Speaker
Description
Among the various processes of cosmic nucleosynthesis, the rapid neutron-capture process (r-process) plays a decisive role in the production of elements heavier than iron. A characteristic feature of the r-process abundance distribution is the so-called “rare-earth peak” around mass number A~160. The origin of this peak has been theoretically attributed to possible changes in nuclear shell structure and the presence of deformed shell gaps near N~100 [1-4]. Furthermore, while β-delayed neutron emission in the neutron-rich rare earth region is thought to play a key role in the formation of the A≈160 peak, experimental data in this mass region are scarce, and the detailed nature of the shell structure remains unclear.
We therefore have systematically investigated decay modes and structures in the neutron-rich rare-earth region using β-γ and isomer spectroscopy of A=150~160 Nd, Ce, Ba nuclei. The experiment was performed using RIBF facility in RIKEN Nishina Center. These neutron-rich nuclei were produced using in-flight fission of a 345 MeV/u $^{238}$U beam. Fission fragments were separated and transported by the BigRIPS and ZeroDegree spectrometer. Nuclei of interest were identified by measuring the time-of-flight and magnetic rigidity in the second stage of BigRIPS and by measuring the energy loss using the ion chamber at the final focal plane, F11. The secondary beam was implanted into an active stopper, WAS3Abi to perform β-γ spectroscopy using EURICA Ge cluster detector array. We obtained the results indicating β-delayed neutron emission in $^{158}$Nd for the first time.
In the presentation, the latest result of beta-gamma spectroscopy of $^{157,158}$Nd will be reported and its relevance to the rare-earth peak will be discussed.
This work is partially supported by JSPS KAKENHI Grant Number 26H01418.
[1] M.R. Mumpower, G.C. McLaughlin, R. Surman: Phys. Rev. C85, L021303 (2012).
[2] R. Surman, J. Engel, J. Bennett, and B. Meyer: Phys. Rev. Lett. 79, 1809 (1997).
[3] S. Shibagaki, T. Kajino, G. J. Mathews, S. Chiba, S. Nishimura, and G. Lorusso: Astrophys. J. 816, 1 (2015).
[4] S. Goriely, et al.: Phys. Rev. Lett. 111, 242502 (2013).