US scientists have made an extraordinary discovery with a roll of sticky tape.
By placing the spool in a vacuum and unrolling the tape with a motor they produced a burst of x-rays. Writing in this week's Nature UCLA scientist Carlos Camara and his colleagues describe how they made the discovery whilst investigating the phenomenon of triboluminescence - the process by which materials give out light when they are squeezed or pulled.
"Triboluminescence is a well-known phenomenon," says Camara, "you can even see it in your mouth when you crunch certain types of candy, but no one had thought that the process could produce even more powerful forms of light light x-rays!"
Triboluminescence occurs in the sticky tape when the glue is pulled away from the underlying tape layer. The molecules stretch out, separating electrical charges in the adhesive. As they are pulled further apart the voltage between the charges increases to the point where it overcomes the natural resistance of the material and discharges, producing a miniature lightning bolt. This is what produces the visible light.
But, the team discovered, things become a lot more interesting if the tape is unwound in a vacuum. Now, the electrons that flow when the material discharges travel much faster because there are no air molecules to slow them down. As a result, when they slam back into the tape surface, they decelerate so quickly that they turn all of their energy into a short, vigorous burst of x-rays strong enough, the team found, to x-ray their fingers!
Although no one is advocating turning tape reels into x-ray machines, this breakthrough does represent the most compact and lightweight way yet discovered to produce x-ray light.
"This could have all kinds of applications for portable x-ray production in cameras and other imaging devices," suggests Camara.