Measurement of Mass and Beta-Lifetime of Stored Exotic Nuclei

Abstract

In this lecture, the basic techniques and concepts of ion storage-cooler rings are first presented, such as storing, beam-focusing and beam-cooling. In particular the main facets of electron cooling will be discussed, the cooling method being most successfully exploited in all operational ion storage-cooler rings. In the second part it will be demonstrated why and how an ion cooler-ring connected with a device producing exotic nuclei -as the coupled experimental storage ring (ESR) and fragment separator (FRS) at GSI in Darmstadt- is a unique tool to provide efficiently, precisely and with unrivalled sensitivity the ground-state properties of exotic nuclei, i.e. mass and (beta) lifetime. They are the basic and necessary ingredients for redrawing the pathways of stellar nucleosynthesis in the s-, rp- and r- processes, and also for exploring the limits of nuclear stability at both the proton and the neutron drip line, which directly reflects the deep entanglement of nuclear astrophysics on the one hand and of nuclear structure on the other. The two complementary methods of mass measurements, ‘Schottky mass spectrometry’ for longer-lived and ‘isochronous mass spectrometry’ for short-lived exotic nuclei, are visualized by plenty of data. Both methods were first developed and successfully applied at the ESR. In the last part of the lecture the unique worldwide potential of the ESR is demonstrated, namely the measurement of beta decays of highly-charged exotic ions, including the first observation of bound-state beta decay. This exotic mode of beta decay, being marginal for neutral atoms, becomes important in hot stellar plasmas during nucleosynthesis. As a striking example the impact of bound-state beta decay for the nuclear ‘eon clock’ ¹⁸⁷Re/¹⁸⁷Os and, connected therewith, for the determination of the age of our milky way galaxy and of the universe will be outlined.