Experimental study on neutron-capture rate using the decelerating and focusing device OEDO in RIBF

In the r-process nucleosynthesis, the neutron capture rates on the nuclei around ¹³²Sn largely contribute to the second peak of the final abundance of the elements. The dominant neutron capture process is the compound reaction, which involves the huge number of quantum levels above the neutron threshold so that the cross section is normally described in a stastitical way. Among several parameters to describe the compound reaction cross sections, the gamma-emision probablity from the unboud state is still challenging to predict for the nuclear theory.

To evaluate experimentally the gamma emission probabiility, the decelerator and focusing device named OEDO was installed at RIBF. OEDO can provide RIB of around 20 MeV/nucleon. By employing the idea of the surrogate reaction, the (d,p) reaction was used to populate the unboud states. Because the reaction takes place in the inverse kinematics, the reaction residues move forward and can be analyzed by the SHARAQ spectrometer which is placed downstream of OEDO. The gamma emission channel stands for no particle emission from the residual nucleus. Namely we can determine the gamma emission probability through identifying the residual nuclei.

The first experiment was coducted for ⁷⁹Se. Although this nucleus is not directly related to the r-process, the cross sections are demanded for the s-process as well as the transmutation of the nuclear waste. The neutron capture reaction cross sections are reasoably in agreement with the theoretical prediction. As for the second experiments, we recently applied this experimental technique to ⁵⁶Ni and ¹³⁰Sn. In this talk, we mainly discuss the details of the experimental setup and result on 79Se. Also, we will show some preliminary results on the ¹³⁰Sn(d,p) reaction in the inverse kinematics.