Prospects for direct neutron capture measurements on s-process branching point isotopes

Abstract

Abstract. The neutron capture cross sections of several unstable key isotopes acting as branching points in the s-process are crucial for stellar nucleosynthesis studies, but they are very challenging to measure directly due to the difficult production of sufficient sample material, the high activity of the resulting samples, and the actual (n, γ) measurement, where high neutron fluxes and effective background rejection capabilities are required. At present there are about 21 relevant s-process branching point isotopes whose cross section could not be measured yet over the neutron energy range of interest for astrophysics. However, the situation is changing with some very recent developments and upcoming technologies. This work introduces three techniques that will change the current paradigm in the field: the use of γ-ray imaging techniques in (n, γ) experiments, the production of moderated neutron beams using high-power lasers, and double capture experiments in Maxwellian neutron beams.Nucleosynthesis of the elements heavier than iron is driven mainly by neutron capture reactions in stars. Half of the total amount of elements between Cu and Bi in the solar system was produced in the slow neutron capture process (s-process) in He-burning layers of low-mass asymptotic giant branch (AGB) stars and during the Heand C-burning phases of massive stars. The corresponding nucleosynthesis studies involve detailed stellar modelling constrained by spectroscopic observations and meteoritic stardust grains in which reliable information on the nuclear physics part, in particular on the basic nuclear data such as half-lives and neutron capture (n, γ) cross sections, constitutes an essential ingredient [1,2].The most crucial and difficult data to obtain in this respect are the stellar neutron capture (n, γ) cross sections of unstable isotopes, which are still very scarce or require substantial improvement. The importance of unstable isotopes is that they give rise to branchings in the s-process path due to the competition between β-decay and neutron capture, which depends on the neutron density, the Maxwellian Averaged Cross Section (MACS) and the mean thermal neutron velocity, i.e. the neutron energy a e-mail: carlos.guerrero@cern.ch distribution at a given temperature. In addition, both halflife and (n, γ) cross section values depend in some cases on the temperature of the stellar site because of the population of some levels above the ground state. This dependence has a sizable effect on s-process abundance calculations for some isotopes [3] and cannot be measured, so measurements have to be combined with models [4,5].Since the isotopic abundance pattern within a branching is sensitive to the neutron density and temperature, knowledge of the neutron capture cross section of the involved unstable isotope provides very valuable information on these parameters at the corresponding stellar site. One such example is the measurement of the 93 Zr(n, γ) cross section [6] which, in combination with spectroscopic observations and met...

Origin of the Heaviest Elements: the Rapid Neutron-Capture Process

“…While the direct measurement of neutron capture reactions on stable and even long-lived radioactive for the s process has been very successful (Guerrero et al, 2017), a similar approach to study neutron capture on shortlived neutron rich isotopes provides considerable challenges. Most of the r-process neutron capture rates rely on theoretical predictions based on the Hauser-Feshbach statistical model formalism (Rauscher et al, 1997;Goriely, 1998).…”

Constraining Neutron Capture Cross Sections for Unstable Nuclei with Surrogate Reaction Data and Theory

“…These can be used in inverse-kinematics experiments to bombard longer-lived target materials. Neutrons are not suitable (stationary) target material [9]; thus new techniques have to be developed to extract the desired cross sections from radioactive-beam experiments [10][11][12][13][14][15][16].…”

Application of kernel ridge regression in predicting neutron-capture reaction cross-sections

“…163 Ho and 171 Tm. Note that the experimental data of 171 Tm have been updated by recent experiments [74], which are displayed with stars in figure 3 and match the KRR predictions better than the old data. These results suggest that the KRR predictions of the other 13 s-process branch-point nuclei, whose experimental values of the MACSs are unavailable, can be trusted.…”