Hendrik Schatz

rp-process nucleosynthesis at extreme temperature and density conditions

We present nuclear reaction network calculations to investigate the influence of nuclear structure on the rp-process between Ge and Sn in various scenarios. Due to the lack of experimental data for neutron-deficient nuclei in this region, we discuss currently available model predictions for nuclear masses and deformations as well as methods of calculating reaction rates (Hauser-Feshbach) and beta-decay rates (QRPA and shell model). In addition, we apply a valence nucleon (NpNn) correlation scheme for the prediction of masses and deformations. We also describe the calculations of 2p-capture reactions, which had not been considered before in this mass region. We find that in X-ray bursts 2p-capture reactions accelerate the reaction flow into the Z greater than or equal to 36 region considerably. Therefore, the rp-process in most X-ray bursts does not end in the Z = 32-36 region as previously assumed and overproduction factors of 10(7)-10(8) are reached for some light p-nuclei in the A = 80-100 region. This might be of interest in respect of the yet unexplained large observed solar system abundances of these nuclei. Nuclei in this region can also be produced via the rp-proces in accretion disks around low mass black holes. Our results indicate that the rp-process energy production in the Z < 32 region cannot be neglected in these scenarios. We discuss in detail the influence of the various nuclear structure input parameters and their current uncertainties on these results. It turns out that rp-process nucleosynthesis is mainly determined by nuclear masses and beta-decay rates of nuclei along the proton drip line. We present a detailed list of nuclei for which mass or beta-decay rate measurements would be crucial to further constrain the models

The Rapid Proton Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star

The temperature and nuclear composition of the crust and ocean of an accreting neutron star depend on the mix of material (the ashes) that is produced at lower densities by fusion of the accreting hydrogen and helium. The hydrogen/helium burning is thermally stable at high accretion rates, a situation encountered in weakly magnetic (B 1011 G) neutron stars accreting at rates > 10-8 M☉ yr-1 and in most accreting X-ray pulsars, where the focusing of matter onto the magnetic poles results in local accretion rates high enough for stable burning. For a neutron star accreting at these high rates, we calculate the steady state burning of hydrogen and helium in the upper atmosphere (ρ < 2 × 106 g cm-3), where T ≈ (5-15) × 108 K. Since the breakout from the hot CNO cycle occurs at a temperature comparable to that of stable helium burning (T 5 × 108 K), the hydrogen is always burned via the rapid proton capture (rp) process of Wallace & Woosley. The rp-process makes nuclei far beyond the iron group, always leading to a mixture of elements with masses A ~ 60-100. The average nuclear mass of the ashes is set by the extent of helium burning via (α, p) reactions and, because these reactions are temperature sensitive, depends on the local accretion rate. Nuclear statistical equilibrium, leading to a composition of mostly iron, occurs only for very high local accretion rates in excess of 50 times the Eddington rate. We briefly discuss the consequences of our results for the properties of the neutron star. The wide range of nuclei made at a fixed accretion rate and the sensitivity of the ash composition to the local accretion rate makes it inevitable that accreting neutron stars have an ocean and crust made up of a large variety of nuclei. This has repercussions for the thermal and electrical properties and structural properties (the shear modulus and viscosity) of the neutron star crust. A crustal lattice as impure as implied by our results will have the conductivity throughout most of its mass set by impurity scattering, allowing for more rapid Ohmic diffusion of magnetic fields than previously estimated for mononuclear mixes.

