Scientists have recently discovered a new isotope of oxygen, oxygen-28, that defies all expectations about its stability. Oxygen-28 has the highest number of neutrons ever observed in the nucleus of an oxygen atom, yet it decays rapidly, challenging our understanding of the “magic” number of particles in atomic nuclei.
The nucleus of an atom is made up of nucleons – subatomic particles consisting of protons and neutrons. While the atomic number of an element is determined by the number of protons, the number of neutrons can vary, resulting in different isotopes. For example, oxygen has 8 protons but can have a different number of neutrons.
Previously, the highest number of neutrons observed in an oxygen isotope was 18 in oxygen-26. However, a team led by nuclear physicist Yosuke Kondo of the Tokyo Institute of Technology discovered two previously unseen isotopes of oxygen: oxygen-27 and oxygen-28, containing 19 and 20 neutrons, respectively.
The research was conducted at the RIKEN Radioactive Isotope Beam Factory, a facility designed to produce unstable isotopes using a cyclotron gas pedal. They first bombarded a beryllium target with a beam of calcium-48 isotopes, which produced lighter atoms, including fluorine-29, a fluorine isotope with 9 protons and 20 neutrons.
To create oxygen-28, the researchers isolated fluorine-29 and collided it with a target of liquid hydrogen to remove a proton. The experiment was successful, but it turned out that both oxygen-27 and oxygen-28 are unstable: in a very short time they decay into oxygen-24 and 3-4 free neutrons.
This instability is surprising because both 8 and 20 are considered “magic” numbers for protons and neutrons, respectively. Magic numbers refer to the number of nucleons that completely fill a shell, with each new shell having a significant energy gap compared to the previous one. A nucleus whose shells contain both protons and neutrons with magic numbers is called a double magic nucleus and is expected to be highly stable.
Oxygen-16, the most common form of oxygen on Earth, is an example of a double magic number isotope. The next double-magic oxygen isotope after oxygen-16 was thought to be oxygen-28, but previous attempts to find it were unsuccessful. However, recent results obtained by Kondo and coworkers indicate that the neutron shell in oxygen-28 is incompletely filled, casting doubt on whether 20 is indeed the magic number for neutrons.
Interestingly, this finding is consistent with a phenomenon known as an “inversion island” that has been observed in the isotopes of neon, sodium, and magnesium. In these elements, the shells with 20 neutrons do not close properly. This phenomenon has now been found to extend to fluorine-29 and oxygen-28 as well.
This discovery is of great importance to our understanding of the stability of atoms and the role of magic numbers in determining the behavior of isotopes. Further research will be required to fully understand the nature of oxygen-28 and its implications for nuclear physics.