[Here's] a novel discovery about a very common piece of stationery, and that’s sticky tape. But who would have thought that just unrolling it can produce intense bursts of X-rays. To tell us more, here’s Carlos Camara.
Carlos Camara: In our group, we’re generally interested in energy-focusing phenomena. In particular, triboluminescence, which is the emission of visible light from relative motion of surfaces; like, for example, when you chew on winter green candy in a dark room, you can see that it glows when it breaks and it rubs.
Chris Smith: I think I’ve seen something similar when you’re tearing the seal of a self-sealing envelope apart and the glue molecules seem to glow a sort of blue colour.
Carlos Camara: Yes, absolutely, and the same thing happens with sticky tape. If you take a roll of regular sticky tape into a dark room, and you let your eyes adapt for about ten minutes, and you peel the tape, you don’t even have to do it fast, just peel it slowly, you’ll see that the peeling verdicts of the tape as you’re separating it glows in this beautiful fluorescent blue.
Chris Smith: And what’s actually the physics that’s making that tape light up like that?
Carlos Camara: Well there are many mysteries that remain to be solved, especially now that we've shown that the energies are so huge at that peeling point, but generally what’s going on is that a tremendous amount of charge is being separated between the two faces of the tape, between the sticky side of the tape and the roll, so very large electric fields get formed in between the two faces at that little interface in about a region that’s not more than a tenth of a millimetre.
Chris Smith: And so, presumably, as you build up this very high potential difference, voltage, eventually you get to a point where the voltage is big enough or sufficiently large that it will discharge?
Carlos Camara: Absolutely.
Chris Smith: And overcome the resistance of the material and you get a sort of spark?
Carlos Camara: Absolutely. You get these miniature lightning strikes, to picture it in some way. You get a spark. You get a tremendous flow of current. And if the conditions are right, then the electrons moving in the strong electric field can attain sufficient velocity that when they strike the tape they emit X-rays.
Chris Smith: Well, so it's not just visible light, it's X-ray light as well that’s coming off?
Carlos Camara: That’s what we have shown, indeed. But for you to get X-rays out of sticky tape, as far as we know, you have to put it in a vacuum. So what happens in the vacuum is that by removing the gas particles from that peeling point, then you allow the electrons to move freely across the gap from one side of the tape to the other without striking any gas molecules. So they attain full speed before they hit the other side. So they hit the other side going very, very fast and emit X-rays when they get stopped.
Chris Smith: But if you didn’t do it in a vacuum, they would bash into air molecules which would slow them down and they wouldn’t be going fast enough to make x-rays?
Carlos Camara: Exactly, but certainly fast enough to make light, and that, I really strongly encourage all your listeners to try to peel sticky tape in a dark room, it's spectacular.
Chris Smith: Spectacular it might be but why is this useful? What does this mean we might be able to do or what do we understand now about this that we didn’t before?
Carlos Camara: When scientists think of triboluminescence, they now must think that the emission can be high enough to make X-rays, so this is new. The X-ray emission is remarkable because it comes out in these bursts, so a lot of X-rays come out together in roughly a billionth of a second. You can get almost a hundred thousand X-rays coming out at once. Now, there’s also the surprising fact that we got so many X-rays out of this effect that we were able to take X-ray images.
Chris Smith: When you actually do this, could you see any other application of doing this and what sort of dose of X-rays come out? What sorts of things could you see? If this were a medical setting, let’s say, to give it some perspective, what sorts of things could you image like this?
Carlos Camara: Well we were able to image the bones in our fingers, for starters, so the dosage is equivalent to a low dose dental X-ray. If you unroll tape in a vacuum, you can get sufficient X-rays to image the bones in your fingers.
Chris Smith: So does this mean then, this could be a very clever way of making X-rays without having to have massive accelerators and high voltage apparatus? You could do this with something obviously a little bit more complicated than a reel of tape but the principle would be pretty much the same?
Carlos Camara: That’s exactly the point, and it is a completely new way. Possibly it has a potential of becoming the least expensive way of generating sufficient X-rays to take an image.
Chris Smith: I wonder how many rolls of sticky tape they got through just doing that research. That was Carlos Camara from the University of California, at Los Angeles, where he and his team are producing X-rays with sticky tape. That work’s published in this week’s Nature.
This conversation was originally broadcast as part of Breaking Science on BBC Radio Five. Listen to the full programme.