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The new 5MV Stable beam Accelerator for Nuclear Astrophysics (St. ANA) to be installed at the Nuclear Science Laboratory at the University of Notre Dame represents a major equipment upgrade. The accelerator will provide high intensity heavy ion beams for experiments with the St. George recoil separator, but it will also increase the intensities for proton and alpha beams, necessary for studying some astrophysical processes.
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Gamma-rays from radioactive by-products of nucleosynthesis ejecta provide a direct measurement of cosmic nucleosynthesis (i.e. the formation of nuclei in stellar environments) as the detection of the decay gamma-rays with satellite-borne telescopes provide direct isotopic constraints to the physics of nuclear burning regions inside stars.
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The new 5 MV accelerator to be installed at the Nuclear Science Laboratory at the University of Notre Dame represents a major equipment upgrade. The accelerator will provide high intensity heavy ion beams for experiments at the St. George recoil separator, but it will also increase the intensities for proton and alpha beams.
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The experiments were performed at the low energy accelerators of the Notre Dame Nuclear Science Laboratory. The cross section was measured over a large energy range from about 0. 25 MeV to 3.5 MeV. Special attention was given to determine the angular distribution of the emitted g radiation. A special detector arrangement consisted of a Ge clover detector at 45° with 5 Ge detectors mounted at different angles. Both implanted ¹⁴N and TiN targets have been used for the experiments. The experiments were successfully completed and are presently being analyzed.
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To test the reliability of the HF calculations and provide a systematic understanding of the alpha optical potential at energies of astrophysical interest, a series of precision alpha scattering measurements were carried out at the Notre Dame FN tandem accelerator. Specifically, ¹⁰⁶Cd, ¹¹⁸Sn, and ^{120,124,126,128,130}Te were studied at energies both below and above the Coulomb barrier: 17, 19, 22, 24.5, and 27 MeV.
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Traditionally, optical model or equivalent square-well optical model (Fowler model) are used to fit the average cross section and predict the reaction cross sections at the energies of astrophysical interest. Recently, a new model, the hindrance model, was proposed to provide systematic fits to fusion reaction data at extreme sub-barrier energies. The goal of this project is to test the models by pushing the lowest measured energies down towards the astrophysics relevant region.
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Recent results from meteoritic grain measurements and stellar models indicate that an enhanced experimental data set of the reactions ²⁵Mg(a,n)²⁸Si, ²⁶Mg(a,n)²⁹Si and ¹⁸O(a,n)²¹Ne would unveil a clearer picture on their role toward stellar nucleosynthesis. The new reaction rates will be implemented into stellar models and the resulting isotopic abundance patterns then compared to measured isotopic abundance patterns in meteoritic inclusions.
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X-ray bursts are some of the most energetic events observed in the cosmos. It is thought that X-ray bursts are the thermonuclear explosions occurring near the surface of accreting neutron stars. The material accreted from the companion star is funneled to the surface of the neutron stars. The extreme density found near the surface of the neutron star provides the environment for explosive Hydrogen and Helium burning.
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Most of the heavy elements in the universe are produced by sequential neutron captures on lighter nuclei during the helium burning stage of stellar evolution. The efficiency of these processes depends on the abundance of neutron sources, of seed nuclei for the buildup towards heavier elements and so called neutron poisons, which remove neutrons from the stellar burning environment.
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The R-matrix analysis code, AZURE, has been successfully used to fit the new data, and extrapolate the calculation down to astrophysically relevant energies. The results demonstrate the predictive power of Rmatrix techniques in evaluating and extrapolating low energy reaction cross sections. Reaction rate calculations have been performed and are significantly reduced compared with current compilations.
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Nuclear reactions are at the heart of stellar structure, evolution and nucleosynthesis from main sequence, giants and super-giants to supernovae, white dwarfs and neutron stars. Depending on fuel, temperature, and density, stellar burning may involve many reactions of different nuclei, from light to heavy and stable to extremely neutron rich ones. The possibility of fusion reactions between low mass nuclei at low temperatures but extremely high densities occurs in the cores of white dwarfs and the crust of neutron stars. In white dwarf matter the ¹²C+¹²C fusion may trigger type I supernovae, in neutron star crusts fusion between very neutron rich nuclei may provide internal heating.
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The recoil mass separator St. George at Notre Dame is primarily designed to study alpha capture reactions in inverse kinematics. Therefore a heavy ion beam is in combination with a ⁴He gas target. We designed a differentially pumped windowless jet-gas target to reduce beam straggling and back ground scattering and provide a point-like source for the recoil separator.
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Abundance calculations for the p-nuclei involve an extended network of about 20,000 nuclear reactions of almost 2000 nuclei with masses ranging from 12 to 210. These associated photodissociation and capture rates are typically calculated with the statistical Hauser Feshbach Model (HF-Model). The (y,a) flow is critical for the processing of heavier elements towards lower masses in the p-process flow.
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²⁰Ne and ²⁴Mg are important products of nucleosynthesis in stellar carbon burning. The ²⁴Mg((a,g)²⁸Si reaction is of significant importance for the possible subsequent depletion of ²⁴Mg. The cross section in the stellar Gamow window for carbon burning between 1.0 MeV and 1.6 MeV is characterized by a number of resonances which are not well known.
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The ¹⁶O((a,g)²⁰Ne reaction is of significant importance for stellar Helium burning. The reaction rate is low at temperatures of stellar helium burning and is therefoe identified as the "end-point" of the main reaction chain ⁴H(2(a,g)¹²C((a,g)¹⁶O((a,g)²⁰Ne which is induced by the triple alpha process.
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A successful new program in the tradition of the JINA PAN program at MSU has been introduced at Notre Dame. The PIXE-PAN program at Notre Dame uses the existing low energy accelerator facility to introduce high school teachers and students to a two week program in material analysis techniques with accelerators.
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The p-nuclei are stable but very rare neutron deficient isotopes of the elements above Z=42. In our present interpretation they are produced by photodisintegration processes of heavy elements in high photon-flux environments. Such conditions are expected in the type II supernova shock front heating and compressing the outer layers of the star.
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The origin of fluorine has been unknown and speculations ran from AGB star inter-shell burning during late stellar evolution to neutrino induced production in supernova type-II explosions. Recent spectroscopic observations of post AGB stars showed strong indications of fluorine abundance in the stellar spectrum.
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In collaboration with the Department of Anthropology a program was initiated at the Notre Dame accelerator facilities to analyze the composition of copper samples of Western Indiana native heritage. The goal is to determine the providence of the material and provide information of 17^th -18^th century cultural connections and trading patterns between Western Indiana and the British and French Colonies.
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Twenty years ago it was predicted that the reaction rate is dominated by a single resonance with 4.03 MeV excitation energy in ¹⁹Ne. Numerous research groups have tried to measure the resonance and its strength and determine the trigger temperature for X-ray bursts. We have performed the first successful measurement of the á-decay of the 4.03 MeV state using the Notre Dame TwinSol system.
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Recirculating windowless gas-target systems are important instruments for low energy measurements of relevance for nuclear astrophysics. The gas provides pure target material conditions, and the beam energy loss is minimized so that the measurements can be pursued towards very low energies. The Rhinoceros gas-target from the University of Stuttgart has been installed at the Notre Dame Nuclear Science laboratory for measuring the low energy cross sections of alpha capture reactions in stellar helium burning.
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