X-Rays, Light

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“We still generate x-rays the same way we did in 1896,” Scott Watson, a Lab fellow and electrical engineer, says in his workshop. “That’s kind of weird, actually.” But over the past seven years, his team gave x-rays a significant upgrade with PHOENIX, a new portable system designed to create stop-motion radiographs in the field. Its applications range from imaging industrial welds on pipelines or bridges to creating radiographs of small-scale explosions, a Lab need that dates back to the Manhattan Project. The key to updating the x-ray machine and shrinking a device that usually fills a room to the size of a 55-gallon barrel was, as Watson put it, “Combining two, century-old ideas into one machine.”

All x-ray machines require a way to generate high-energy electrons, a place to store them, and a mechanism for dispatching them into a tiny space. The novel pieces that Watson and colleagues Nicola Winch, Lauren Misurek, Dave Platts, and Chris Romero put together are a Cockcroft-Walton generator, a circuit that multiplies voltages from low to very high, and a Van de Graaff dome, essentially a capacitor that stores a charge at a million volt potential. Charges stored in Van de Graaff domes have traditionally been generated with a belt rubbing against a metal comb—moving parts that render the system incompatible with vacuums. But x-rays demand a vacuum. To retain the storage capacitor while keeping the system vacuum compatible, the team replaced the belt with a Cockcroft-Walton circuit, which has no moving parts. On demand, it converts the low-voltage and high-current of a drill battery to the 1 million volts and microampere currents needed for x-rays. Once the team had stored a million volts in a vacuum, their next clever trick was developing a mechanism for dispatching them.

 
They used a cathode, essentially a switch, no bigger than the head of a pin. When remotely triggered with a laser, the cathode is heated to the temperature of the sun, releasing the stored energy. The resulting electron beam traverses a gap, hits a heavy metal target, and creates a pulse of x-rays that is captured by a digital detector. Because the charge is released from the Van de Graaff storage dome, rather than from pulsed power sources like most other x-ray machines, PHOENIX can create flash radiographs every nanosecond, every second, every hour, or anywhere between. The team designed two versions of the device. One is the 150-pound machine that caters to commercial applications, and the other, which fits on a flatbed trailer, is designed for national security purposes.