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The elements in the universe are mainly produced by charged-particle fusion reactions and neutron-capture reactions. About 35 proton-rich isotopes, the p-nuclei, cannot be produced via neutron-induced reactions. To date, nucleosynthesis simulations of possible production sites fail to reproduce the p-nuclei abundances observed in the solar system. In particular, the origin of the light p-nuclei 92Mo, 94Mo, 96Ru and 98Ru is little understood. The nucleosynthesis simulations rely on assumptions about the seed abundance distributions, the nuclear reaction network and the astrophysical environment. This work addressed the nuclear data input.
The key reaction 94Mo(g,n) for the production ratio of the p-nuclei 92Mo and 94Mo was investigated via Coulomb dissociation at the LAND/R3B setup at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. A beam of 94Mo with an energy of 500 AMeV was directed onto a lead target. The neutron-dissociation reactions following the Coulomb excitation by virtual photons of the electromagnetic field of the target nucleus were investigated. All particles in the incoming and outgoing channels of the reaction were identified and their kinematics were determined in a complex analysis. The systematic uncertainties were analyzed by calculating the cross sections for all possible combinations of the data selection criteria. The integral Coulomb dissociation cross section of the reaction 94Mo(g,n) was determined to be (571 +- 14 (stat) +- 46 (syst) ) mb. The result was compared to the data obtained in a real photon experiment carried out at the Saclay linear accelerator. The ratio of the integral cross sections was found to be 0.63 +- 0.07, which is lower than the expected value of about 0.8.
The nucleosynthesis of the light p-nuclei 92Mo, 94Mo, 96Ru and 98Ru was investigated in post-processing nucleosynthesis simulations within the NuGrid research platform. The impact of rate uncertainties of the most important production and destruction reactions was studied for a Supernova type II model. It could be shown that the light p-nuclei are mainly produced via neutron-dissociation reactions on heavier nuclei in the isotopic chains, and that the final abundances of these p-nuclei are determined by their main destruction reactions. The nucleosynthesis of 92Mo and 94Mo was also studied in different environments of a Supernova type Ia model. It was concluded that the maximum temperature and the duration of the high temperature phase determine the final abundances of 92Mo and 94Mo.
XIII Nuclei in the Cosmos, 7-11 July, 2014 Debrecen, Hungary.
As an alternative production scenario to the so-called g process, the most abundant p nucleus 92Mo may be produced by a chain of proton-capture reactions in supernovae type Ia. The reactions 90Zr(p,g) and 91Nb(p,g) are the most important reactions in this chain. We have measured the first reaction using high-resolution in-beam g-spectroscopy at HORUS, Cologne, Germany, to contribute to the existing experimental data base. So far, we only investigated the high-energy part of the Gamow window and the analysis is still in progress. We plan to study the second reaction in standard kinematics at the FRANZ facility, Frankfurt, Germany. Current developments at FRANZ will be explained in detail.
Neutron capture cross sections of unstable isotopes are important for neutron-induced nucleosynthesis as well as for technological applications. A combination of a radioactive beam facility, an ion storage ring and a high flux reactor would allow a direct measurement of neutron induced reactions over a wide energy range on isotopes with half lives down to minutes. The idea is to measure neutron-induced reactions on radioactive ions in inverse kinematics. This means, the radioactive ions will pass through a neutron target. In order to efficiently use the rare nuclides as well as to enhance the luminosity, the exotic nuclides can be stored in an ion storage ring. The neutron target can be the core of a research reactor, where one of the central fuel elements is replaced by the evacuated beam pipe of the storage ring. Using particle detectors and Schottky spectroscopy, most of the important neutron-induced reactions, such as (n,γ), (n,p), (n,α), (n,2n), or (n,f), could be investigated.
