Sticky tape generates X-rays

Christmas could bring with it a new hazard as you wrap your gifts – X-ray-emitting sticky tape.

Researchers at the University of California, Los Angeles, have shown that simply peeling ordinary sticky tape in a vacuum can generate enough X-rays to take an image — of one of the scientists' own fingers (see videos).

"At some point we were a little bit scared," says Juan Escobar, a member of the research team. But he and his co-workers soon realized that the X-rays were only emitted when the kit was used in a vacuum. "We don't want to scare people from using Scotch tape in everyday life," Escobar adds.

This kind of energy release — known as triboluminescence and seen in the form of light — occurs whenever a solid (often a crystal) is crushed, rubbed or scratched. It is a long-known, if somewhat mysterious, phenomenon, seen by Francis Bacon in 1605. He noticed that scratching a lump of sugar caused it to give off light.

The leading explanation posits that when a crystal is crushed or split, the process separates opposite charges. When these charges are neutralized, they release a burst of energy in the form of light.

As long ago as 1953, a team of scientists based in Russia suggested that peeling sticky tape produced X-rays. But "we were very sceptical about the old results," says Escobar. His team decided to look into the phenomenon anyway, and found that X-rays were indeed given off, in high-energy pulses.

When the researchers placed a small plastic window in their vacuum chamber, they were even able to take an X-ray image of a finger, using a dental X-ray detector. Their results are published in Nature¹

Mechanoluminescent mystery

"Of the total electron discharges, only one in ten thousand makes X-rays," says Escobar. The energies of the individual X-ray pulses, typically a few nanoseconds long, are about 15 kiloelectron volts.

The energy of the X-rays is directly related to the amount of charge that builds up at the surface of the tape as it is peeled. The scientists calculate that this charge was ten times greater in their study than typically seen in similar experiments. "We are not exactly sure why the tape is so heavily charged," Escobar says.

The sticky-tape X-ray machine is also baffling others in the field. "You wouldn't have thought that so much of the mechanical energy would come out as X-rays," says Ken Suslick, an expert in mechanoluminescence at the University of Illinois at Urbana-Champaign. "The adhesive on the tape is an amorphous liquid, not crystalline. What's causing the transfer of charge, of electrons or protons, what the accepting and donor groups are — these things are much less clear."

The researchers suggest that the high charge density generated by peeling the tape could be great enough to trigger nuclear fusion. Michael Loughlin, a nuclear analyst at the international nuclear fusion experiment, ITER, in Cadarache, France, is sceptical. But he adds that if he is proved wrong, a system that could provide fusion reactions at the flick of a switch would be very useful. ... - nature