Which are the unstable elements

When atomic nuclei become unstable


When atomic nuclei contain too many neutrons, they break apart. For the first time, an international team of physicists has developed a method that enables an exact calculation of the point at which the nuclei become unstable. The results allow a more detailed insight into the structure of atomic nuclei. The scientists hope, among other things, that this will enable them to better understand the formation of the elements after the Big Bang. In addition to the Jülich Institute for Nuclear Physics, the universities in Bonn and Bochum as well as various US universities were involved in the study. The calculations were carried out on the Jülich supercomputer JUQUEEN. The study has now appeared in the Physical Review Letters.

Atoms consist of a shell and a core. The shell is formed by the negatively charged electrons. They are responsible for enabling atoms to form chemical bonds. The core, on the other hand, is positively charged. It holds the electrons firmly due to the electrostatic attraction, so to speak.

The protons ensure the positive nuclear charge. There are always as many of them as there are electrons. Atoms are therefore electrically neutral overall. A carbon atom, for example, consists of six electrons and six protons. In addition, the core of the carbon atom also contains uncharged particles, the neutrons. However, when the nucleus of an atom contains too many neutrons, it becomes unstable. The atom can then break - it disintegrates.

When exactly this happens differs from atom to atom. "So far it has not been possible to calculate exactly how many neutrons this point is reached," explains Prof. Ulf Meißner from the Jülich Institute for Nuclear Physics and the Helmholtz Institute for Radiation and Nuclear Physics at the University of Bonn. Reason: At the core, different forces are at work. The current algorithms can calculate some of them exactly, but only determine others approximately.

The method that Meißner and his colleagues have now published is different. This is initially based on a kind of "deprivation of liberty". In reality, the protons and neutrons can be anywhere in space. However, the scientists restricted this freedom for their calculations: They arranged the core particles at the nodes of a three-dimensional lattice and thus only allowed them to have certain, strictly defined positions. For such a lattice configuration, the binding energy between the particles can be determined relatively easily.

In the next step, the core particles were allowed to swap places. This created a new grid configuration. If this was energetically more favorable than the first, it served as the basis for a renewed exchange of places. "We have repeated this step a million times," explains Meißner. "This brought us closer and closer to the core configuration, which is energetically optimal. And on this basis we were then able to calculate whether or not the core is stable with the specified number of protons and neutrons."

Experts also speak of a Monte Carlo procedure. It does provide exact results on the bonding in the atomic nucleus. However, the assignment of the core particles to certain positions also results in disadvantages. Normally it is not possible to calculate the exact density distribution of the core. However, the researchers modified the process so that this is also possible.

»Original publication

Source: Research Center Jülich

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