Speaker
Description
Neutron-capture cross sections are a key nuclear-physics input for modelling the slow neutron-capture process, which is responsible for the production of about half of the elements heavier than iron in red-giant and massive stars [1]. In particular, unstable branching-point nuclei provide a sensitive probe of the physical conditions in stellar interiors, because neutron capture competes with $\beta$ decay and the resulting isotopic abundance pattern depends on the local temperature and neutron density. The long-lived isotope $^{79}$Se, with a terrestrial half-life of 3.27(8) $\times$ 10$^{5}$ y [2], is one of the most relevant and debated s-process branching points [3]. Located in the transition region between the weak s-process in massive stars and the main s-process in asymptotic giant branch stars, the $^{79}$Se branching controls the flow toward the s-only isotope $^{80}$Kr through $\beta$ decay or toward $^{82}$Kr through neutron capture [4,5]. Owing to the strong thermal dependence of the $^{79}$Se $\beta$-decay rate, an accurate $^{79}$Se(n,$\gamma$) cross section can provide stringent constraints on the stellar temperature by comparison of model predictions with Kr isotopic ratios measured in presolar SiC grains [5,6]. In addition, $^{79}$Se is relevant for nuclear-transmutation studies because of its contribution to the long-term radiotoxicity of spent nuclear fuel [7].
Despite this relevance, no experimental data existed so far for $^{79}$Se(n,$\gamma$), and the available MACS values relied entirely on theoretical estimates [8]. Direct activation is not feasible because the reaction product $^{80}$Se is stable, making time-of-flight the only direct experimental approach. In this context, the first-ever measurement of this cross section was proposed at CERN n_TOF [9]. Among the experimental challenges, only about 2.7 mg of $^{79}$Se were available in a PbSe alloy sample produced from neutron irradiation of enriched $^{78}$Se at ILL [9]; the $^{79}$Se/$^{78}$Se ratio is very small, with $^{78}$Se representing about 99.7% of the selenium content; the sample activity produces a large background, with an important contribution from $^{60}$Co; and the large amount of $^{208}$Pb in the sample leads to a strong neutron-scattering background relative to the capture signal. These limitations have only recently become tractable thanks to the combination of the high instantaneous neutron flux of the upgraded CERN n_TOF-EAR2 beam line and advanced detection systems developed for high-sensitivity capture measurements [10-13].
In this contribution, we present the results of the successful n_TOF measurement, which took place in both time-of-flight beam lines EAR2 (19 m) and EAR1 (185 m). The EAR2 measurement using the new segmented sTED array [13] in a compact ring configuration [14] provided the sensitivity required to overcome the activity background and made it possible to achieve low statistical uncertainties [10]. Moreover, the high-resolution EAR1 measurement with the Compton-imaging i-TED [11,12] array enabled improved control of neutron-induced background and systematic effects related to dead time and energy resolution [10]. The analysis of the capture yield in the resolved-resonance region has led to the observation and analysis of more than ten resonances of the $^{79}$Se(n,$\gamma$) cross section for the first time. The resulting resonance parameters have been used to derive a semi-empirical cross section, thereby providing the first experimental constraints on the Maxwellian-averaged cross section over stellar temperatures relevant to AGB and massive-star nucleosynthesis. The results from this analysis will be presented for the first time in this contribution.
References
[1] F. Käppeler et al., Reviews of Modern Physics 83, 157 (2011).
[2] G. Jörg et al., Applied Radiation and Isotopes 68, 2339--2351 (2010).
[3] F. Käppeler et al., Reports on Progress in Physics 52, 945 (1989).
[4] G. Walter, H. Beer, F. Käppeler et al., Astronomy \& Astrophysics 167, 186 (1986).
[5] R. S. Lewis et al., Nature 348, 293 (1990).
[6] N. Klay and F. Käppeler, Physical Review C 38, 295--306 (1988).
[7] W. S. Yang, Y. Kim, R. N. Hill, T. A. Taiwo, and H. S. Khalil, Nuclear Science and Engineering 146, 291--318 (2004).
[8] Z. Y. Bao, H. Beer, F. Käppeler et al., Atomic Data and Nuclear Data Tables 76, 70 (2000).
[9] J. Lerendegui-Marco et al., CERN-INTC-2020-065 / INTC-P-580 (2020).
[10] J. Lerendegui-Marco et al., EPJ Web of Conferences 279, 13001 (2023).
[11] C. Domingo-Pardo, Nuclear Instruments and Methods in Physics Research A 825, 78--86 (2016).
[12] V. Babiano, J. Lerendegui-Marco et al., European Physical Journal A 57, 197 (2021).
[13] V. Alcayne et al., Radiation Physics and Chemistry 217, 111525 (2024).
[14] J. Balibrea-Correa et al., Nuclear Instruments and Methods in Physics Research A 1072, 170110 (2025).