X-ray Emission

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Vol 455 | 23 October 2008 | doi:10.1038/nature07378

L ETTERS
Correlation between nanosecond X-ray flashes and
stick–slip friction in peeling tape
Carlos G. Camara1*, Juan V. Escobar1*, Jonathan R. Hird1 & Seth J. Putterman1

Relative motion between two contacting surfaces can produce
visible light, called triboluminescence1. This concentration of diffuse mechanical energy into electromagnetic radiation has previously been observed to extend even to X-ray energies2. Here we report that peeling common adhesive tape in a moderate

vacuum produces radio and visible emission3,4, along with nanosecond, 100-mW X-ray pulses that are correlated with stick–slip peeling events. For the observed 15-keV peak in X-ray energy, various models5,6 give a competing picture of the discharge process, with the length of the gap between the separating faces of the tape being 30 or 300 mm at the moment of emission. The intensity of X-ray triboluminescence allowed us to use it as a source for X-ray imaging. The limits on energies and flash widths that can be achieved are beyond current theories of tribology.

When a continuous medium is driven far from equilibrium, nonlinear processes can lead to strong concentrations in the energy density. Sonoluminescence7 provides an example in which acoustic energy concentrates by 12 orders of magnitude to generate subnanosecond flashes of ultraviolet radiation. Charge separation at contacting surfaces8,9 is another example of a process that funnels diffuse mechanical energy into high-energy emission. Lightning10, for instance, has been shown to generate X-rays with energies of more than 10 keV (ref. 11). Although triboelectrification is important in many natural and industrial processes, its physical explanation is still debated10,12.

By peeling pressure-sensitive adhesive tape one realizes an everyday example of tribocharging and triboluminescence1: the emission of visible light. Tape provides a particularly interesting example of these phenomena because it has been claimed that the fundamental energy that holds tape to a surface is provided by the van der Waals interaction13. This energy—the weakest in chemistry—is almost 100fold smaller than the energy required for generating a visible photon, yet, as demonstrated in 1939 (ref. 3), light emission from peeling tape can be seen with the unaided eye. That even more energetic processes were at play had already been suggested in 1930 (ref. 14); it was observed that when mica is split under vacuum ‘‘the glass of the vessel fluoresces like an X-ray bulb’’. This insight led to the discovery in 1953 (ref. 2) that peeling tape is a source of X-rays. The simultaneous emission of visible and X-ray photons from peeling tape is shown in Fig. 1a, in which the blue glow is due to a scintillator responsive to X-ray energies and the red patch near the peel point is neonenhanced triboluminescence3. Figure 1b shows that when the vacuum pressure is 1023 torr the high-energy emission is so strong that the photo is illuminated entirely with scintillations.

Motivated by these photos, we interpret triboluminescence1, a phenomenon known for centuries, as being part of an energy-densityfocusing process that can extend four orders of magnitude beyond visible light to X-ray photons. To learn about the processes occurring

in peeling tape, we employed efficient high-speed X-ray detection equipment. Our measurements indicated that the scintillations in a

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Figure 1 | Apparatus for studying high-energy emission from peeling tape. a, Photograph of the simultaneous emission of triboluminescence (red line) and scintillations of a phosphor screen sensitive to electron impacts with energies in excess of 500 eV (under neon at a pressure of 150 mtorr). b, Photograph of the apparatus (under a pressure of 1023 torr) illuminated entirely by scintillations. c, Diagram of the apparatus used...
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