Louis_J_Sheehan

Even the lowliest kind of sticky tape can leave physicists befuddled. Unrolling tape in a vacuum produces X-rays — enough of them to do X-ray imaging, researchers have found. No current theory can explain such intensity of X-ray emissions, the scientists write in the Oct. 23 Nature.

Unrolling anything from regular sticky tape to duct tape produces a glow that, although faint, is easy to see in a completely dark room. The fact that X-rays, which are thousands of times more energetic than ordinary visible-light photons, can also be produced was first hinted at in a 1953 Russian experiment but seems to have been little-known.

Carlos Camara and his colleagues at the University of California, Los Angeles, have now shown that the X-rays produced from tape are much more intense than current theories about unsticking can explain, so much so that tape-unrolling machines could become cheap, commercially-available X-ray sources, Camara says.

Benoît Roman of the National Center for Scientific Research in Paris says he finds the effect surprising. “That you can use that for X-ray imaging is even more surprising,” he says.

The UCLA team built a mechanical tape-unrolling system and put it in a small vacuum chamber. As the tape peeled off, the team’s instruments recorded intense X-ray bursts, each lasting about a billionth of a second. The team even used this newly discovered radiation source to image Camara’s finger bones on plates of the type normally used by dentists.

Peeling tape leaves some electrons behind, Camara explains, so that the surface of what the tape was sticking to becomes negatively charged. The peeled-off tape, meanwhile, becomes positively charged due to a deficit of electrons. The electrostatic attraction of opposite charges makes electrons leap from the surface to the tape.

The electrons accelerate the closer they get to the tape, and when they reach the tape’s surface they bounce off other electrons or atomic nuclei. These sudden jerks make an electron lose some of its energy in the form of photons. Camara says that one in every 10,000 electrons produces a highly energetic X-ray photon.

Each burst contained up to 100,000 X-ray photons. The researchers expected to detect some X-rays, but not that many, Camara says. “No current theory predicts that there will be as many charges as are required for our observation.”

Enrique Cerda, a physicist at the University of Santiago in Chile, says he had never heard of such an effect. “I would bet that this emission is not constrained only to peeling tape,” he says. “For instance, it should be observed also when a film is torn apart.”

At normal atmospheric pressure, the electrons are slowed down by air particles, so they never reach the speeds and energies required for making X-rays.

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