Determination of proton- and neutron- induced cross sections for [rho]-process studies : construction of an online database for the [rho] process

Abstract

The nucleosynthesis of elements beyond iron is dominated by neutron captures in the s and r processes. However, 32 stable, proton-rich isotopes between 74Se and 196Hg cannot be formed in that way, because they are shielded from the s process flow and r process β-decay chains. These nuclei are thought to be produced in the so-called ”p process”, where proton-rich nuclei are made by sequences of photodisintegrations on existing r- and s-seed nuclei and following β+ decays. Since the largest part of the p-process reaction network lies in the region of proton-rich unstable nuclei, most of the reaction rates are not yet accessible by experimental techniques and have to be inferred from statistical model calculations, e.g. by using the Hauser-Feshbach codes NON-SMOKER and MOST. The parametrization of these models has to be constrained by measurements on as many nuclei as possible. However, the number of experimental data available for the p process is very scarce. For example, (γ, n) measurements were up to now mainly performed for 13 isotopes beyond 181Ta, whereas the bulk of (p, γ) and (α, γ) reactions was only measured – with exception of 144Sm(α, γ) – for isotopes up to Sn. The database for particle exchange reactions is much more extensive. In contrast to this, the database for the stellar (n, γ) cross sections of the 32 stable p isotopes is also surprisingly scarce. Before the measurements described in this thesis, 12 cross sections were not known experimentally, and further 9 exhibit uncertainties ≥9%. Thus, a series of (n, γ) activation measurements on stable p isotopes were carried out at the Karlsruhe Van de Graaff accelerator using the 7Li(p, n)7Be source for simulating a Maxwellian neutron distribution of kT= 25 keV. These studies included measurements of 7 total and 3 partial neutron capture cross sections of the stable isotopes 74Se, 84Sr, 102Pd, 120Te, 130Ba, 132Ba, and 174Hf (see Chapter 4). Chapter 5 is related to proton-induced reactions of palladium isotopes between 2.7 MeV≤Ep≤5 MeV, the energy range relevant for the p process. These measurements were performed using the cyclotron and Van de Graaff accelerator at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig/ Germany. In these experiments we determined the total (p,γ) cross sections for 102,104Pd, the total (p, n) cross section of 105Pd, as well as the partial cross sections for 105Pd(p, γ), 106Pd(p, n), and 110Pd(p, n). Chapter 6 describes the update of the previous stellar neutron cross section compilation of Bao et al. from 2000 with recent measurements. The updated sequel to this compilation is available online and is part of the ”Karlsruhe Astrophysical Database of Nucleosynthesis in Stars” (KADoNiS) project, which was started in April 2005 under http://nuclear-astrophysics.fzk.de/kadonis. In 2006 this project was extended with a first collection of experimental cross sections relevant for p-process studies. This part of KADoNiS is still under construction, but a first layout is given here. The updated KADoNiS database for stellar neutron capture cross sections was further used in Chapter 7 for an update of the local version of a reaction rate library for astrophysics. Where available, this library already contained experimental rates, but neutron capture rates up to 81Br were still based on the first Bao et al. compilation from 1987. With the updated reaction library p-process network calculations were performed (Chapter 8) with the program ”pProSim” to examine the influence of the new experimental neutron rates. Surprisingly the abundances of almost all p-process isotopes got smaller with the updated reaction library. This effect can be mainly traced back to much lower experimental cross sections of nuclei around the shell closures compared to previous NON-SMOKER calculations. It is well known that statistical model predictions cannot be applied here and tend to overpredict neutron cross sections. Since the s-process seed nuclei used for these simulations have larger abundances around the shell closures the influence of these decrease is global and the reaction flow to almost all p isotopes is affected. By comparing the abundance before and after the simulations we additionally realized that the isotopes 152Gd, 164Er, 113In, and 115Sn are destroyed rather than produced in our simulations. Together with possible different contributions from different astrophysical processes to the abundance of 180Ta, it is possible that in future we might have to speak of only ”30 p isotopes”.