Measurement of $\beta$-delayed neutrons around the third r-process peak

Abstract

This proposal aims at the measurement of both β-decay half-lives and β-delayed neutron emission probabilities of a number of nuclei near the third r-process peak. Assuming a U beam intensity of 2×10 ions per second we have estimated that the isotopes Tl, 213,214Hg, Au, 208,209Pt and 205,206Ir can be implanted in sufficient intensity for such studies at the final focal plane of the GSI fragment separator. This will allow, for the first time, the measurement of their β-decay halflives and neutron emission branching ratios. The high primary beam energy of 1 GeV/u available at GSI will be crucial for such measurements, in order to avoid contaminations due to incompletely stripped charge states. The β-delayed neutron emission probability of these isotopes is expected to be at least 5%, and their implantation rates between ∼ 10 s and 10 s, which should enable their measurement using a high-efficiency neutron detector. The detection system will also include an state-of-the-art array of DSSSDs for the detection of both implanted ions and β-decays. HPGe-detectors will be used for γ-ray tagging and will help for a precise A/q-calibration by measuring isomers in the neighbouring nuclei. 1 Motivation and introduction The rapid neutron capture process (r process) is characterised by extremely high temperatures of T∼10 K and very large neutron densities of 10 to 10 cm. Although the astrophysical site for this process has not been identified yet, it seems clear that it must be related to explosive scenarios such as type II Supernovae [1], where such cataclysmic conditions are presumably encountered. Many of the uncertainties in our understanding of the r process arise from the vast number of neutron-rich nuclei involved (see Fig. 1), where experimental information is rather scarce, uncertain and in most cases non-existent. Provided that the relevant nuclear physics input parameters could be measured with sufficient accuracy, the observed r-process abundance distribution would reflect the history of the r-process nucleosynthesis, its dynamics and the cosmic site or sites where it takes place. Although the nuclear physics input is rather poorly known it is clear that, the r process shows a prominent influence in the evolution and composition of our Universe, thus it accounts for roughly half of the isotopic abundances observed in the solar system (beyond Iron) and it seems to be the unique mechanism responsible for the existence of the actinide nuclei. Furthermore, nucleochronometry based on the long lived isotopes Th and U has recently attracted great interest since UV spectroscopy observations made with the Hubble Space Telescope [2], SUBARU [3], KeckHIRES [4, 5] and the detailed survey from the Hamburg/ESO HERES project [6] revealed the signature of neutron rich heavy elements in ultra metal poor stars, where one can assume that only one single (or few) nucleosynthesis event has contributed to its composition. The age of these ancient stars represents not only a lower limit for the age of the Milky Way Galaxy and of the Universe, but also provides an important constraint on the time of onset of stellar nucleosynthesis, with further implications for galaxy formation and evolution. Radioactive decay ages can be determined by comparing the observed abundances of the Thorium and Uranium elements (relative to a stable r-process element), to the production ratio expected from theoretical r-process yields. Using