An approximation for the rp-process

Hot (explosive) hydrogen burning, or the rapid proton capture process (rp-process), occurs in a number of astrophysical environments. Novae and X-ray bursts are the most prominent ones, but accretion disks around black holes and other sites are candidates as well. The expensive and often multidimensional hydrocalculations for such events require an accurate prediction of the thermonuclear energy generation while avoiding full nucleosynthesis network calculations. In the present investigation we present an approximation scheme that leads to accuracy of more than 15% for the energy generation in hot hydrogen burning from 108-1.5 × 109 K, which covers the whole range of all presently known astrophysical sites. It is based on the concept of slowly varying hydrogen and helium abundances and assumes a kind of local steady flow by requiring that all reactions entering and leaving a nucleus add up to a zero flux. This scheme can adapt itself automatically and covers low-temperature regimes, characterized by a steady flow of reactions, as well as high-temperature regimes where a (p, γ)-(γ, p)-equilibrium is established, while β+-decays or (α, p)-reactions feed the population of the next isotonic line of nuclei. In addition to a gain of a factor of 15 in computational speed over a full-network calculation and energy generation accurate to more than 15% this scheme also allows the correct prediction of individual isotopic abundances. Thus, it delivers all features of a full network at a highly reduced cost and can easily be implemented in hydrocalculations.

O-17(n, alpha)C-14 - Closure of a primordial CNO bi-cycle?

The cross section of the 17 O(n, α) 14 C reaction was investigated in the neutron energy range from 10 to 250 keV. Compared to previous measurements, our results are about 40 times lower around 130 keV but show reasonable agreement in other energy regions. In terms of the astrophysical reaction rate, this discrepancy gives rise to a correction of up to a factor of 2. The present results imply that the 17 O(n, α) 14 C reaction playsindeed an important role in inhomogeneous big bang models, since it hampers significantly the synthesis of heavier elements, which could act as seed for a primordial r-process

The rp-process in x-ray bursts

The rp-process was first suggested by Wallace and Woosley (1981) as the dominant nucleosynthesis process in explosive hydrogen burning at high temperature and density conditions. The process is characterized by a sequence of fast proton capture reactions and subsequent β-decays. The reaction path of the rp-process runs along the drip line up to Z≈50. Within a sufficiently long time scale the associated nucleosynthesis produces N=Z even-even nuclei. Extended model calculations indicate a large production of in particular mass A=80 isotopes. The abundances are highly sensitive to the lifetimes and decay pattern of these nuclei. We present the implications of recent experimental results for the nucleosynthesis of mass 80 isotopes.

The endpoint of the rp-process

Abstract The endpoint of rp-process nucleosynthesis in X-ray bursts determines the fuel consumption, the energy generation, and the abundance pattern of the produced nuclei. To investigate the time structure of rp-process nucleosynthesis, we used a nuclear reaction network including nuclei from H to Sn. We found that if 2p-capture reactions are included, the synthesis of nuclei heavier than Kr proceeds faster than previously thought. Therefore, in most X-ray bursts large amounts of nuclei in the A=80–100 region are expected to be produced. With an escape factor of about 1%, X-ray bursts could account for the large observed solar system abundances of the light p-nuclei like 92 Mo and 96 Ru that cannot be produced sufficiently in other p-process scenarios.

Tz = −1 nuclei in the rp-process — astrophysical implications and a new experimental approach

The ( α , 8 He) reaction is used to measure level energies in very proton rich nuclei. The results allow a more reliable calculation of the reaction flow in the rp and αp processes in Novae, X-ray pulsars and X-ray bursts.

NUCLEAR PHYSICS FAR FROM STABILITY AND EXPLOSIVE NUCLEOSYNTHESIS PROCESSES

SummaryIn this paper, we discuss the astrophysically relevant nuclear-physics input for a selected set of explosive nucleosynthesis scenarios leading to rapid protonand neutron-capture processes. Observables (like,e.g., luminosity curves or abundance distributions) witness the interplay between nuclear-structure aspects far from β-stability and the appropriate astrophysical environments, and can give guidance to and constraints on stellar conditions and/or key features of reaction and decay data for radioactive isotopes.

New Reaction Rates Of 64Ge(p,γ)65As(p,γ)66Se And The Impact On Type I X-ray Bursts

The nucleosynthesis occurring in type-I X-ray bursts (XRBs) and the respective energy released in these thermonuclear explosions are sensitive to nuclear masses and reaction rates around the

Design and Construction of a Gas Jet Target for RIB Experiements

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