Stellar(n,γ)cross sections ofp-process isotopes Part I:

Abstract

We have investigated the (n, γ) cross sections of p-process isotopes with the activation technique. The measurements were carried out at the Karlsruhe Van de Graaff accelerator using the 7 Li(p, n)7 Be source for simulating a Maxwellian neutron distribution of kT = 25 keV. I. INTRODUCTIONAstrophysical models can explain the origin of most nuclei beyond the iron group by a combination of processes involving neutron captures on long (s-process) or short (r-process) time scales [1,2].However, 32 proton-rich stable isotopes between 74 Se and 196 Hg cannot be formed in these neutron capture processes, because they are either shielded by stable isotopes from the r-process decay chains or lie outside the s-process flow (Fig. 1). These isotopes, which are ascribed to the so-called "p-process", are 10 to 100 times less abundant than their s-and r-process neighbors. So far, the astrophysical site of the p-process is still under discussion, since the solar p-abundances can not be completely described by current models.Historically, the p-process was thought to proceed via proton captures, but a plausible site with the required amount of free protons could not be identified. Moreover, elements with large Z cannot be produced by proton captures because the temperatures necessary to overcome * Electronic address: iris.dillmann@ph. the Coulomb repulsion favor photodisintegration rather than charged-particle capture.The most plausible astrophysical site is the explosively burning Ne/O layer in core collapse supernovae, which is heated to ignition temperatures by the outgoing shock front [3][4][5]. In this high-temperature environment proton-rich nuclei are produced by sequences of photodissociations and β + decays. In stars 20 times more massive than the sun the p-process temperatures for efficient photo-disintegration are already reached at the end of hydrostatic Ne/O burning [6]. This mechanism is also called "γ process" because proton-rich isotopes are produced by (γ, n) reactions on pre-existing seed nuclei from

Activation Experiments forp-Process Nucleosynthesis

“…Approaches for the determination of reliable particle-nucleus potentials as well as a systematic study of (γ,n) reaction rates were shown. An interesting approach is also to use neutron-capture reactions for the determination of (γ,n) cross sections as shown in [37,38]. However, the most interesting reactions concerning the production of the heavy p nuclei involve unstable nuclei.…”

Neutron capture reactions near theN=82shell-closure

“…The reverse process, i.e., (n, γ) reactions may serve the purpose. For example, Dillmann et al [3] studied a number of (n, γ) reactions with neutrons from the 7 Li(p, n) 7 Be reaction to simulate a Maxwellian neutron distribution at 25 keV thermal energy. Neutron capture reactions have been studied in various methods.…”

Experimental Signals for Broken Axial Symmetry in Excited Heavy Nuclei from the Valley of Stability

“…This is depicted in Fig. 4 for the Maxwellian average cross sections compiled recently [44] for E n = 30 keV. An essential feature here is our global ansatz for the spreading width which fixes the important tail of the E1-resonance; it depends on the IVGDR pole energies only and their dependence on exact deformation parameters is nearly unimportant.…”