Fat silicon atoms are doubly magic
For protons, fourteen is the new magic number.
A form of silicon that is bloated with extra neutrons has revealed a 'magic number' for the protons in its nucleus.
Atomic nuclei are only stable when packed with certain combinations of positive protons and uncharged neutrons. Some combinations are more stable than others, and the numbers of protons or neutrons in such cases are called 'magic'. This extra stability is achieved when the subatomic particles fill up certain energy levels within the nuclei, leaving no stray particles hanging around at higher energies.
Physicists have long known of a series of such magic numbers, including 2, 8, 20, 28, 50, 82 and 126. Some elements have quantities of subatomic nuclei that match these numbers and are more stable. Some even hit these scores in both their neutrons and protons, making them doubly magic (see 'Double bind').
University of Surrey, UK
"Between 8 and 20 there are some energy sublevels," explains Jeff Tostevin, a theoretical nuclear physicist from the University of Surrey, UK, who was part of the team. "Normally, these sublevels are very close together, so they don't obviously stand out as a magic number," he explains. This is the case in the most common form of silicon, which has 14 protons and 14 neutrons.
But nuclei grow and change shape as more subatomic particles are packed in, and this changes the relative location of their energy levels. Paul Cottle, a nuclear physicist from Florida State University, Tallahassee, and the rest of the study's international team had reason to believe that as silicon gets beefed up with neutrons, this would alter the energy levels in a way that would make 14 a magic number.
To test this idea, the team fired a high-energy beam of sulphur-44 at a beryllium target. This forced the sulphur nuclei to lose two protons, transmuting them to silicon. They counted how much silicon-42 was produced by the collisions, and compared this with quantum mechanical calculations that assumed 14 was magic. The numbers matched up perfectly, says Tostevin.
The calculations throw up some surprises. It seems odd, for example, that 28 remains a magic number for neutrons in the silicon-42 nucleus, says Robert Janssens, a nuclear physicist at Argonne National Laboratory, Illinois. Finding out where the magic stops working is the key to exploring the most neutron-rich isotopes, Janssens explains.
Understanding how the energy levels of nuclei are arranged is an important test of quantum theory, says Tostevin.
It could help to reveal the sequence of nuclear reactions that occur in supernova explosions, he adds. These violent stellar deaths are the source of all elements heavier than iron. Huge amounts of neutrons are released in these explosions, which collide with atoms to make very short-lived, neutron-rich isotopes similar to silicon-42.
"Understanding neutron-rich nuclei helps you to understand the fundamentals of the creation of the Universe," says Tostevin.
- Fridmann J., et al. Nature, 435. 922 - 924 (2005).