The 2ZNe(c~,7)26Mg and 22Ne(cq n)2SMg reactions were investigated for E~(lab) from 0.71 to 2.25 MeV. Neon gas enriched to 99% in 22Ne was recirculated in a differentially pumped gas target system of the extended type. The 7-ray transitions were observed with Ge(Li) detectors and the neutrons with 3He ionization chambers. A previously known resonance at ER(lab)= 2.05 MeV was verified and 15 new resonances were found in the energy range covered, with the lowest at ER(lab)=0.83 MeV. Information on resonance energies, widths, strengths, 7-ray branching ratios, as well as J" assignments, is reported. The energy range investigated corresponds to the important temperature range of T9 from 0.3 to 1.4 (109 K), for which the astrophysical rates were determined for both reactions. The results show that the ratios of the rates for 22Ne(c~,n)2SMg and 22Ne(c~, 7)26Mg are significantly smaller than the previously adopted values, e.g., by at least a factor of 60 near T9=0.65. Thus, the 22Ne(e, n)25Mg reaction will likely play a smaller role as a neutron source for s-process nucleosynthesis, than has frequently been assumed.

The Ne(n, n) 'Mg reaction has been investigated in the energy range E"" =0.73-2. 10 MeV using high-resolution 'He ionization chambers. Sixteen resonances have been observed in the three neutron channels to the ground state and the first two excited levels of 'Mg. From the analysis of neutron angular distributions, spin and parity assignments could be made for eight resonances. Furthermore, partial widths as well as absolute cross sections have been obtained yielding resonance strengths for the observed levels. The role of the Ne(a, n) reaction as a neutron source for the s process is brieAy discussed.

No abstract

No abstract

In contrast to the conventionaI picture, the r-process yields in the A~-80 abundance peak ezhibit a pronounced oddeven staggering. It is proposed that this behaviour may be due to strong fl-delayed neutron branching from a few odd-mass isotopes located in or close to the r-process path.