Neutron capture measurements for s-process nucleosynthesis: A review about CERN n_TOF developments and contributions

Abstract

This article presents a review about the main CERN n_TOF contributions to the field of neutron-capture experiments of interest for s-process nucleosynthesis studies over the last 25 years, with a special focus on the measurement of radioactive isotopes. A few recent capture experiments on stable isotopes of astrophysical interest are also discussed. Results on s-process branching nuclei are appropriate to illustrate how advances in detection systems and upgrades in the facility have enabled increasingly challenging experiments and, as a consequence, have led to a better understanding and modeling of the s-process mechanism of nucleosynthesis. New endeavors combining radioactive-ion beams from ISOLDE for the production of radioisotopically pure samples for activation experiments at the new NEAR facility at n_TOF are briefly discussed. On the basis of these new exciting results, also current limitations of state-of-the-art TOF and activation techniques will be depicted, thereby showing the pressing need for further upgrades and enhancements on both facilities and detection systems. A brief account of the potential technique based on inverse kinematics for direct neutron-capture measurements is also presented.

Fig. 1

From left to right, the three generations of spallation sources at n_TOF. The latest version [24] delivers a superior neutron-beam quality for capture experiments at both measuring stations EAR1 (185 m) and EAR2 (20 m) [25]

Fig. 2

Over the past two decades, advancements in facility upgrades (EAR1, EAR2, Target#1-3) and detection systems (C$_6^{}$D$_6^{}$, sTED, i-TED) at CERN n_TOF have enabled increasingly challenging experiments on s-process branching nuclei (horizontal axis). The figure of merit (FOM, vertical axis), defined in Eq. (1), provides a visual representation of this progress. For details on various s-process branching cross-section measurements, refer to the main text. Additional information on Target#1–3 can be found in Fig. 1, while details on detection systems are in Figs. 3 and 4. Abbreviations: temperature (T), Solar System (SS), Thermally-Pulsing Asymptotic Giant-Branch star (TP-AGB). Credits for figure insets: TP-AGB drawing adapted from [46]; Hubble-Space Telescope image of a TP-AGB (U. Camelopardalis) with a SiC grain (NASA/Nan Liu/Andrew Davis), massive-star illustration (NASA/CXC/M. Weiss)

Fig. 3

Evolution of the capture set-up at EAR1, from left to right, two first-generation C-fiber based C$_6^{}$D$_6^{}$ detectors for the measurement of $_{}^{151}$Sm($n,\gamma$) [26], four Bicron C$_6^{}$D$_6^{}$ detectors with lead shields for the $_{}^{204}$Tl($n,\gamma$) experiment [63] and four latest-generation C-fiber C$_6^{}$D$_6^{}$ detectors [39] plus the i-TED Compton array [40] for the $_{}^{79}$Se($n,\gamma$) cross-section measurement [43]

Fig. 4

Left: Conventional (large-volume) C$_6^{}$D$_6^{}$ detectors for capture measurements in EAR2. Notice that due to their large volume and the high neutron-flux the detectors had to be placed far from the sample in order to limit the count-rate per detector. Right: Upgraded setup with nine small volume C$_6^{}$D$_6^{}$ sTED detectors in a compact configuration surrounding the capture sample

Fig. 5

Left: Part of the nuclear chart showing the s-process path at the branching in $_{}^{79}$Se. The $\beta$-decay towards s-only $_{}^{80}$Kr is strongly enhanced at higher stellar temperatures (red arrow) due to the population of the $1/2^{-}$-isomeric state at low energy. Right: Effective half-life of $_{}^{79}$Se as a function of stellar temperature. The inset indicates levels involved

Fig. 6

Comparison of the capture yields measured at n_TOF EAR2 [115] for the first capture-resonance in $_{}^{209}$Bi+n with a thick and a thin sample. The double-peak structure arises from enhanced multiple neutron-scattering effects inside the thick sample

Fig. 7

R-matrix fit and its expected shape based on the ENDF/B-VIII.0 parameters of the first resonance in $_{}^{30}$Si+n measured in EAR1

Fig. 8

Drawings showing the location and different elements of the new NEAR station (left) and some quasi-Maxwellian neutron distributions obtained via MC-Simulation for different B$_4^{}$C- filter thicknesses, leading to mean kT values of about 1 keV and 35 keV