13.12.2017

Superheavy Elements

Superheavy elements can be found at the extreme "North-East" of the nuclear chart, and contain more than one hundred protons in the nucleus. The strong Coulomb repulsion between the protons acts to overcome the strong force holding the nucleons together, resulting in a reduction of the barrier to fission. Indeed, if the nucleus is considered to behave like a drop of liquid, then the fission barrier drops to zero when a sufficient number of protons is added. This condition provides a rough definition for "superheavy elements". Such nuclei only exist due to the fact that the nucleons in the nucleus move in a confining potential which gives rise to quantal "shell effects" (the spacing between energy levels in the nucleus is non-uniform). These shell effects give rise to additional binding, resulting in the well-known magic numbers at closed shells. One long-standing prediction is that there should exist an "island" of spherical superheavy nuclei located at the next magic numbers for protons and neutrons beyond the currently known Z=82 and N=126, for protons and neutrons, respectively. Different nuclear models predict different magic numbers, but the island should be in the region of Z=114-126 and N=184. Superheavy elements can only be produced in the laboratory, and to date elements up to Z=118 have been created. Superheavy elements are often identified through the observation of long chains of alpha decays which end in regions of the nuclear chart where the nuclear decay properties are well known. Most experiments to produce superheavy elements may last several months, in which time it may be that only a few atoms of the element of interest are created. Members of the Nuclear Spectroscopy group participate in experiments to study superheavy elements in GSI, Darmstadt, and were involved in the discoveries of elements 108, 110, 111 and 112 (Matti Leino).

Further reading:fission_graphs_web.jpg