of cooked spaghetti.
That’s what comes to mind off-hand – there might be more uses, but these are the ones I have some experience with.