Strange things happen in large nuclei
A single proton or neutron in the outer shell of a large nucleus moves differently than modern theoretical models predict, determined an international team of researchers, including physicists from the Faculty of Physics, University of Warsaw. Source: Faculty of Physics UW
Single protons and neutrons in atomic nuclei behave differently than theoretical models predicted - discovered physicists, including researchers from the University of Warsaw. Their research forces to revise the way in which large nuclei have been described for decades.
Nuclei shape our reality: they contain as much as 99.9 percent mass of surrounding matter. Although they are so ubiquitous, there are still many gaps in the knowledge of them. The main obstacle in the construction of a coherent theoretical description of atomic nuclei is the complexity of interactions between their components: protons and neutrons. The situation becomes especially complicated when there are a lot of particles in the nucleus.
In the publication in the prestigious journal "Physical Review Letters", a group of scientists from Poland (Faculty of Physics, University of Warsaw), Finland and Sweden demonstrated the need to modify the decades-old model of nuclei with specific, large numbers of both protons and neutrons. Representatives of the Faculty of Physics UW reported the discovery in a release sent to PAP.
Until now, researchers describing the interactions in the nucleus used the models similar to those describing electrons. It is generally assumed that the electron moves in the electrostatic field formed by the other electrons and distant atomic nuclei. This model predicts that electron shells in different sizes are formed in the atom: the first contains a maximum of 2 electrons, the second 8 electrons, the third 18 electrons and so on. However, the use of a similar model for the nuclei may not necessarily be correct - the interactions are different. "Protons and neutrons in atomic nuclei are very close to each other. Each one moves in the field, which it also actively shapes" - explains another co-author, Dr. Dimitar Tarpanov (Faculty of Physics UW).
The model used until now predicts the existence of shells in the nucleus - the areas with highest probability of the presence of proton or neutron. Subsequent nuclei shells can accommodate a maximum of 2, 8, 20, 28, 50, 82 and 126 protons (the same numbers apply to shells with neutrons). Completely filled shells also appear at 114, 120 and 126 protons and 184 neutrons. Chemists call these the magic numbers. A nucleus is double magic, if it contains the magic number of protons and the magic number of neutrons.
Researchers were particularly interested in situations where the nucleus is almost double magic: one shell is completely filled, and another has only one single proton or neutron. The question was: under what interactions will this single particle move.
In the old model - to prevent it from contradicting the experimental data - for decades additional effects have been taken into account: vibrations and movements of protons and neutrons that occur due to quantum effects. In special cases, these vibrations could even change the appearance of the nucleus: slightly flatten it or give it the shape of a pear. Such alterations would also affect the field in which the "loner" moves - a single proton or neutron in the outermost shell of the nucleus.
The researchers used the experimental data available for, among others, double magic nuclei of oxygen 16O, calcium 40Ca and 48Ca, nickel56Ni, tin 132Sn and lead 208Pb, and the nearly double magic nuclei, such as 207Pb and 209Pb. Theoretical analysis has left no doubt that the quantum effects and associated vibrations have a much smaller impact on the movement of a single particle in the nucleus shell than previously expected.
"We have demonstrated that one of the two main physical factors previously included in the large nuclei model, is in fact not particularly important. In practice, this means that the physics of such nuclei works somewhat differently than we all thought" - said Prof. Jacek Dobaczewski from the Institute of Theoretical Physics, Faculty of Physics UW. He noted that the achieved result is "very, very interesting". "Since the quantum effects in such a large nucleus as lead 209Pb are not particularly important, this means that the contemporary averaged field model does not fully correspond to reality. There is something we are not taking into account. The question is: what?" - wonders Prof. Dobaczewski.
Work on the creation of an accurate and uniform description of phenomena occurring in the light, heavy and superheavy nuclei are of particular practical importance. Knowledge about the physics of the atomic nucleus is used in the construction of nuclear power plants and in the work on future thermonuclear power plants, in the military, as well as in nuclear medicine in imaging of tissues, diagnosing diseases and anti-cancer therapies.
Moreover, nuclear processes and interactions are an essential element of the description of the stars in the universe. Theoretical methods developed to describe the interaction of many particles in the nucleus also find numerous applications in atomic physics and condensed matter physics, and quantum chemistry - the spectral analysis of the excited states of nuclei, atoms and molecules.
This research was funded by the European project ENSAR, Polish National Science Centre, Finnish academic program FIDIPRO and the Bulgarian Science Fund.
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