Measurement of very neutron-rich r-process nuclei with the BELEN setup

Full text: Very neutron-rich nuclei can emit neutrons after β--decay when their reaction Q-value is larger than the (one/two/three) neutron separation energy. This decay mode is called -delayed (one/two/three)-neutron emission and was discovered in 1939 by Roberts et al. [1], shortly after the discoveries of fission by Meitner, Hahn, and Strassmann in 1938, and the neutron by Chadwick in 1932. Delayed in this context means, that the neutron is emitted with the -decay half-life of the precursor AZ, ranging from few milliseconds for the most neutron-rich isotopes up to 55.65 s for the (up to now) longest-lived βn-precursor 87Br. These delayed neutrons have to be distinguished from the prompt neutrons evaporated immediately (in the order of 10-14 s) after a fission event from a neutron-rich nucleus. βn measurements are commonly carried out since many decades, especially for fission fragments. With present and future nuclear physics facilities more and more neutron-rich isotopes outside the fission region become accessible which can also emit multiple neutrons. This exotic decay mode is of special interest for the astrophysical rapid neutron capture process (r process) since it leads to a deviation of the reaction flow to lower mass chains during the freeze-out phase. The exact knowledge of the beta-delayed neutron emission probability (Pn) is required to interpret the observed solar abundance peaks and draw conclusions from this about the participating r-process progenitor isotopes and their contributions. The BELEN (BEta-deLayEd Neutron) detector with its state-of-the-art digital electronics is one of the most powerful setups for the measurement of β-delayed neutrons in the world. Its present version consists of 48 3He-filled long counters and achieves - depending on the design - a neutron detection efficiency of up to 60%. This high efficiency is necessary for the detection of very neutron-rich n-emitters as well as for the detection of multiple neutronemitters via nn coincidences. Its flexible design allows measurements at both, ISOL labs and in-flight fragmentation facilities. It was successfully used for measurements of fission fragments at JYFL in Jyvaeskylae, Finland and for r-process isotopes at N=82 and N>126 at the FRS at GSI Darmstadt. Presently BELEN is upgraded towards its 96 counters-version for the FAIRDESPEC campaign in a few year. Until then, measurements with an intermediate version are being performed in various labs.