Superradiance occurs when an atomic nucleus reaches a high excitation energy.
Mirror nuclei, such as oxygen-18 and neon-18, have the same number of protons and neutrons (18), but while oxygen-18 has 8 protons and 10 neutrons, neon-18 has 10 protons & 8 neutrons. When they absorb enough energy, they can decay and emit an alpha particle (2 protons and 2 neutrons). Image credit: M. Barbui.
Nuclear physicists refer to atomic nuclei as quantum many-body systems because they are formed by many particles that interact with each other in complex ways.
Nuclei can absorb energy, placing them into excited states. These states then lose energy through decay and may emit different particles.
The various processes of decay and particle emission are called decay channels.
The interplay between the internal characteristics of the excited states and the different decay channels gives rise to interesting phenomena.
One of these phenomena is superradiance, which occurs when a nucleus reaches a high excitation energy.
“Resonances in unstable quantum systems are radiating states that despite decaying overall normalization have a well-defined structure which is being balanced by outgoing radiation,” said Dr. Alexander Volya, a physicist with the Cyclotron Institute at Texas A&M University and Florida State University, and his colleagues.
“Such an interplay between outgoing wave and internal quantum many-body dynamics leads to several unique effects.”
“One of those is known as superradiance, or alignment, where due to decay or virtual coupling to the continuum the states undergo restructuring so that their wave functions align towards the decay channels thus facilitating the decay.”
“This effect is well understood theoretically and is closely related to the fundamental properties of reaction physics.”
“Direct observation of superradiance in open quantum many-body systems is difficult because it is hard to find identical complex quantum systems that are different only in their coupling to the continuum of reaction states describing the decay.”
To find evidence of superradiance in nuclei, nuclear physicists look for two systems that have the same internal structure but different decay channels.
Mirror nuclei have the same total number of protons and neutrons, but the number of protons in one equals the number of neutrons in the other.
The internal structure of mirror nuclei is the same since the nuclear force is the same whether between two protons, two neutrons, or a proton and a neutron. This makes the nuclear force charge independent.
However, the decay channels are different due to the different electric charge repulsion in the two systems because of the difference in each system’s number of protons.
In the new research, Dr. Volya and co-authors found evidence of the superradiance effect in the differences between the alpha decaying states in oxygen-18 and neon-18.
They studied the structure of neon-18 by scattering a radioactively unstable beam of oxygen-14 on a thick helium-4 gas target.
The gas target allowed the authors to measure the tracks of the incoming and outcoming particles and produce a complete reconstruction of the nuclear events.
As expected from the charge independence of the nuclear force, the researchers found a correspondence between mirror states in the two nuclei, although some differences emerged when comparing the strength of mirror states.
If the internal structure of the nuclei is the same one would expect mirror levels to have the same strength, but in these cases alignment with slightly different decay channels produces observed differences.
“We interpreted these differences as evidence of the superradiance effect,” the physicists said.
Their results appear in two papers (paper #1 and paper #2) in the journal Physical Review C and the journal Communications Physics.
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M. Barbui et al. 2022. α-cluster structure of 18Ne. Phys. Rev. C 106: 054310; doi: 10.1103/PhysRevC.106.054310
A. Volya et al. 2022. Superradiance in alpha clustered mirror nuclei. Commun Phys 5, 322; doi: 10.1038/s42005-022-01105-9
