Machines that take x rays need a lot of power and as a result are generally large, cumbersome contraptions. Anything that needs to be x rayed has to brought to the machine. But there are plenty of reasons develop a portable x-ray machine. A mobile device could be carried into the sports field or battlefield to diagnose injured people or it could be used by security personnel to analyze packages at airports or check concealed shipments at seaports for illegal contents.
Scott Kovaleski, an associate professor of electrical and computer engineering, and some of his graduate students, found a way to make a lower-power x-ray machine that’s only about the size of a stick of gum. That means instead of bringing objects to an x-ray lab for analyze, technicians can bring the x rays to the field.
The key to the small machine is a crystal of lithium niobate, which exhibits a particular property known piezoelectricity. Piezoelectric crystals generate a small electrical current when put under mechanical stress, such as being squeezed. The effect also works in reverse. Running a current through a piezoelectric crystal generates a mechanical action, like a vibration.
Kovaleski capitalized on this property by attaching an electrode to each side of the lithium niobate crystal, and then hitting it with alternating current. But instead of using 120 volts alternating at 60 times per second — the standard for household currents — Kovaleski’s group used 10 volts alternating at 40,000 times per second. That frequency is specially tuned to the lithium niobate crystal: it makes it vibrate in a very specific way. “It makes it ring like a bell,” Kovaleski told Discovery News.
All that vibrating generated an electric field equal to 100,000 volts. Kovaleski was able to turn 10 volts into 100,000 because he and his team modified the ends of the crystals with tiny bits of wire shaped like sharp points. The pieces were so small, the points were at the scale of atoms. But electric fields tend to build up at sharp points and so even though the amount of current going in was small, enough energy gathered on those wires to pull electrons from the crystal at strengths of 100,000 volts.
Electrons moving at that speed produce x rays when they hit anything because the atoms in the material slow or deflect the electrons. That deflection or slowdown takes energy away from the electron, and the energy takes the form of an x-ray photon. To make a portable x-ray generator, all that is needed is a block of dense material with lots of atoms for the electrons to hit — lead will do. Voilà, you have x rays.
In addition to being small, Kovaleski’s x-ray machine is cheap. Just about all the parts can be had at the local electronics supply store, and even lithium niobate crystals are common in telecommunications equipment.
Credit: Peter Norgard, University of Missouri