Direct Experiments of Neutron Capture on Stable and Unstable Isotopes for Stellar Nucleosynthesis Studies

Abstract

Neutron-capture reactions provide essential nuclear-physics input for modeling the synthesis of heavy elements in stars. The growing precision of stellar spectroscopy and isotopic measurements in presolar SiC grains now demands cross sections with improved accuracy over the full energy range, and access to unstable nuclei relevant to slow (s-) process branchings and the intermediate (i-) process. This article reviews recent progress in direct neutron-capture measurements, focusing on time-of-flight (TOF) experiments at CERN n_TOF and complementary activation techniques. Substantial advances have been achieved for stable s-only and bottleneck isotopes, significantly improving constraints on s-process models. In parallel, the combination of high instantaneous neutron fluxes and advanced detector systems has facilitated first-time measurements of several radioactive branching-point nuclei. Feasibility studies, however, reveal current limitations related to sample availability, background conditions, and restricted energy coverage. In this context, the complementarity between TOF and activation emerges as a central strategy. Future developments, including high-flux facilities and novel inverse-kinematics experiments in ion storage rings, are expected to extend the boundaries of neutron-capture measurements, overcoming current limitation

Figures (12)

• Figure S1: Top: Maxwell-Boltzmann spectrum of neutrons at the reference stellar temperature $kT$ = 30 keV (blue curve), example of an (n,$\gamma$) cross section as available in JEFF-3.3 (black curve) and the convolution of the two (red curve), illustrating the contribution to the MACS. The units in the vertical axis are relevant only for the cross section. Bottom: Energy range covered by the two mostly employed beams and techniques for stellar (n,$\gamma$) measurements (see text for details).
• Figure S2: MACS$_30$ in KADoNis v0.3 KADONIS (top) and their present accuracy (bottom). Nuclei acting as s-process bottlenecks and branching points are displayed in blue and red, respectively. Figure taken from Ref. Domingo:22.
• Figure S3: Left: $\delta$-values (per mil deviations from solar system ratios) of $^{146}$Nd/ $^{144}$Nd relative to $^{150}$Nd/ $^{144}$Nd from AGB star calculations (solid lines) compared with SiC grain data (black dots) from Richter Richter:92 and Liu LiuPhD), showing the impact of varying the $^{146}$Nd(n,$\gamma$) rate of ASTRAL ASTRAL in $\pm$20% in the full energy range and only below $kT$=20 keV. Right: Example of the counting rate measured at n_TOF EAR2 in a small range of neutron energies compared to the expected results based on JEFF-3.3.
• Figure S4: Left: Total counts and background components as a function of the neutron energy measured with the PbSe ($^{78}$Se+$^{79}$Se) sample in n_TOF EAR2. The first resonances of $^{79}$Se are highlighted. Right: Background-subtracted $^{79}$Se(n,$\gamma$) yield showing examples of observed resonances compared to the expected results based on JEFF-3.3.
• Figure S5: Expected capture yield with 1250 bins per decade (bpd) after the background subtraction compared to a the expected yield based on the simulated resonance parameters for a $^{155}$Eu sample of 5$\times10^{17}$ atoms (left) and a $^{179}$Ta sample of 10$^{18}$ atoms.