Session 3WPB: The Origin of the Heavy Elements II

The diversity of stellar nucleosynthesis sites in the s- to i-process and n-process range

Based on modern stellar simulations and observations, at least four different neutron-capture processes are needed to explain the production of heavy elements beyond iron in stars. The slow neutron-capture process (s-process) and the rapid neutron-capture process (r-process) were introduced decades ago in order to explain the solar system abundances, even before detailed stellar computational simulations were available. Recently, the existence of the intermediate neutron-capture process (i-process) was confirmed, with typical neutron densities in the order of 10¹³-10¹⁶ neutrons cm^-3 in-between the s-process and the r-process. The i-process explains a number of puzzling observations in stars with different metallicities and in presolar grains, all of them not compatible with neither the s-process and the r-process signatures. Finally, the n-process is a neutron-capture process activated in Core-Collapse Supernovae, with peak neutron density generated typically larger than 10¹⁸ neutrons cm^-3. Its signature has been identified in presolar grains. The ¹³C(α,n)¹⁶O reaction is a crucial source of neutrons to power the s-process and the i-process production, and the ²²Ne(α,n)²⁵Mg is a neutron source for the s-process and the n-process. In this contribution I will give an introduction to the diversity of the nucleosynthesis sites where the s-process, the i-process and the n-process are activated. I will also discuss their observational signatures and current needs from stellar theory and nuclear astrophysics to improve our present understanding of these processes.   

Better understanding of nu-p process with experimentally determined 56Ni(n,p) reaction rate via direct measurements at LANSCE

For reaching out radioactive isotopes relevant for the nu-p process, a new capability of producing radioactive isotopes at Isotope Production Facility and performing direct measurements for (n,p) and (n,a) reactions has been commissioned at Los Alamos Neutron Science Center. We studied the ⁵⁶Ni(n,p) reaction, which has been identified as one of critical reactions for understanding  heavy element productions in core-collapse supernovae. Using the hotLENZ (radioactive Low Energy NZ) instrument, the first directly measured cross sections of ^56,59Ni(n,p), ⁵⁶Co(n,p), and ⁵⁹Ni(n,a) reactions were obtained for deducing reaction rates. Updated ⁵⁶Ni(n,p) and ⁵⁶Co(n,p) reaction rates are compared with the current rates from REACLIB (Cyburt 2010), and used for hydrodynamics calculations with the parameters from Wanajo et al. (2011) and  Nishimura et al. (2019) along with our parameters. The summary of this effort will be discussed in terms of potential further constrains on the nu-p process. The optimized solenoidal spectrometer development for radioisotope reaction studies at LANSCE will be presented.   

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.