X-rays from peeling adhesive tape

Settled in 2008 by Carlos G. Camara, Juan V. Escobar, Jonathan R. Hird, Seth J. Putterman (UCLA).

Triboluminescence - light emitted when crystals are scratched, crushed, or adhesive is peeled - was already noted by Francis Bacon in the early 1600s and studied qualitatively for centuries, but the exact charge-separation mechanism resisted quantitative explanation. In 2008 Carlos Camara, Juan Escobar, Jonathan Hird and Seth Putterman at UCLA peeled ordinary Scotch tape in a moderate vacuum and showed that stick-slip peeling separates charge across the tape/substrate interface strongly enough to accelerate electrons and produce nanosecond bursts of 15-keV X-rays, intense enough to X-ray a finger. This resolved the mechanism (electron acceleration and bremsstrahlung across the peeling gap) for this common case, though the general triboelectric charging process for arbitrary materials remains an active research topic.

An everyday piece of Scotch tape peeled in a vacuum turns out to be a crude but genuine X-ray generator, powered by nothing but friction and electrostatics.

Explanations that circulated

• Simple piezoelectric charging of the crystal alone was long assumed sufficient to explain triboluminescent light; it does not explain the very high-energy (X-ray) emission seen from tape peeled in vacuum.
• Some early accounts attributed the glow purely to static-electric sparks in air, without recognizing that in a vacuum, accelerated electrons striking the tape substrate can generate genuine bremsstrahlung X-rays.

References

• Correlation between nanosecond X-ray flashes and stick-slip friction in peeling tape, C. G. Camara, J. V. Escobar, J. R. Hird, S. J. Putterman, Nature, 2008