MORE NUKES: This illustration of the nuclear landscape shows atomic isotopes arranged by an increasing number of protons (up) and neutrons (right). The dark blue blocks represent stable isotopes, while the lighter blue blocks are unstable isotopes. Image: Andy Sproles, Oak Ridge National Laboratory
Scientists have long wondered whether there is a limit to the number of protons and neutrons that can be clustered together to form the nucleus of an atom. A new study comes closer than ever to finding the answer by estimating the total number of nucleus variations that can exist.
The periodic table of elements includes 118 known species of atoms, and each of these exists (either naturally or synthetically) in several versions with differing numbers of neutrons, giving rise to a total of about 3,000 different atomic nuclei. As technology has improved over the years, physicists have been building heavier and heavier atoms — element 117 was created only last year, and researchers are hot on the trail of 119. New projects are in the works to add and subtract neutrons to known elements to create ever more exotic variations, known as isotopes.
But where does it end?
In a paper published in tomorrow's (June 28) issue of the journal Nature, researchers report that roughly 6,900 nuclides (variations of atomic nuclei), plus or minus 500, should be possible. [Infographic: Nature's Tiniest Particles Dissected]
"Beyond the 7,000, we are talking about nuclides whose lifetimes can be so short that they can't form," said research team member Witold Nazarewicz of the University of Tennessee, the Oak Ridge National Laboratory in Tennessee and Warsaw University in Poland. "The system would decay instantly."
Even within those 7,000, the vast majority would be unstable, lasting only a tiny fraction of a second. Of the 3,000 known nuclides, only 288 are stable.
Atoms are limited in the number of protons they can contain, because each proton is positively charged, and because "like repels like," they want to push each other away. Even neutrons, which have no charge, are slightly repulsive to each other. A mysterious force called the strong interaction, which is about 100 times stronger than electromagnetism, is what binds protons and neutrons together in nuclei.
"The nature or the exact form of the strong force, especially in heavier nuclei, is still a subject of very intense experimental and theoretical research," Nazarewicz told LiveScience. [Top 10 Unexplained Phenomena]
To create the new estimate, Nazarewicz and his colleagues, led by Jochen Erler of the University of Tennessee and Oak Ridge, examined what's called the drip line, a theoretical boundary on the number of neutrons that can be combined with a given number of protons to form a nucleus. (The idea is that if more neutrons are added beyond this line, they will "drip," or fall out, from the nucleus.)
To plot out the drip line, the researchers extrapolated from the best available models of nuclear interactions in heavy nuclei. By including various models, the scientists were able to estimate the first reliable error bars on their predictions, showing just how precise the estimate is.
"This is the first study which really gave an error bar and showed what is the best theoretical guess for this limit," Nazarewicz said. "It is not enough that you provide a number. You need to provide a number with [an estimate of the] uncertainty."
Supernovas and neutron stars
The new estimate isn't just a theoretical quantity — the number would represent all the possible species that might be created inside astrophysical phenomena, such as supernova explosions or neutron star mergers.