Inside the glove box, the technician’s hands carefully swirl liquid in a small vial. She and her supervisor have rehearsed every movement, pinpointed where every piece of equipment will be placed on the other side of the leaded glass.
These precise movements, captured in a training video, are so carefully choreographed because they have to be. The berkelium-rich compound that the technicians are working to purify is expensive and highly radioactive. There’s only about 20 mg of it, and its short half-life means they risk ending up with nothing.
Once technicians Clarice Phelps and Shelley Van Cleve are finished, the ultrapure berkelium-249 will be shipped in a lead-lined container to Russia, Germany, or Japan. There it will be exposed to a beam of ions and, if all goes well, transformed—briefly—into a brand-new element. The work of the team members at Oak Ridge National Laboratory (ORNL) in Tennessee isolating the berkelium led to their recognition in 2016 as codiscoverers of element 117, tennessine.
Anyone with access to a periodic table and a calculator can propose routes to a new element. What is your target proton number Z? Find two elements with that number of protons between them and smash them together. Berkelium (Z = 97) plus calcium (Z = 20), for example, yields tennessine. That final step of element discovery, the fusing of proton-rich nuclei to produce a substance never before observed by humans, is the one that gets the most attention (see the article by Yuri Oganessian and Krzysztof Rykaczewski, Physics Today, August 2015, page 32). Yet perhaps the hardest part of the whole process is obtaining the starting materials. Unlike the previous feedstocks used to make superheavy elements, with half-lives in the hundreds of years, 249Bk has a half-life measured in hundreds of days. And as a transuranium element, berkelium isn’t naturally occurring; you have to make it in a laboratory.
In fact, there’s only one place in the world that can create enough 249Bk to put under an ion beam. The process isn’t quick: Scientists have to leave the decaying product sitting around for several months after making it because the radioactivity is too high. The 249Bk target is very much a minority component in a messy reaction, so its manufacturers must then spend several more months separating and cleaning the material.
This isn’t your grandmother’s element-discovery process. It can’t be done by lone-wolf scientists in horn-rimmed glasses and white shirts. You need training videos. You need ex-navy officers who worked on nuclear submarines. You need 50-ton cranes. In short, you need Oak Ridge National Lab.
Berkelium and californium
Located about 40 kilometers west of Knoxville in Tennessee, ORNL is the place to go when you require rare, radioactive material. One of the lab’s primary research areas is neutron scattering, which has applications in medicine and materials science. It’s also a way to make rare elements from more common ones. What makes ORNL uniquely good at this task is its High Flux Isotope Reactor (HFIR), which emits on the order of 1015 neutrons per square centimeter per second, an order of magnitude higher than most reactors. “The neutron flux gives ORNL the ability to transmute isotopes at a rate better than any other,” says Chris Bryan, HFIR experiments manager.
Berkelium is a by-product in the production of californium-252, a “top priority” campaign performed at ORNL. A natural neutron emitter, 252Cf is used to jump-start nuclear reactors and detect contaminants in coal. Few compounds or elements can match its strong, sustained neutron emission power. Oak Ridge is thought to account for about 70% of the world’s californium supply. (Russia is coy about its 252Cf production capacity.) Due to its rarity, californium is often labeled the world’s most expensive element. One microgram costs an estimated $27.
Applicants can request small quantities of californium and berkelium for research purposes through the laboratory’s National Isotope Development Center. The US Department of Energy has the final say over who gets what.
The half-life of 252Cf is 2.6 years, so production of the isotope takes place on a two-year schedule. Currently, on 1 February of every odd year the californium campaign begins. Julie Ezold, the program manager for 252Cf production, ensures that all aspects of the program go according to DOE expectations and that ORNL stays on schedule. “I equate myself to the conductor of an orchestra,” she says. “I can’t play the instruments, but I know how to make the pieces fit together to work.” She arrived at Oak Ridge as a graduate student, performing research at the HFIR as part of her master’s program in nuclear engineering.
To make californium (Z = 98), Oak Ridge scientists start with a mixture of curium isotopes (Z = 96) that was transferred to ORNL in the 1970s from a reactor at the Savannah River Site in South Carolina. The curium targets are about a meter long, have a mass of roughly 40 g, and are shaped like arrows. After absorbing neutrons, some nuclei undergo beta decay to form higher-Z elements; others undergo fission (see the article by Alvin Weinberg, Physics Today, June 1967, page 23). For each campaign, up to 10 targets are irradiated within the HFIR over at least four 25-day cycles.
Of the fission by-products, the least welcome is iodine-131, one of the most radioactive compounds the facility produces. Only the HFIR pool offers adequate shielding. So once the cycles are completed, the target is held in underwater storage while the 131I decays. Despite 131I’s half-life of eight days, it takes three months before the overall radioactivity of the target is low enough for scientists to work with it in a hot-cell facility, which can shield against 1000 roentgen/hr of gamma and neutron radiation—doses strong enough to kill. By that point, the 249Bk is already depleting.
Moving those dangerously radioactive materials around the reactor pool requires what Bryan calls “laparoscopic surgery with 20-foot poles.” The operations staff work from a gangway above the pool, holding hollow metal tubes with grippers, claws, and other tools attached at the other end.
After three months in the pool, the californium targets are still highly radioactive, but they can be carefully moved to the cleanup facilities. Each target is sealed in a 25-ton carrier, moved out of the bay with a 50-ton crane, and loaded onto a truck. It takes half a day to complete this maneuver and secure the carrier in place. “It goes through the same rigor of transportation as if it were going across the United States—but it’s actually going 500 yards down the road,” says Porter Bailey, operations manager of the hot-cell facility.
