First conceived in the combustion of a hydrogen bomb on the South Pacific island of Elugelab in 1952, the heavy element einsteinium is one of the shier members of the Periodic Table; it doesn’t naturally occur and is so unstable that it’s difficult to get enough of the stuff, for long enough, to actually study it.
Now, a team of chemists at Lawrence Berkeley National Laboratory, Los Alamos National Laboratory, and Georgetown University have managed to do just that. They inspected a microscopic amount of einsteinium-254 in order to better understand the elusive element’s fundamental chemical properties and behavior. Their research is published today in the journal Nature.
Einsteinium is made at the Oak Ridge National Laboratory’s High Flux Isotope Reactor as a by-product of biannual californium-252 production (another heavy, lab-synthesized element, but one that has commercial utility.) Technological advancements have meant that these radioactive elements can be made in laboratory settings, without the destructive pyrotechnics of the mid-20th century. The reactor in Oak Ridge, Tennessee, is one of an extremely few suppliers of californium-252.
“The reason that they can create these elements is because they have this really high flux of neutrons, so they can just kind of push further and further and further out [of their nucleon shells],” said Katherine Shield, a chemist at the Lawrence Berkeley National Laboratory and co-author of the paper, in a video call. The initial product of the reactor is “just an absolute mess, a combination of all sorts of things,” Shield said, explaining that “it’s not just about making the element or making the isotope, but also purifying it so that we can do chemistry with it.”
Such heavy, radioactive elements as einsteinium and californium, as well as household names like uranium and plutonium, are part of the actinide group: elements 89 to 103 on the Periodic Table. Only some of them, like einsteinium and californium, are synthesized. Once a research team gets past the logistical work of safety protocols (to ensure the radioactive elements, like any other lab material, are handled safely), the issues are primarily ensuring they have enough of the material to work with and that the material is pure enough to offer useful results. Extracted from the process of californium production, einsteinium can often be contaminated by the former.
The research team was working with a mere 200 nanograms of einsteinium, an amount about 300 times lighter than a grain of salt. According to Korey Carter, a chemist now at the University of Iowa and lead author of the study, a microgram (1,000 nanograms) was previously thought to be the lower limit for a sample size.
“There were questions of, ‘Is the sample going to survive?’ that we could prepare for as best as we possibly could,” Carter said in a video call. “Amazingly, amazingly, it worked.”
The team managed to measure the bond distance of einsteinium-254 using X-ray absorption spectroscopy, in which you bombard the sample with X-rays (this line of inquiry also required building a specialized holder for the sample, one that wouldn’t crumble under X-ray bombardments over the course of about three days). The researchers looked at what happened to light that was absorbed by the sample and found that the light that was subsequently emitted was blueshifted, meaning the wavelengths were slightly shortened. This was a surprise, because they had expected a redshift—longer wavelengths—and this suggests einsteinium’s electrons may couple differently than other elements near it on the Periodic Table. Unfortunately, the team was unable to get X-ray diffraction data due to a californium contamination in their sample, which would muddy their results from the method.
Previously, researchers assumed they could extrapolate certain trends seen in lighter elements to the heavier actinide elements, such as how they absorb light and how the size of the atoms and ions of other elements, called lanthanides, decrease as their atomic numbers go up. But the new results suggest that extrapolation might not hold true.
“There’s been a lot of great work over the last 20 years up moving progressively farther into the actinide series, showing that...actinide chemistry has more going on,” Carter said. “The rules that we’ve kind of developed for smaller things, maybe they don’t work quite as well.”
Radioanalytical work had been done on einsteinium shortly after its discovery in the 1950s, but at the time, little was studied about actinides in general beyond their radioactive properties). The recent research showed that einsteinium’s bond distances—the average length of the connection between the nuclei of two atoms in a molecule—were a little shorter than expected. The result, Carter said, is a “meaningful first data point.”
Like so many other scientists during this pandemic, the team wasn’t able to conduct the follow-up experiments they had planned. When they finally got back into the lab, most of their sample had decayed. But as with any first step, this one is sure to be followed by strides. It’s just a matter of when.