Tale of the tape

Humble desk adhesive is a tiny particle accelerator

Forget fancy particle accelerators — a cheaper tool for emitting X-rays is right there in the office supply cabinet. Pulling back Scotch tape emits X-rays, the same high-energy light emanating from airport security scanners and the interiors of galaxy clusters, and scientists now have a better understanding of why.

INVISIBLE TAPE Peeling back Scotch tape emits not only blue light from breaking chemical bonds, but also invisible X-rays from separating positive and negative charge. Carlos Camara, Juan Escobar and Seth Putterman

Physicists were dumbfounded two years ago when UCLA researchers produced quick flashes of X-ray light by peeling Scotch tape in a vacuum. Scientists have known since the dawn of 3M Scotch tape in the 1930s that pulling the adhesive emits blue light. But to discover that X-rays also fly out was perplexing because X-rays are a hundred thousand times more energetic than the chemical bonds holding the sticky side down.

Not only that, says UCLA physicist Seth Putterman, the light can pulse a billion times a second from a region only 100 micrometers in size, or about the width of a human hair.

“Just try to purchase a device like this,” Putterman says. Now, two teams of scientists have an explanation: Peeling tape separates positive and negative charges, creating an electric field. The field jump-starts free electrons in the neighborhood, accelerating them fast enough to emit X-ray photons. This bremsstrahlung radiation is like that created in the bellies of particle accelerators as they whip charged particles around near the speed of light.

No need to worry about radiation exposure at the office — at atmospheric pressure, where air molecules bustle, the electrons quickly run into other particles before they can radiate X-rays.

To track where the X-rays travel, Australian scientists rigged up Scotch tape on a spool driven by a motor (the lab’s first prototype spun on an electric drill). The X-rays mostly sprayed at a right angle to the direction the tape was pulled, the researchers report in the Sept. 29 Applied Physics Letters. That’s a convenient property, because herding light into a straight line normally absorbs the light’s energy, but the tape naturally emits X-rays in a straight line to within 5 degrees.

“Tape is an even better use as an X-ray source than we thought,” says Putterman, who first observed the phenomenon and reported in May that bremsstrahlung radiation is the X-ray source in a May paper in Applied Physics B: Lasers and Optics.

Other materials can generate X-rays using the same principle, says Putterman. He imagines that soldiers and medical workers in the field could use a hand crank to peel off adhesives and create X-rays. The light is powerful enough to image a human finger.

“This research is useful in a broader area,” said physicist Josip Horvat at the Institute for Superconducting and Electronic Materials in Wollongong, Australia. “People are trying to understand bremsstrahlung and they might be able to use this.”

But it’s still a mystery how tape could separate enough charge to create a strong electric field, the same way physicists don’t know how charge separates in clouds to create bolts of lightning.

“That is not explained yet, not by this paper, not by our research, or anyone else,” said Putterman.

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