Beta-delayed neutrons for astrophysics: Understanding the most neutron-rich nuclides

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A large fraction of the heavy nuclei (elements heavier than iron) that are produced in the "rapid neutron capture process" (r process) are not (yet) experimentally accessible and are located in the so-called "Terra Incognita". With the next generation of radioactive ion beam (RIB) facilities presently under construction worldwide, the main motivation of all projects is the investigation of these very neutron-rich nuclei which are in or at the reaction path of the r process. However, until the new facilities will become operational within the next 5-10 years, the presently operating facilities are continuing to push the borders of known nuclei as far out as possible. Whereas the proton dripline has been reached for many elements up to mass A~200, the neutron dripline is only accessible up to A~40 (Na, Mg). Theoretical predictions show that on top of the 3000 known nuclei one can expect ~4000 more nuclei to be discovered, the largest fraction of them on the neutron-rich side.******For these neutron-rich nuclei, a new decay mechanism appears when the neutron separation energy is lower than the reaction Q value: beta-delayed neutron (bdn) emission. Two important physical quantities can be measured with the investigation of bdn emitters: the beta-decay half-life and the beta strength above the neutron separation energy from the neutron-branching ratio. ******The more neutron-rich a nucleus becomes, the more important bn emission gets until it will be the dominant decay mechanism for almost all newly discovered neutron-rich nuclei. So far about 600 bn-emitters are known, but for only 300 of them the neutron-branching ratio has been measured. In 2016/17 we have (re)measured 270 bn-emitters alone in the BRIKEN campaign. For 183 of them (68%) the neutron-branching ratio was measured for the first time, and for 60 of them the beta-decay half-life for the first time. ******With this proposal I will continue and extend my ongoing program about the investigation of even more exotic bdn emitters in the next 5 years. In 2019/20 we are planning to run at least three more experiments at RIKEN, one led by my group. These measurements are aiming at a better understanding of the competition between delayed one- and multi-neutron emission. In addition, we will provide data for the very rare processes of delayed three- and four-neutron emission. ***After the BRIKEN campaign we aim to extend our experimental scope by moving our detection setup to another facility to try to investigate yet inaccessible nuclei like e.g. heavy isotopes at the N=126 shell closure with the multi-nucleon transfer. This promising reaction mechanism will allow for the first time access to those nuclei that are responsible for the third peak of the r-process around mass A~195.******A detailed knowledge of the bdn process is an important key ingredient for a better understanding of even more neutron-rich nuclei that will not be accessible experimentally and have to be inferred from theoretical models.********