¹⁷⁶Lu/¹⁷⁶Hf: A Sensitive Test ofs‐Process Temperature and Neutron Density in AGB Stars

Abstract

The s-process branching at A ¼ 176 has been analyzed on the basis of significantly improved experimental cross sections. This work reports on activation measurements of the partial (n; ) cross section of 176 Lu feeding the isomeric state in 176 Lu. In total, six irradiations were performed at the Karlsruhe 3.7 MV pulsed Van de Graaff accelerator, and the induced activities were measured with HPGe clover detectors. In combination with previous data, partial cross sections of 3185 AE 156 and 1153 AE 30 mbarn were deduced at kT ¼ 5:1 and 25 keV, respectively. With these results and a recent time-of-flight measurement of the total stellar (n; ) cross section, the isomeric ratio was found to be constant in the relevant thermal energy range of the main s-process component. Based on these new data, a comprehensive analysis of the branching at 176 Lu was carried out for testing the temperature and neutron density conditions during He shell flashes in thermally pulsing low-mass asymptotic giant branch stars. It was found that the long-standing problem of the mother/daughter ratio of the two s-only isotopes 176 Lu and 176 Hf could be solved, if the temperaturedependent -decay half-life of 176 Lu was considered with sufficient resolution over the temperature profile of the convective He shell flashes.

Radiative neutron capture cross sections onLu176at DANCE

“…in Ref. [50]. Nevertheless, at 30 keV stellar temperature, the total MACS value found for the 176 Lu(n,γ) reaction in this work is 1647±5 mb which is still in agreement with the KADONIS [51,52] adopted value from [5], 1639±14 mb, and relatively higher than Bao's value, 1532±69 mb [13].…”

Nuclear Excitation Processes in Astrophysical Plasmas

“…The 175 Lu(n,) 176 Lu reaction produces most of 176 Lu in the low-K isomeric state which decays by -emission to 176 Hf. 176 Lu can only survive if the isomer is coupled to the high-K ground state (Heil et al, 2008). The most relevant IMS for this coupling is located at 839 keV with J π ;K=5 -;4, although other IMS have been suggested very recently (Gintautas et al, 2009;Dracoulis et al, 2010).…”

Databases and tools for nuclear astrophysics applications

“…4) for 1 ≤ A ≤ 16 targets; -the NACRE data (Angulo et al 1999), when not superseded by NACRE II, or by other more recent compilations than NACRE listed below; -the compilation of the rates of Big-Bang reactions based on the R-matrix model (Descouvemont et al 2004); -the 2010 evaluation of Monte Carlo-based rates of resonant proton capture reactions on targets in the 20 ≤ A ≤ 40 range (Iliadis et al 2010); -the proton-induced reaction rates on stable and unstable target nuclei in the 20 < A < 40 region (Iliadis et al 2001); -when not available in the above-mentioned compilations, the reaction rates proposed by Caughlan & Fowler (1988); -the compilation of Bao et al (2000) for radiative neutron capture rates; -various radiative neutron capture rates made available between 2000 and 2011 that supersede the compilation of Bao et al (2000) (Abbondanno et al 2004;Aerts et al 2006;Best et al 2001;Blackmon et al 2002;Borella et al 2007;Dillmann et al 2006bDillmann et al , 2009Dillmann et al , 2010Domingo-Pardo et al 2006a,b, 2007aEsch et al 2008;Guber et al 2002Guber et al , 2010Heil et al 2005Heil et al , 2008aKatoh et al 2003;Koehler et al 1998Koehler et al , 2000Koehler et al , 2001Lederer et al 2011;Marrone et al 2006;Marganiec et al 2009aMarganiec et al ,b, 2010Meierhofer et al 2010;Mohr et al 1999;Mosconi et al 2010;Nakamura et al 2003;Noguere et al 2006;…”