Atomic nuclei are often referred to as “quantum many-body systems” due to their complex nature. They consist of multiple particles, known as nucleons (neutrons and protons), which interact with each other in intricate ways. These nuclei can absorb energy, resulting in excited states, and subsequently release energy through decay, often accompanied by the emission of various particles. These decay processes, along with the interplay between excited states and different decay channels, give rise to fascinating phenomena.
One intriguing phenomenon in nuclear physics is superradiance, which occurs when a nucleus reaches a high level of excitation. According to the nuclear shell model, nucleons are promoted to higher energy levels to achieve excitation. These excited states are termed excited states. As the available excitation energy increases, the number of ways nucleons can be promoted also increases, leading to a greater number of excited states. Superradiance can manifest when neighboring excited states become closely spaced, resulting in their overlap. In such cases, instead of observing multiple states, only one “superradiant” state is apparent.
To detect evidence of superradiance in nuclei, nuclear physicists study systems with identical internal structures but different decay channels. These systems, known as mirror nuclei, possess an equal total number of protons and neutrons, but the number of protons in one nucleus corresponds to the number of neutrons in the other. Since the nuclear force behaves uniformly between protons, neutrons, or a proton and a neutron, the internal structure of mirror nuclei remains the same. However, the decay channels differ due to the repulsion resulting from differences in electric charge.
In a recent study published in Physical Review C, a team of scientists from Texas A&M University, the CEA research institute in France, the University of Birmingham in the UK, and Florida State University presented evidence of the superradiance effect in the discrepancies between the alpha decaying states in Oxygen-18 and Neon-18.
The researchers examined the structure of Neon-18 by directing a beam of radioactive Oxygen-14 onto a thick Helium-4 gas target. The gas target enabled them to track the incoming and outgoing particles, facilitating a complete reconstruction of the nuclear events. The structure of Oxygen-18 had previously been investigated at Florida State University by scattering Carbon-14 on a Helium-4 target using a particle accelerator. This earlier experiment yielded favorable results, enabling the researchers to employ the Oxygen-18 excited state data to determine the initial parameters for the analysis of the Neon-18 data.
Consistent with the charge independence of the nuclear force, the researchers discovered a correspondence between mirror states in the two nuclei. However, some differences emerged when comparing the strengths of these mirror states. While one would anticipate mirror levels to exhibit equal strengths if the nuclei possess identical internal structures, the observed differences can be attributed to slight variations in alignment with different decay channels. The researchers interpreted these differences as evidence of the superradiance effect.
Additional related research has also been published in Communications Physics.
Source: US Department of Energy