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One of the outstanding unsolved riddles of nuclear astrophysics is the origin of the so called "p-process" nuclei from A = 92 to 126. Both the lighter and heavier p-process nuclei are adequately produced in the neon and oxygen shells of ordinary Type II supernovae, but the origin of these intermediate isotopes, especially 92,94 Mo and 96,98 Ru, has long been mysterious. Here we explore the production of these nuclei in the neutrino-driven wind from a young neutron star. We consider such early times that the wind still contains a proton excess because the rates for ν e and positron captures on neutrons are faster than those for the inverse captures on protons. Following a suggestion by Fröhlich et al. (2005), we also include the possibility that, in addition to the protons, α-particles, and heavy seed, a small flux of neutrons is maintained by the reaction p(ν e , e + )n. This flux of neutrons is critical in bridging the long waiting points along the path of the rp-process by (n,p) and (n,γ) reactions. Using the unmodified ejecta histories from a recent two-dimensional supernova model by Janka, Buras, & Rampp (2003), we find synthesis of p-rich nuclei up to 102 Pd. However, if the entropy of these ejecta is increased by a factor of two, the synthesis extends to 120 Te. Still larger increases in entropy, that might reflect the role of magnetic fields or vibrational energy input neglected in the hydrodynamical model, result in the production of numerous r-, s-, and p-process nuclei up to A ≈ 170, even in winds that are proton-rich.

Indirect methods play an important role in the determination of nuclear reaction cross sections that are hard to measure directly. In this paper we investigate the feasibility of using the so-called surrogate method to extract neutron-capture cross sections for low energy compound-nuclear reactions in spherical and near-spherical nuclei. We present the surrogate method and develop a statistical nuclear-reaction simulation to explore different approaches to utilize surrogate reaction data. We assess the success of each approach by comparing the extracted cross sections with a predetermined benchmark. In particular, we employ regional systematics of nuclear properties in the 34 ≤ Z ≤ 46 region to calculate (n, γ) cross sections for a series of Zr isotopes, and to simulate a surrogate experiment and the extraction of the desired cross section. We identify one particular approach that may provide very useful estimates of the cross section, and we discuss some of the limitations of the method. General recommendations for future (surrogate) experiments are also given.PACS numbers: 24.10. 24.60.Dr, 25.40.Lw, 98.80.Ft

We compare the nuclear energy generation rate and abundance levels given by an a-chain nuclear reaction network that contains only seven isotopes with a standard 13 isotope a-chain reaction network. The energy generation rate of these two small networks are also compared to the energy generation rate given by a 489 isotope reaction network with weak reactions turned on and o †. The comparison between the seven isotope and a-chain reaction networks indicate the extent to which one can be replaced by the other, and the comparison with the 489 isotope reaction network roughly indicates under what physical conditions it is safe to use the seven isotope and a-chain reaction networks. The seven isotope reaction network reproduces the nuclear energy generation rate of the standard a-chain reaction network to within 30%, but often much better, during hydrostatic and explosive helium, carbon, and oxygen burning. It will also provide energy generation rates within 30% of an a-chain reaction network for silicon burning at densities less than 107 g cm~3. Provided there remains an equal number of protons and neutrons over the course of the evolution, and that Ñows through a-particle (Y e \ 0.5) channels dominate, then both of the small reaction networks return energy generation rates that are compatible with the energy generation rate returned by the 489 reaction network. If is signiÐcantly Y e di †erent from 0.5, or if there are substantial Ñows through neutron and protons channels, then it is not generally safe to employ any a-chain based reaction network. The relative accuracy of the seven isotope reaction network, combined with its reduction in the computational cost, suggest that it is a suitable replacement for a-chain reaction networks for parameter space surveys of a wide class of multidimensional stellar models.