Tikalon Blog by Dev Gualtieri

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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.

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