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Generating X-rays

July 5, 2011

Gen-X would have been a good name for an X-ray equipment company. Unfortunately, the term has been usurped as the name of the generation born after the baby boom of which I was a part. Gen-X a fuzzy term, meaning different things to different people. It's most commonly applied to people born in the 1960s and 1970s. They missed Sputnik, but many of them experienced the first manned moon landing. That's quite a leap in technology in just one generation!

Like most materials scientists, I did a lot of work with X-rays. There was routine powder diffraction analysis of phases, but there was also the more exotic linewidth determination by a double-crystal diffractometer to determine the quality of single crystals. Having worked with X-rays for many years, I have two interesting anecdotes.

The first comes from the time when my fellow graduate students and I were first learning X-ray technique. Since we were working with X-rays, we each needed X-ray dosimeter badge. One of the students didn't quite understand the badge concept, so he asked how a small piece of plastic could protect him from the X-rays.

Then there was the month when all the dosimeter badges in our laboratory showed an exposure. We checked all three machines in the room, and there were no stray X-rays emitted by any. As it turned out, someone who was working with a harmless tracer chemical had left a vial of it on the shelf where everyone kept their exposure badges.

The generation of X-rays is quite simple. So simple, in fact, that equipment not designed to emit X-rays can emit them. Electrons of sufficiently high energy impacting materials will generate X-rays. All you need is a source of high energy electrons and a target. There aren't that many cathode ray tubes out there, now, but the introduction of color television caused a problem.

Color television tubes incorporate a shadow mask to define the color fields on the phosphor screen, and electrons impacting on this screen generate X-rays. The electrons impacting on the phosphor are less of a problem, since the phosphor elements needed higher electron energies to produce X-rays. Not surprisingly, such emissions are limited by law, but CRT televisions are now a rarity.

Wilhelm Röntgen was awarded the first Nobel Prize in Physics in 1901 for his discovery of X-rays in 1895. X-rays soon became a common tool for scientific and medical investigations. By 1933, a patent was issued for the first rotating anode X-ray tube for generating extremely high X-ray power.[1] One of my aunts had X-ray treatments for acne in the 1930s, a medical procedure that's not currently recommended!

Hand mit Ringen: Wilhelm Röntgen's x-ray of his wife's hand

The first medical X-ray.

Hand mit Ringen: Wilhelm Röntgen's x-ray of his wife's hand, taken on December 22, 1895.

(Via Wikimedia Commons)

Stefan Kneip of the Blackett Laboratory, Imperial College (London, UK) has written a review of novel methods of X-ray generation in a recent issue of Nature.[2] One method of special interest is that pioneered at Seth Putterman's laboratory at UCLA.

Putterman is a wellspring of novel ideas,[3-4] and I've mentioned his work in a previous article (Pyroelectric Energy Harvesting, October 15, 2010). Putterman's group discovered that the electrostatic charge developed when something as common as unrolling pressure-sensitive tape will generate X-rays.[5-8] This work made the cover of the October 23, 2008, issue of Nature.[5]

The principle is called triboluminescence, and Putterman's UCLA team found that peeling common adhesive tape in a moderate vacuum will produce emission throughout the electromagnetic spectrum, from radio through X-rays. The X-rays were concentrated in 100-mW pulses that were correlated with the stick–slip action involved in the peeling. The X-ray emission was found to occur at a gap between the separating faces of the tape of 30 - 300 μm with an emission peak at 15-keV. They were able to use the emission for X-ray imaging.[3] In homage to Röntgen, the Nature cover image was of a human finger imaged using X-rays from Scotch-brand tape.

The study was done with a device that unspooled tape at 1.3 inches per second.[6] One observation was that duct tape did not produce X-rays.[7] The production of X-rays required the tape to be unspooled in a vacuum, since the mean free path of electrons is too short in air, or humidity short-circuits the electric field.[7]

In a follow-up study published in May, 2011,[9] the UCLA group improved their apparatus to have silicone and a metal-filled epoxy repeatedly contact each other in vacuum. The apparatus uses a solenoid to make, and break, contact twenty times a second, but they are investigating a piezoelectric actuator that will operate at 300 Hz.[2] Other material couples may be better at creating triboelectricity.[2]

According to calculations, pulling apart this silicone-epoxy couple generates 1010 electrons per cm2.[2] The machine was able to produce X-rays from molybdenum and silver target materials at a rate of 105 photons per contact cycle. The electron energy was 40 keV. They predict that an X-ray flux of up to 108 photons per second would be possible, and such a device could be an inexpensive source of X-rays.


  1. Albert Bouwers, "X-Ray Tube," US Patent No. 1,933,005, Oct 31, 1933
  2. Stefan Kneip, "Applied physics: A stroke of X-ray," Nature, vol. 473, no. 7348 (May 26, 2011), pp. 455-456.
  3. Seth Putterman, James K. Gimzewski, Brian B. Naranjo, "High energy crystal generators and their applications," US Patent No. 7,741,615, June 22, 2010.
  4. Brian Naranjo, James Gimzewski, Seth Putterman, "Method And Apparatus For Generating Nuclear Fusion Using Crystalline Materials,"US Patent Application No. 11/745,556, Publication number: US 2008/0142717 A1, May 8, 2007.
  5. Carlos G. Camara, Juan V. Escobar, Jonathan R. Hird and Seth J. Putterman, "Correlation between nanosecond X-ray flashes and stick–slip friction in peeling tape," Nature, vol. 455, no. 7216 (October 23, 2008), pp. 1007-1148.
  6. Thomas H. Maugh II, "UCLA researchers' surprising finding could lead to applications in medicine, other fields," LA Times, October 25, 2008.
  7. Kenneth Chang, "Scotch Tape Unleashes X-Ray Power," The New York Times, October 28, 2008
  8. Dave Bullock, "Gallery: Take an X-Ray With Your Office Sticky Tape," Wired, October 22, 2008 .
  9. J. R. Hird, C. G. Camara, and S. J. Putterman, "A triboelectric x-ray source," Applied Physics Letters, vol. 98, no. 13 (March 28, 2011), Document No. 133501 (3 pages).

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