Scientists finally detected O-28. It's instability surprised them

https://www.sciencenews.org/article/oxygen-28-instability-surprise-physics

Using a powerful particle accelerator, researchers have spotted an elusive variant of oxygen for the first time. The isotope, oxygen-28, was predicted to be stable thanks to its eight protons and 20 neutrons — “magic” numbers associated with extra stability in atomic nuclei.

Atomic nuclei are made up of protons and neutrons, each of which are thought to occupy their own “shells” — discrete energy levels that are separated by large energy gaps. Atomic nuclei with full outer shells are bound extra tightly, making them very stable. Shells fill up when they hit two, eight, 20, 28, 50, 82 and 126 subatomic particles (SN: 10/9/13).

Finding the isotope took a combination of brute force and experimental elegance. Physicist Yosuke Kondo of the Tokyo Institute of Technology and colleagues used a particle accelerator to smash calcium-48 atoms against a beryllium target. This fragmented the calcium-48 atoms into lighter isotopes, including fluorine-29. Throwing the fluorine-29 against a liquid hydrogen target knocked off a single proton, producing oxygen-28.”

Scientists expected the isotope to be stable, but it isn’t: it sloughs off 4 neutrons in a femtosecond (1E-21 sec). O-28’s surprising instability indicates there’s something we don’t understand about the strong nuclear force, which binds together protons and neutrons in the atomic nucleus.

A little more in the article.