In 1945, scientists in a remote part of East Tennessee discovered a new element in the byproduct of their work developing the atomic bomb. Named after the god who gifted fire to mortal man, Promethium’s properties remained widely unknown until nearly 80 years after its discovery.
Ilja Popovs, a synthetic organic chemist at Oak Ridge National Laboratory who co-led the study, spoke about how the study started. “Our senior co-author, Alex Ivanov, will tell the story. As per usual, a good endeavor starts with a big cup of coffee and a conversation while looking at the periodic table,” said Popovs.
Promethium is part of the series of metallic chemical elements called lanthanides, also known as rare earth elements. Lanthanides are primarily used in green technologies like electric motors for wind turbines, atomic batteries and various parts for nuclear power plants. But the elements in the lanthanide series are nearly identical, making separating them difficult.
According to the National Library of Medicine, promethium is only naturally found on a star in the Andromeda galaxy two million light years away. But small traces of the element can be found in uranium ore as a result of nuclear fission. As a result, little is known about Promethium, including its atomic structure. In a breakthrough nearly 80 years after promethium was first discovered, a group of scientists at the Oak Ridge National Laboratory in Oak Ridge, Tennessee used a combination of experimentation and computer simulation to synthesize the electronic structure of promethium.
This discovery will help scientists study the lanthanide series and help in the production and stabilization of promethium for future use.
“We’re working simultaneously on many different aspects of the chemistry of promethium itself, as well as trying to discover and develop new technologies that would allow separation of individual lanthanides. These can have substantial implications. For instance, it would secure the supply of these elements that are vital for the nation,” said Popovs.
Summit Supercomputer’s role in the discovery
Oak Ridge Laboratory is home to multiple of the world’s fastest supercomputers, including the world’s fastest, Frontier. Dmytro Bykov, a group lead in computing for chemistry and materials at Oak Ridge Laboratory, tested the theoretical science of uncovering promethium’s properties. Bykov used the Summit supercomputer, the seventh fastest supercomputer in the world, to model the molecular structure of promethium.
“As a scientist, you always try to find something unique. [It] cannot be more unique than, yeah, I work on the biggest machine in the world. No questions asked,” Bykov said.
Bykov used previous studies and Schrodinger equation, which is specifically designed for atomic structuring, to write a theoretical equation for a model of promethium. “We sit on the shoulders of giants,” Bykov said. After a few hours of Summit running the simulation, Bykov had his results.
“You know, as a chemist, you look at the [periodic] table like every day. So suddenly, you realize you are a guy who has actually made a contribution into characterizing one of those boxes there,” Bykov said. “That’s it. I mean, that’s how the universe works. There will be nothing else.”
The process leading up to the discovery
Promethium is often found entangled with other elements, like uranium. Without knowledge of the atomic structure, it can be incredibly difficult to separate promethium from the other elements. “Our outstanding separation sciences and radio chemists are able to take essentially what I call a soup of different elements, and pull out specifically an element or radio isotope that they’re interested in. And in this case, it was promethium,” Popovs said
Scientists at the Oak Ridge Lab used a high flux isotope reactor to create a pure isotope of the element called promethium-147. Promethium-147 has a short half-life of two and a half years, after which the chemical structure would alter into samarium, the next element in the lanthanide series, and need to be repurified.
“Here in the United States, we have very limited rare Earth [element] separation capabilities. All the separation takes place in the People's Republic of China,” said Santa Jansone-Popova, a co-leader of the study and a senior research scientist in the chemical sciences division at Oak Ridge Laboratory. “I think the important thing is to develop a technology that’s efficient, selective and environmentally friendly so that we can bring that separation of rare Earth elements, of these critical minerals, back to the United States.”
Promethium is an unstable element, so promethium-147 was bonded to a ligand, a molecule that bonds to another molecule to create a stable complex, or a chemical compound. The ligand would need to fit with promethium perfectly, like a glove.
“You want that glove to fit perfectly, because if it’s too big…too small, it’s uncomfortable. So, you need the perfect fit between the glove and the hand,” Jansone-Popova said. “So that’s what we’re trying to do, design those ligands that are perfect.”
The team then used an optical emission spectrometer and a mass spectrometer to measure the amount of metal ions in the complex. This allowed the scientists to know how much of each element was in the solution.
The full experiment utilized various equipment in multiple different buildings across the 4400-acre campus requiring 18 specialized scientists to focus on each section of the process. “We’re enabled by the expertise of the outstanding team and people here, as well as the state-of-the-art facilities. Only a few places on this planet would be able to conduct the research that we’re doing here. And I think it’s also important for folks in East Tennessee to also feel some pride because this is not an everyday occasion where this type of research has been performed,” Popovs said.
The scientists ran multiple trials before promethium was introduced to the lab. In its place, pure water was used. As promethium is rare, short-lived and radioactive, the scientists only had one shot to do everything correctly.
“We needed to figure out what type of sample holder we’re going to use, what kind of capillary, what type of ceiling epoxy resin, what kind of syringe, how much of the material we’re going to actually inject in the capillary, how would you seal it using the gloves, we also had to use, essentially a puncture resistant gloves, and the needle was actually not sharp,” said Popovs.
Oak Ridge has a history of safety and Popovs did not want to jeopardize that record or the safety of his team. “We have absolutely outstanding safety staff that enable us to work with some of the most interesting elements in the periodic table. We leaned on maybe 10 people, at any given moment, to figure out how to do it. We had brainstorming sessions and meetings every day for three weeks straight, just to perform an experiment with a sample of pure water. When we were comfortable doing it with just pure water, we are ready for the work with promethium,” Popovs said. “There is quite a lot of safety regulations that we need to follow. And it’s for a good reason. Therefore, everybody on the team is safe and we performed absolutely stellar work without any incidents.”
The sample was sent for x-ray absorption spectroscopy measurements at Brookhaven National Lab where the electronic structure of promethium was revealed. “We were super excited, extremely excited,” Jansone-Popova said.