p-process nucleosynthesis via proton-capture reactions in thermonuclear supernovae explosions
(2015)
Model calculations within the framework of the so-called γ process show an underproduction of the p nucleus with the highest isotopic abundace 92Mo. This discrepancy can be narrowed by taking into account the alternative production site of a type Ia supernova explosion. Here, the nucleus 92Mo can be produced by a sequence of proton-capture reactions. The amount of 92Mo nuclei produced via this reaction chain is most sensitive to the reactions 90Zr(p,γ) and 91Nb(p,γ). Both rates have to be investigated experimentally to study the impact of this nucleosynthesis aspect on the long-standing 92Mo-problem. We have already measured the proton-capture reaction on 90Zr using high-resolution in-beam γ-ray spectroscopy. In this contribution, we will present our preliminary results of the total cross sections as well as the partial cross sections. Furthermore, we plan to measure the 91Nb(p,γ) reaction soon. Due to the radioactive target material, the 91Nb nuclei have to be produced prior to the experiment. The current status of this production will be presented in this contribution.
Partial wave analysis of the reaction p(3.5 GeV) + p → pK +Λ to search for the "ppK−" bound state
(2015)
Employing the Bonn–Gatchina partial wave analysis framework (PWA), we have analyzed HADES data of the reaction p(3.5 GeV) + p → pK +Λ. This reaction might contain information about the kaonic cluster “ppK −” (with quantum numbers J P = 0− and total isospin I = 1/2) via its decay into pΛ. Due to interference effects in our coherent description of the data, a hypothetical KNN (or, specifically “ppK −”) cluster signal need not necessarily show up as a pronounced feature (e.g. a peak) in an invariant mass spectrum like pΛ. Our PWA analysis includes a variety of resonant and non-resonant intermediate states and delivers a good description of our data (various angular distributions and two-hadron invariant mass spectra) without a contribution of a KNN cluster. At a confidence level of CLs = 95% such a cluster cannot contribute more than 2–12% to the total cross section with a pK +Λ final state, which translates into a production cross-section between 0.7 μb and 4.2 μb, respectively. The range of the upper limit depends on the assumed cluster mass, width and production process.
We present a nucleosynthesis sensitivity study for the γ-process in a Supernova type II model within the NuGrid research platform. The simulations aimed at identifying the relevant local production and destruction rates for the p-nuclei of molybdenum and at determining the sensitivity of the final abundances to these rates. We show that local destruction rates strongly determine the abundance of 92Mo and 94Mo, and quantify the impact.
The neutron capture cross section of the s-process branch nucleus 63Ni affects the abundances of other nuclei in its region, especially 63Cu and 64Zn. In order to determine the energy-dependent neutron capture cross section in the astrophysical energy region, an experiment at the Los Alamos National Laboratory has been performed using the calorimetric 4πBaF2 array DANCE. The (n,γ) cross section of 63Ni has been determined relative to the well-known 197Au standard with uncertainties below 15%. Various 63Ni resonances have been identified based on the Q value. Furthermore, the s-process sensitivity of the new values was analyzed with the new network calculation tool NETZ.
he study of the resonant structures in neutron-nucleus cross-sections, and therefore of the compound-nucleus reaction mechanism, requires spectroscopic measurements to determine with high accuracy the energy of the neutron interacting with the material under study.
To this purpose, the neutron time-of-flight facility n_TOF has been operating since 2001 at CERN. Its characteristics, such as the high intensity instantaneous neutron flux, the wide energy range from thermal to few GeV, and the very good energy resolution, are perfectly suited to perform high-quality measurements of neutron-induced reaction cross sections. The precise and accurate knowledge of these cross sections plays a fundamental role in nuclear technologies, nuclear astrophysics and nuclear physics.
Two different measuring stations are available at the n_TOF facility, called EAR1 and EAR2, with different characteristics of intensity of the neutron flux and energy resolution. These experimental areas, combined with advanced detection systems lead to a great flexibility in performing challenging measurement of high precision and accuracy, and allow the investigation isotopes with very low cross sections, or available only in small quantities, or with very high specific activity.
The characteristics and performances of the two experimental areas of the n_TOF facility will be presented, together with the most important measurements performed to date and their physics case. In addition, the significant upcoming measurements will be introduced.