Fighting for Subatomic Independence
Figure 1 Proton knockout from Oxygen-16, Oxygen-22 and Oxygen-24. The thickness of the arrow showing the removed proton reflects the spectroscopic factor for this process, or in other words how “free” the protons can be considered in neutron rich oxygen isotopes. Illustration by Andy Sproles, Oak Ridge National Laboratory.
Protons and neutrons typically enjoy a peaceful coexistence, bound together in the nuclei of all matter. In the nuclear shell model, which has been very successful in explaining how nuclei are structured, there is an assumption that this peace comes with a certain level of detachment: each proton or neutron is an independent spirit, moving on its own within the mean field they generate as a whole. Deviations from an independent motion, may reveal important features of how nucleons communicate under both normal and more extreme conditions. Correlations play a crucial role and proper understanding of correlations conveys information about the underlying laws of motion. In quantum mechanical systems, the concept of independent particle motion has played, and continues to play, a fundamental role in studies of complex many-particle systems. Within such a picture, the various constituents in a complicated many-particle system are assumed (as in the nuclear shell model) to move in an average mean field set up by the other interacting particles. When scientists observe any kind of deviation from that model, they expect those observations to reveal important features of both the structure and the dynamics of a many-particle system. Large-scale microscopic coupled-cluster calculations of neutron-rich oxygen isotopes demonstrate a surprising and intimate relationship between the enhanced correlations of the outnumbered protons and the freedom of the abundant neutrons to partly escape from the nucleus: if the neutrons are forbidden to enter the free continuum, the protons will fight harder for their own independence.
“Quenching of Spectroscopic Factors for Proton Removal in Oxygen Isotopes“
Ø. Jensen, G. Hagen, M. Hjorth-Jensen, B. Alex Brown, and A. Gade, Phys. Rev. Lett. 107, 032501 (2011)