The atomic nucleus 100Sn: doubly magic and extremely exotic

This talk is intended for a general audience of physicists: Nuclei have – like atoms – a shell structure, and nuclei with closed shells of protons and neutrons have special properties: they are inert against internal excitations, however, their beta decay may show particularly large strength. 100Sn is such a doubly magic nucleus, with 50 protons and 50  neutrons. It has a large excess of protons – stable nuclei with 100 nucleons have only 42 or 44 protons. Due to the strong Coulomb repulsion it is only marginally bound against one-proton or two-proton emission and the production  cross section is very small. We succeeded in producing and identifying 259 such nuclei at the accelerators of GSI in Darmstadt. The nuclei were stopped in a detector system and beta-decays at the same position as well as gamma rays were detected. Thus we could determine the halflife, the beta-decay
energy and the energies of the succeeding gamma transitions. It turns out that 100Sn has the largest beta-decay strength of all nuclei (after dividing out the trivial energy dependence). This proves the pure shell model character
of this nucleus.