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- The isotope lead-208 was predicted to be extremely stable and perfectly spherical because of the “magic” numbers of electrons and protons orbiting its nucleus.
- When researchers blasted lead-208 with ions to excite the subatomic particles and determine the shape of the nucleus, they found that it was instead sort of oblong, as if it had been squashed.
- While the exact reason for the odd shape of the nucleus of such a stable isotope is still being guessed at, it might be able to explain more physical processes, such as the formation of heavy metals in space.
The lead isotope known as lead-208 was considered a “double magic” atom because of its subatomic particles—until the reveal of something completely unexpected.
It was supposed to be a physics fairy tale. Having a “magic” number of protons or neutrons means there are enough to fully occupy the nuclear valence shells (outermost orbits around the nucleus) in an atom and give it stability. Magic numbers mostly occur in noble gases, which explains why these elements are so stable. More stable isotopes can also be identified by magic numbers, and double magic means an extra level of stability, providing resistance against nuclear decay.
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In the case of lead-208—the heaviest stable isotope known so far—a team of researchers led by physicist Jack Henderson expected a spherical and stable nucleus. After all, it has magic numbers of 82 protons and 126 neutrons.
“Studying this nucleus is thus key to understanding not only the structure of doubly magic nuclei, but also the entire region of the nuclear landscape,” the researchers said in a study recently published in Physical Review Letters.
The team used the GRETINA gamma-ray spectrometer to bombard the lead-208 with ions at 30,000 km per second (19,000 miles per second)—ten percent the speed of light. This excited its quantum state by making the particles in the sample vibrate furiously. The shape of the nucleus could then be inferred from the quantum states of those particles.
What Henderson’s team discovered ended up defying their expectations. They realized that what was supposed to be a perfect orb of a nucleus was actually somewhat warped. Lead-208 exhibited a prolate deformation, which meant that it lengthened in the direction of its poles (as opposed to oblate, or flattened at the poles).
This might sound like an incredibly niche experiment, but the research could actually have a significant number of implications, including for the cosmos at large. The neutron magic number in lead-208 might be associated with a process that involves capturing neutrons and forming about half the elements in the galaxy (which are heavier than iron).
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“Understanding the properties of nuclei in this [process] is of paramount importance for our understanding of the production of heavy elements,” the researchers said in the same study.
Why the nucleus of lead-208 was not nearly as spherical as expectations thought it would be is still a mystery. Even a nucleus that appears to be a perfect sphere is not rigid, and therefore can be reshaped. The researchers think it’s possible that the way subatomic particles vibrated while being blasted was much more irregular than they predicted, and may have had something to do with the warping of the nucleus.
“The findings directly challenge results from our colleagues in nuclear theory,” Henderson said in a press release, “presenting an exciting avenue for future research.”
Elizabeth Rayne is a creature who writes. Her work has appeared in Ars Technica, SYFY WIRE, Space.com, Live Science, Den of Geek, Forbidden Futures and Collective Tales. She lurks right outside New York City with her parrot, Lestat. When not writing, she can be found drawing, playing the piano or shapeshifting.
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