The same thing that makes X-rays helpful for diagnosing fractures and cavities is what makes them pesky in other areas. (Don’t search for “X-ray burns” unless you have a strong stomach and a morbid curiosity.)
X-rays photons have plenty of energy, and the body — save all those stalwart calcium atoms in your bones — does a poor job absorbing it. When you get X-rayed by a doctor, what you seeing on the screen are the shadows of calcium-dense parts of your body as the X-rays pass clean through softer tissue. And that works out great when X-rays are used in short bursts, but in longer exposures, they can prove harmful.
Just like they X-rays hurt you if your dentist gets thoroughly distracted by YouTube videos during an exam, they can damage even inorganic matter. When X-ray photons collide with atoms and make them take on more energy, it can excite the electrons in those atoms to move to a high-energy orbit around the nucleus. It can cause an atom to ionize; those electrons could careen out of the atom; or they could relax to their natural state.
And in that space between excitation and relaxation is, perhaps, everything.
“For most physicists and other scientists that use X-rays, it's a nuisance. But what we realized is that we could actually use the X-rays as a means to harness novel chemistry,” physics professor Micahel Pravica said. “Chemistry is usually done where we’re adding [energy] in different ways. In our case we're using the X-rays.
“Hard [higher energy] X-rays are very penetrating and they are very highly ionizing. It's like taking a bull in a china shop just wreaking havoc. But when you heat something the whole sample gets heated up. X- rays, you can focus them to less than a millionth of a meter. So, even though it's a bull in a china shop it's a targeted bull. That can wreak havoc but over the course of that wreaking of the havoc you rebuild things. There's a healing process. What we're doing is we're using hard X-rays to drive the chemistry. We're finding that as a consequence of this you get new compounds that are synthesized that nobody's ever synthesized before.”
Breaking Bonds
In 2011, Pravica and his team were studying X-rays and potassium perchlorate at the Department of Energy’s outside of Chicago. While observing shadows from the X-rays, they noticed an occasional popcorn effect – the result of oxygen being formed which broke up crystals in the sample. So they started using the process on other chemicals. In the course of the experiments, Pravica had to leave Chicago and return to ӽ紫ý, so he left an assistant in charge of an experiment of trying to synthesize water, with strict instructions not to leave a cap on the container, or else gasses would build up and explode a $3,000 crystal detector.
He didn’t; it did.
“I realized, my gosh, we have an interesting way to ignite things without heat. We called that X-ray induced combustion, which was driven by useful hard x-ray photochemistry. When I first saw this, I had this whole vision: what if we could make complex molecules?”
X-ray photochemistry wasn’t, in itself, new. What was new was using hard X-rays and pressure to synthesize new materials that have never before been created, like new types of cesium superoxide, and stable doped (chemically altered) polymeric carbon monoxide.
It opens up entire avenues for materials science to explore and create in ways that previously hadn’t been available to researchers. The former could theoretically be used to help create entire new structures of silicon that could revolutionize electronics; the latter could lead to new types of radiation-hardened sensors for extreme environments like deep space or in solar cells.
“We've actually for the first time been able to make a new structure just with X-rays that nobody could make with normal chemical methods,” Pravica said. “That's what we're really excited about”
The applications go beyond materials, too. The work has already generated two patents, one for the synthesis of polymers using X-ray chemistry and one for bond destruction.
If the Post Office were trying to get rid of anthrax found in a letter, right now they’d use a neutron beam, which can irradiate materials. Using Pravica’s method of bond destruction, an X-ray could be “tuned” to a particular bond, so it could be used to denature anthrax without any harmful side-effects.
If the chemical questions are robust, the epistemological ones are downright massive. The process could even be used to explain the most enduring of mysteries: how did life come to exist in the universe?
The Sweet Spot: X-rays
Human beings have been able to produce amino acids with just sparks, but we can’t make complex polymers without complex means, and the primordial earth didn’t have the complex means to form polymers. And with the early, harsh atmosphere, few complex organic molecules could be made. In the “civilized” conditions of space, though, it’s another story.
Biological systems need molecular precursors like oxalate salts and oxalic acid. There have been longstanding questions about how these compounds could have been synthesized in space. Useful hard X-ray photochemistry may be the answer.
“There may have been some initial complex polymer that came onto the Earth and then for whatever reason on an asteroid or some interstellar medium you have billions of years of these X-rays. Over time they're interacting with other molecules and maybe through some of the same mechanisms that we're observing in our research they formed larger quantities of these materials and more complex molecules that may have led to the original of life on our planet. We found people talking about gamma rays, but nobody really was looking in the sweet spot of hard X-rays.”
This kind of photochemistry could also could have had an effect on our celestial neighbor. The surface of Mars is believed to contain oxalate salts. Its atmosphere is thin enough to allow hard X-rays above a certain energy to penetrate, and though Mars’ red hue is often attributed to iron oxide rust, it’s also possible that it could be partially attributable to these types of chemical reaction products. Pravica’s polymer carbon monoxide can turn that kind of red with the right dose of hard X-ray radiation. Mars could be a verifiable proof that these processes were happening, under the right conditions.
It’s possible, at the beginning of understanding this catalyst, that hard X-rays could turn out to be the building block that explains processes deep into the past, and pave the way toward techniques and materials that serve us well into the future.
“I want to make something useful for humanity,” Pravica said. “I want to make a whole tailored family of polymers using this extra X-ray-induced polymerization. And that I think would be the most exciting because it's a whole class of compounds we can make. We're showing X-rays can actually build complex molecules.”