Precise purification
After a few minutes in transit, the carrier is unloaded with the same degree of caution at the Radiochemical Engineering Development Center (REDC). The carrier interfaces directly with the REDC hot-cell facilities through an airlock, bringing the mixed californium/berkelium target to the next stage of the production cycle. The composition of the target now includes several transcurium elements, lanthanides, and the unconverted curium, along with about 200 mg of 252Cf, 20–30 mg of 249Bk, 1 or 2 micrograms of einsteinium-254, and about a picogram of fermium-257.
The concrete walls of the REDC hot-cell facilities are designed to shield against high levels of radioactivity. Workers stand behind multiple panes of leaded glass interspersed with mineral oil and use robotic manipulator arms to move samples and equipment. Nine hot cells are lined up together, linked by a conveyor belt. The samples become progressively cleaner as they move through the cells.
First a researcher drops the target in a caustic solution to dissolve the aluminum casing, leaving behind only the superheavy elements. A subsequent set of chemical reactions isolates the curium and then the transcurium actinides, which are further separated into berkelium, californium, einsteinium, and fermium. It takes three to four months of around-the-clock work to complete the hot-cell purification process, with each target individually processed, purification columns re-run, and equipment prepared.
The unconverted curium is converted into an oxide, mixed with aluminum powder and pressed into pellets, put into an aluminum tube, and shipped back to the HFIR for use in the next production campaign. The 252Cf is processed into thin wires for the industrial consortium members.
The isolated 249Bk is taken upstairs to the purification group. At this point there’s less than 30 mg of faintly radioactive blue-green liquid, which is put in a glass bottle and walked through the building. That precious liquid contains less than a milligram of impurities, which is still far too much for the lab’s element-forging clients. That’s where Rose Boll comes in. As leader of the Medical, Industrial, and Research Isotopes Group, Boll oversees the final removal of impurities from berkelium and other actinides. “I tell people I’m the dishwasher,” she says. “I’m the one at the end who gets the dishes really clean.”
Boll came to ORNL following a decade of work in a hospital clinical laboratory, where she analyzed blood and tissue samples. After obtaining a PhD in inorganic chemistry, she transitioned into the medical isotopes team first as a postdoc, then as a collaborator, and finally as a full-time research scientist. Her team includes Phelps, one of many Oak Ridge scientists who came to the lab by way of the US Navy. Phelps grew up within striking distance of ORNL in Tennessee, but after earning her BS she decided the navy’s nuclear submarine program would be her best practical training ground. That’s one of the few careers in which technicians gain experience directly applicable to the work Oak Ridge carries out.
One complicating element of the berkelium purification process is the “people factor.” For instance, the two scientists who performed the final purification procedures in 2009 left Oak Ridge before the next production run in 2011. Boll, who had shadowed the work of the scientists in 2009, became the project lead for the purification team and guided Phelps and Van Cleve, who were handling berkelium for the first time. To minimize the impact of future staff turnover, Boll and the team recorded every step of their procedure. The resulting 30-minute instructional video will be used to train new hires. “I don’t know if you can see it in the video, but our hands were definitely shaking,” says Phelps.
The big challenge for Boll’s team is separating berkelium from remaining traces of californium. The two elements are neighbors on the periodic table and have near-identical physical and chemical properties. The only thing differentiating them is the size of their ionic radii. “You have to tickle them apart,” Boll says.
In their glove boxes, Boll and her team use an alpha hydroxyisobutyric acid (AHIB) column for the separation. After the material is loaded onto the column, the team changes the pH gradually, which alters the ability of the AHIB reagent to hold on to the transcurium ions. Due to their higher charge density, the californium ions are pulled off first, followed by the weaker berkelium ions. Analytical spectra tell the researchers if the berkelium is clean. Because of the mixture of isotopes, the spectra are sometimes ambiguous; Boll says it requires a certain amount of intuition on top of general radioanalytical knowledge to work out which trace impurities remain.
At the end of a successful 14-month campaign, the team achieves purity levels as high as one atom of contaminant for every 10 000 atoms of berkelium.
Assigning credit
In June 2009 a vial with a drop of green liquid berkelium salt was loaded into a lead-lined drum and shipped to Dimitrovgrad, Serbia, where it was applied in a thin coat to a titanium foil mount. The mounted sample was shipped to Dubna, Russia, and placed under a calcium-ion beam in an experiment that culminated in the creation of atoms with 117 protons. In February 2012 another vial was sent to a team in Germany, where the process was repeated for confirmation experiments. Four years later the International Union of Pure and Applied Chemistry recognized the Oak Ridge technicians as codiscoverers of element 117, named tennessine in honor of their state.
According to Ezold, 61 people from Oak Ridge played a role in the discovery of tennessine, an equal mix of operations staff, support personnel, and researchers—including Phelps, who is thought to be the first African American woman to help discover a chemical element. That doesn’t fit into the textbook narrative of brilliant individuals working solo to forge new elements.
Ezold says there’s a reason the International Union of Pure and Applied Chemistry credits institutions, rather than individual scientists, with discoveries: Every player on the team is key; you couldn’t switch any of them with someone off the street and expect the project to succeed. “There was a story about a janitor working at NASA in the 1960s,” says Ezold. “When people asked what he was doing, he said, ‘I’m helping to put a man on the Moon.’”
The services of Oak Ridge remain in high demand. “There’s been a resurgence of basic chemistry surrounding superheavy elements and actinides,” Ezold says. “The demand for berkelium is increasing.” The 2019 production and purification campaign is under way. More 249Bk has been requested by a Russian team that plans to bombard it with titanium ions in an attempt to form element 119. Another team, in Japan, is trying to reach element 119 through a different pair of elements: curium and vanadium. Oak Ridge sent 248Cm to the Japanese researchers in late 2017. That experiment is ongoing.
