Protons don’t like to stay close to one another for very long. But if you’ve got the right number balanced neatly among enough neutrons, they just might build an atom that won’t crumble apart in the blink of an eye.
Theorists had suggested 114 could be one such ‘magic’ number of protons – but a recent experiment conducted at the GSI Helmholtz Centre for Heavy Ion Research in Germany now makes that incredibly unlikely.
In 1998, Russian experimenters finally succeeded in building an element with 114 protons in its nucleus. It was later named flerovium after its birthplace, the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research.
Creating mammoth-sized atoms is by no means easy, achieved only by starting with heavyweight elements like plutonium and pelting them with slightly smaller ones such as calcium, until something sticks.
By ‘sticks’, we mean ‘pauses long enough to technically pass for an atom’, which for mountain-sized nuclei is rarely more than a fraction of a second. For example, at 112 protons in size, the transuranic element of copernicium has little chance of lasting more than 280 microseconds.
Atomic nucleons hold onto one another as an effect of the strong force shared between the trios of sub-atomic quarks that make them up.
At the same time, the repulsive nature of positive charges in protons push them apart, meaning the whole structure teeters on the brink of collapse, should they come too close together. This is why we see some combinations of nucleons, or isotopes, more often than others.
Once an atom gets to a certain size, a slew of other factors to do with energy and mass also weigh in, making it harder and harder for the atom to hold itself together, not to mention more difficult for physicists to predict its characteristics.
Yet physicists are confident that there are islands of stability in the upper reaches of the periodic table, where arrangements of protons can form patterns and shapes that allow them to hold onto life a little longer than neighbouring elements.
Nihonium, or element 113, has an isotope with a half-life of around 20 seconds, for example.
When signs of flerovium were first sifted out of a debris of plutonium and calcium more than 20 years ago, however, it looked like a real keeper. The signature in the data suggested atoms were remaining stable for as long as 30 seconds before spitting out an alpha particle and crumbling briefly into copernicium.
The excitement was short lived. In 2009, Berkeley scientists managed to recreate two different isotopes of the element. One lasted a tenth of a second. The second hung around a touch longer, falling apart after half a second.
The odds weren’t looking good for element 114, but physicists aren’t the types to leave well enough alone. So the University of Mainz went big, using upgraded detectors to study dozens of possible flerovium decay events.
In the end, two were confirmed as bonafide isotopes. One resulted in an isotope of copernicium that was seen breaking down in a way that hadn’t been previously observed.
In that event, the flerovium decay chain occurred inside 2.4 seconds, in a shedding of alpha particles. The second isotope was gone in 52.6 milliseconds. Importantly, the efficient way each of the two isotopes decayed made it clear that 114 wasn’t stable in the least.
As exciting as a stable flerovium might have been, the novel findings of an excited state of copernicium provides solid ground for exploring islands of stability higher up the periodic table, giving theorists vital information for modelling this phenomena further.
“The existence of the state provides yet another anchor point for nuclear theory, because it seems to require an understanding of both shape coexistence and shape transitions for the heaviest elements,” the researchers note in their report.
While we can now all but rule out 114 as one of the magic numbers of the periodic table, there are more giants left to slay.
Physicists are yet to create the hypothetical element tentatively called unbinilium, or element 120. Crafting one of those monsters would take some powerful technology and advanced knowledge of nuclear physics.
There are plans in the works for pushing the limits on atomic masses, with RIKEN in Japan making steady progress at its Nishina Center for Accelerator-Based Science, so we might not need to wait long.
Like explorers of old, researchers are still confident that there are stable islands just over the horizon. We’re bound to see some mirages along the way.
This research was published in Physical Review Letters.