By Ron B. Davis Jr., Georgetown University
Neutron bombardment is a very convenient way to build up a larger nucleus just one mass unit or two at a time. The problem is that yields of stepping-stone elements like element 95, americium, are sometimes vanishingly small, not providing enough target material to attempt the next iteration of neutron bombardment. So, then, how did physicist, Glenn Seaborg, find a way to leapfrog multiple elements with a single impact and discover new heavy elements?
Neutron Bombardment
Neutron bombardment of large atoms proved a very useful technique in the 1940s, allowing physicists to synthesize transuranic elements like neptunium, plutonium and americium.
Indeed, the process of neutron capture had proven a highly effective synthetic method over those years, but as the 1950s rolled around and the Manhattan project was shuttered, those few physicists who aspired to continue their crusade to make larger and larger elements were facing a problem.
However, neutron bombardment offered diminishing returns as each new heavy product becomes the target for the next step to the right on the table. It was becoming clear that if scientists were going to go much higher in atomic number, they would need a way to skip over more elements in one go.
This led the scientists to explore using full-sized nuclei as their projectiles, nuclei that contained protons as well as neutrons.
Coulombic Barrier
Even very small nuclei like alpha particles have protons and their positive charge is strongly repelled by the positive charge of all the protons in a big target nucleus. This repulsive force between two positive nuclei, that must be overcome to crash to atomic nuclei into one another and have them merge into one, is known as the Coulombic barrier.
Naturally occurring alpha particles, like those emitted from uranium samples, generally don’t have sufficient kinetic energy to overcome the Coulombic barrier. But in 1944, this problem was solved using a recent invention called the cyclotron.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
A Cyclotron
A cyclotron is a device that uses powerful, shaped magnetic fields to accelerate charged particles to extremely high speeds by directing them in a circular or spiral path. It is done so that they can be accelerated to mind-boggling speeds in a relatively confined space. So, using the cyclotron at Berkeley, Glen T. Seaborg, and a group of other physicists, collaborated to successfully crash a helium nucleus into plutonium-239 with sufficient force to overcome the Coulombic barrier.
After assimilating the helium nucleus and rapidly losing a single neutron, in that instant, element 96 was observed for the first time. Having discovered yet another element, Seaborg continued his practice of naming the element in homage to the lanthanide above it in the table- gadolinium, named for Johan Gadolin who first isolated many of the lanthanoid elements.
Curium
Naming it curium, Seaborg decided to pay homage to the wife-and-husband team of Marie and Pierre Curie, whose pioneering work in isolating radioactive elements seemed to make this an appropriate choice.
Seaborg’s curium was abundant enough, and the half-life of over 150 days; long enough that a sample of pure curium was produced and analyzed back in Chicago at Seaborg’s laboratory.
Two decades later, curium-242 would find itself on the surface of the moon, acting as source of alpha particles used in experiments conducted by the NASA Surveyor missions, a series of robotic lunar landings intended to characterize and explore the surface of the moon.
In the latter half of the 1940s, as World War II came to a conclusion, many Americans, including the scientists participating in the Manhattan Project, were returning to their civilian lives. Glenn Seaborg returned to Berkeley, California, to continue his work on nuclear chemistry.
Elements 97, Berkelium
Having discovered americium and curium, and developed ways to produce them in quantities that would allow Seaborg to experiment with these new elements, he set about pushing the limits of the periodic table yet again.
Seaborg and his colleagues—including Albert Ghiorso, who would much later become leader of the Berkeley group—took the next logical step. They used their new heavy-element products as targets yet again. Firing helium nuclei at their new creations, 95-americium and 96-curium, would, they hoped, help them to produce elements 97 and 98, respectively.
Unbelievably, the strategy was a success! In 1949, 7 milligrams of americium was bombarded with helium nuclei to produce element 97. The team rushed to claim their right to name this new element, naming it berkelium in recognition the town of its origin, much like terbium just above it in the lanthanoids was named for Ytterby.
Element 98, Californium
Shortly thereafter came element 98. This time they had just eight micrograms of element-96 curium, which they bombarded with helium nuclei to produce a scant 5,000 atoms of element 98. Put another way, that’s only about 0.002 femtograms of material. If it were possible to line up all of these atoms side by side, they would stretch only about 1.3 microns across—about one-hundredth the width of a human hair.
The team published their findings in rapid succession in the Journal Physical Review. The element, dysprosium, just above it, came from the Greek dysprositos, meaning ‘hard to get at’. Had Seaborg stuck to his tradition of naming new actinoids in the same vein as their lanthanoid homologs, he would need to name element 98 after one of its properties.
Perhaps he felt that five thousand atoms weren’t enough material to determine any properties of his new element. Or maybe he just thought it would be fun to continue immortalizing the address of his lab where all these elements were being discovered—as had happened with Scandium and all the other element names pointing to Ytterby. For whatever reason, the group responsible for americium and berkelium recommended the name californium for their third element.
Common Questions about Curium, Berkelium, and Californium
Neutron bombardment of large atoms proved a very useful technique in the 1940s, allowing physicists to synthesize transuranic elements like neptunium, plutonium and americium.
The repulsive force between two positive nuclei, that must be overcome to crash to atomic nuclei into one another and have them merge into one, is known as the Coulombic barrier.
In 1949, 7 milligrams of americium was bombarded with helium nuclei to produce element 97.
