X-ray technology is a vital testing tool for industrial applications. Industrial radiography is used as a non-destructive testing method to inspect materials for unseen cracks or flaws.
These inspections can include the grading of welds in pressurized piping, high-capacity storage containers, and some forms of structural welds. Non-metal components such as aerospace ceramics are also commonly tested.
The machines currently used for this kind of testing are large and expensive, but new research into producing compact, tuneable X-rays could optimize this process.
A new theory backed by exacting simulations shows that a two-dimensional graphene sheet could be used to generate plasmon surface waves when struck by photons from a laser beam.
The plasmons can in turn be triggered to generate a sharp pulse of radiation, tunable to wavelengths anywhere from infrared light to X-rays.
According to MIT professor Marin Soljaĉić, “coherent X-rays are particularly hard to create.” However, they are also the highest energy wavelength with the widest applications.
In principle, this new system could lead to ultraviolet light sources on a microchip, or table-top X-ray devices that can produce images of the same strength and quality as the current generation of multimillion-dollar X-ray and CT scanning machines.
Creating High-Power X-Ray Beams with Graphene Sheets.
“The usual approach [to make focused, high-power X-ray beams] is to create high-energy charged particles and ‘wiggle’ them,” explained MIT postdoc Ido Kaminer. “The oscillations will produce X-rays.”
But facilities capable of doing this are few and the machinery is expensive. “The dream of the community is to make them small and inexpensive,” Kaminer said.
Most X-ray sources rely on hard-to-produce high-energy electrons. But the new theoretical approach presents a possible way around this.
When a specially patterned graphene sheet is hit by photons from a laser beam, it produces wave-like plasmons with tightly confined power. These plasmons can be made to release their energy in a tight X-ray beam when triggered by an electron gun pulse. This electron gun is similar to those found in electron microscopes.
“The reason this is unique is that we’re substantially bypassing the problem of accelerating the electrons,” Kaminer said. “Every other approach involves accelerating the electrons. This is unique in producing X-rays from low-energy electrons.”
The team indicates that their system can also offer unique tunability. They believe that they will be able to generate and deliver beams of single-wavelength light tuned to come from across the spectrum from infrared through visible light, and into ultraviolet and X-rays.
Three different input variables control the tuning of the generated wave: the frequency of the laser beam to initiate the plasmons, the energy of the triggering electron beam, and the “doping” of the graphene sheet.
The key feature the team sees is that the radiation produced will have a uniform wavelength and tight alignment, similar to that of a laser beam. The team believes this will eventually lead to lower-dose X-ray systems. This would be safer for machine operators in industrial settings, as well as recipients of medical and dental X-rays.
Practical X-Ray Devices May Come in Three Years, Or Never.
The work is based on precise simulations, but is still highly theoretical. However, the group’s past simulations have tended to match quite well with experimental results. “We have the ability in our field to model these phenomena very exactly,” said Soljačić
The team is currently in the process of building a device to test the system in the lab. They plan to start by producing ultraviolet light sources and working up to higher-energy X-rays. “We hope to have solid confirmation of the principles within a year, and X-rays, if that goes well, optimistically within three years,” Soljačić said.
But as with any drastically new technology, he acknowledges, the devil is in the details, and unexpected issues could – and likely will – crop up. So his estimate of when a practical X-ray device could emerge from this is “from three years, to never.”
The full paper is published in the journal Nature Photonics and is available to read here.