Nuclear Physics I - Nuclear Structure

The Figure shows a plot of the known atomic nuclei as a function of their proton and neutron number. The black squares mark stable nuclei, while the green color indicates nuclei which are unstable and eventually transform into the stable ones by radioactive decays.

The nuclear structure research program at GSI has always been focused on studying nuclei at extreme proton and neutron numbers, i.e. at the borderline of the green area. Already in the 1980s proton radioactivity was discovered at the UNILAC. For 151Lu and 147Tm one could show that these nuclei decay by emitting protons from their ground state.

In the nineties, the first synthesis of the doubly-magic nuclei 100Sn and 78Ni was achieved at SIS. The main thrust of the research program goes into two directions: towards heavy and superheavy nuclei and towards the study of nuclei with very large neutron excess. It was a great achievement of our theoretical colleagues to predict already in the late sixties the existence of superheavy elements.

In fact, the search for these superheavy elements was the main motivation to found GSI and to build the UNILAC. But, as it turned out, it was indeed quite difficult to artificially produce nuclei which are heavier than those already known. The probability to form a superheavy nucleus is of the order of 1 in 10 to the power of 18: this means that the odds are of similar magnitude as to win in the German lottery '6 aus 49' with 6 numbers right in three consecutive weeks.

If one nevertheless wants to do such an experiment, one needs extremely high beam intensities and an extremely powerful detector system like SHIP, which represents a complex arrangement of electric and magnetic fields which filter out the rare superheavy fusion products and separate them from beam particles and other background events.

Elements with proton numbers of 107 to 112 were synthesized at GSI and identified by their characteristic a decay chains. Competitive experiments, aiming at the synthesis of even heavier elements, are being performed also in other laboratories like Berkeley (USA), Dubna (Russia), GANIL (France) and RIKEN (Japan).

Leaving the valley of the stability in the direction of very large neutron numbers, one encounters nuclei which are increasingly unstable. A direct way to proof the existence of a nucleus is to measure its mass. It was a highlight of the nuclear structure program at GSI to develop a technique for measuring masses of nuclei even if their half-lives are as short as some tens of microseconds.

This scheme works as follows: the short lived isotopes produced by fragmentation using very intense heavy-ion beams are separated in the fragment separator, and then are transported into the experimental storage ring ESR. There they circulate and by measuring their revolution times one can determine their mass.

Using this technique, masses were determined for a large number of nuclei. In future, these measurements will be extended to the very neutron-rich region of the nuclear chart. On the neutron rich side, many surprising features of nuclei have been discovered.

Some of the shells associated with magic nucleon numbers, which give nuclei an increased stability, have been found to fade out when going to nuclei with very large neutron excess. Other magic numbers come up. Nuclei which have a compact core of nucleons with rather loosely bound neutrons circulating around, so-called halo nuclei, have been discovered. Nuclei of this type do not exist along the valley of stability where most of the nuclear structure studies had been performed so far.

These observations are of importance for nuclear physics in their own right; but they also have a major impact on our understanding of the universe. All heavy elements are produced via nuclear fusion or by capture of protons and neutrons in the interior of stars.

Of particular interest is the so-called r-process which occurs in supernova explosions and proceeds via exotic nuclei whose properties are still unknown. During these cataclysmic events nuclei heavier than iron are formed and at the same time the element inventory of the star is disseminated into space. Thus, all matter surrounding us, each and every atom in our bodies, originated as dust from star explosions.