Decay of heaviest oxygen isotope casts shadow on physicists' understanding of nuclear stability

Heaviest oxygen isotope was observed to quickly decay and challenge physicists' understanding of nuclear forces, scientists report in a study published in the journal Nature.

Isotopes are elements sharing the same atomic number but differing in their atomic weights.

Atomic number is equal to protons in the atom's nucleus, whereas atomic weight comes from the protons and neutrons, collectively called nucleons and bound by strong nuclear forces, in the nucleus.

The most commonly available form of oxygen, 16O, has 8 protons (atomic number 8) and 8 neutrons in its nucleus and thus, has an atomic weight of 16.

Its isotopes, oxygen-28 and oxygen-27, have the same protons (8), but differ in the number of neutrons, which is 20 and 19, respectively.

The neutron-rich isotopes, oxygen-28 (28O) and oxygen-27 (27O), were observed as they were in the process of decaying into oxygen-24 (8 protons, 16 neutrons), after emitting 4 and 3 neutrons, respectively, and thus, proving to be neutron-unbound, the international team of researchers led by Yosuke Kondo, assistant professor at the Department of Physics at Tokyo Institute of Technology, Japan, said in their study.

The nucleus 28O was of significant interest as it was expected to be one of the few "doubly magic" nuclei in the standard shell-model picture of nuclear structure, they said.

According to the nuclear shell-model, a shell inside a nucleus filled up with just the right number or "magic number" of nucleons results in strong binding forces. These numbers are thought to include 2, 8, 20, 28, 50, 82 and 126 particles.

The most abundant form of oxygen, having 8 protons and 8 neutrons, is considered "doubly magic".

However, the decay of 28O, with 8 protons and 20 neutrons and initially thought of as "doubly magic", has challenged scientists' understanding of principles governing nuclear forces.

Both 28O and 27O were found to be generated when thick liquid hydrogen was bombarded with intense beams of 29F (flourine-29 isotope), having unstable nuclei, from a particle accelerator at the RIKEN Radioactive Isotope (RI) Beam Factory, Japan. The incident beam knocked out a proton from the liquid hydrogen's nucleus, thereby generating 28O.

The researchers observed these isotopes and studied their properties by directly detecting their decay products.

Further, they found that the existence of these isotopes was very short-lived, as they decayed through spontaneous neutron emission.

The present findings enhance our understanding of nuclear structure (and forces) by offering new insights, especially for extremely neutron-rich nuclei, the researchers said.