Soft structures in the average -decay strength with possible implications for r-process (n,) reaction rates

26 June 2014

11:00 AM

1200 FRIB Laboratory

Ann-Cecilie Larsen

University of Oslo, Norway

An enhanced probability for low-energy -emission at high excitation energies (the upbend) has been observed for several light and medium-mass nuclei close to the valley of stability [1-3]. Very recently, this unexpected enhancement has been proven to be of dipole nature for 56Fe [4]. Also, two recent theoretical works have proposed it to be of E1 [5] or M1 [6] nature; at present neither of these explanations can be excluded. The impact of this enhancement, if present also in very neutron-rich, exotic nuclei, could greatly increase the (n,) reaction rates relevant for r-process conditions away from (n,)-(,n) equilibrium [7].
Moreover, the so-called M1 scissors mode represents another mechanism for enhancing -decay probabilities for low-energy -rays, and has been rather well studied for stable nuclei [8,9]. However, it has so far not been studied in neutron-rich nuclei, although mass models (e.g. Ref. [10]) predict rather large ground-state deformations, for example for neutron-rich Kr, Sr, and Ru isotopes. Thus, it is possible that both the upbend and the M1 scissors mode are present in neutron-rich, deformed nuclei, consequently increasing the (n,) reaction rates and potentially changing the achieved element abundances in large-network calculations.
In this talk, the present status of the upbend will be presented. Experiments at radioactive-beam facilities that allow for the study of the upbend and possibly the M1 scissors mode will be briefly discussed. Preliminary reaction-rate calculations for (n,) r-process rates, including the M1 scissors mode, will also be shown.
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