Search

The rate of the carbon-nitrogen-oxygen (CNO) cycle of hydrogen burning is controlled by the {sup 14}N(p,{gamma}){sup 15}O reaction. The reaction proceeds by capture to the ground states and several excited states in {sup 15}O. In order to obtain a reliable extrapolation of the excitation curve to astrophysical energy, fits in the R-matrix framework are needed. In an energy range that sensitively tests such fits, new cross-section data are reported here for the four major transitions in the {sup 14}N(p,{gamma}){sup 15}O reaction. The experiment has been performed at the Laboratory for Underground Nuclear Astrophysics (LUNA) 400-kV accelerator placed deep underground in the Gran Sasso facility in Italy. Using a composite germanium detector, summing corrections have been considerably reduced with respect to previous studies. The cross sections for capture to the ground state and to the 5181, 6172, and 6792 keV excited states in {sup 15}O have been determined at 359, 380, and 399 keV beam energy. In addition, the branching ratios for the decay of the 278-keV resonance have been remeasured.

The {sup 15}N(p,{gamma}){sup 16}O reaction represents a breakout reaction linking the first and second cycles of the CNO cycles redistributing the carbon and nitrogen abundances into the oxygen range. The reaction is dominated by two broad resonances, at E{sub p}=338 and 1028 keV, and a direct capture contribution to the ground state of {sup 16}O. Interference effects between these contributions both in the low-energy region (E{sub p}

Proton captures on Mg isotopes play an important role in the Mg-Al cycle active in stellar $H$-shell burning. In particular, the strengths of low-energy resonances with $El200$ keV in $^{25}\mathrm{Mg}$($p,\ensuremath{\gamma}$)$^{26}\mathrm{Al}$ determine the production of $^{26}\mathrm{Al}$ and a precise knowledge of these nuclear data is highly desirable. Absolute measurements at such low energies are often very difficult and hampered by $\ensuremath{\gamma}$-ray background as well as changing target stoichiometry during the measurements. The latter problem can be partly avoided using higher-energy resonances of the same reaction as a normalization reference. Hence the parameters of suitable resonances have to be studied with adequate precision. In the present work we report on new measurements of the resonance strengths $\ensuremath{\omega}\ensuremath{\gamma}$ of the $E=214$, $304$, and $326$ keV resonances in the reactions $^{24}\mathrm{Mg}$($p,\ensuremath{\gamma}$)$^{25}\mathrm{Al}$, $^{25}\mathrm{Mg}$($p,\ensuremath{\gamma}$)$^{26}\mathrm{Al}$, and $^{26}\mathrm{Mg}$($p,\ensuremath{\gamma}$)$^{27}\mathrm{Al}$, respectively. These studies were performed at the LUNA facility in the Gran Sasso underground laboratory using multiple experimental techniques and provided results with a higher accuracy than previously achieved.

The paper describes a multilevel, multichannel R-matrix code, AZURE, for applications in nuclear astrophysics. The code allows simultaneous analysis and extrapolation of low-energy particle scattering, capture, and reaction cross sections of relevance to stellar hydrogen, helium, and carbon burning. The paper presents a summary of R-matrix theory, code description, and a number of applications to demonstrate the applicability and versatility of AZURE.

The rate of the hydrogen-burning carbon-nitrogen-oxygen (CNO) cycle is controlled by the slowest process, 14N(p,γ )15O, which proceeds by capture to the ground and several excited states in 15O. Previous extrapolations for the ground state contribution di

The reaction {sup 24}Mg({alpha},{gamma}){sup 28}Si is involved in the nucleosynthesis of {sup 24}Mg and {sup 28}Si in massive stars. The reaction has not been examined with sufficient sensitivity at {alpha}-particle energies below 1.5 MeV. Several {sup 28}Si states appear favorable for formation by this reaction in the {alpha}-particle energy range of 1.0-1.5 MeV, motivating a new study of this reaction at the University of Notre Dame. To maximize experimental sensitivity, a high efficiency coincidence detection system was developed. Several previously unknown resonances were observed between 1.1 and 1.5 MeV, and an upper limit for any lower energy resonances was obtained. Resonance parameters were determined and reaction rates were calculated. The astrophysical implications of these results are discussed.

Solar neutrino fluxes depend both on astrophysical and on nuclear physics inputs, namely on the cross sections of the reactions responsible for neutrino production inside the Solar core. While the flux of solar $^{8}\mathrm{B}$ neutrinos has been recently measured at Superkamiokande with a 3.5% uncertainty and a precise measurement of $^{7}\mathrm{Be}$ neutrino flux is foreseen in the next future, the predicted fluxes are still affected by larger errors. The largest nuclear physics uncertainty to determine the fluxes of $^{8}\mathrm{B}$ and $^{7}\mathrm{Be}$ neutrinos comes from the $^{3}\mathrm{He}$($\ensuremath{\alpha},\ensuremath{\gamma}$)$^{7}\mathrm{Be}$ reaction. The uncertainty on its S-factor is due to an average discrepancy in results obtained using two different experimental approaches: the detection of the delayed \ensuremath{\gamma} rays from $^{7}\mathrm{Be}$ decay and the measurement of the prompt \ensuremath{\gamma} emission. Here we report on a new high precision experiment performed with both techniques at the same time. Thanks to the low background conditions of the Gran Sasso LUNA accelerator facility, the cross section has been measured at ${E}_{\mathrm{c}.\mathrm{m}.}=170$, 106, and 93 keV, the latter being the lowest interaction energy ever reached. The S-factors from the two methods do not show any discrepancy within the experimental errors. An extrapolated $S(0)=0.560\ifmmode\pm\else\textpm\fi{}0.017$ keV barn is obtained. Moreover, branching ratios between the two prompt \ensuremath{\gamma}-transitions have been measured with 5--8% accuracy.

The flux of {sup 7}Be and {sup 8}B neutrinos from the Sun and the production of {sup 7}Li via primordial nucleosynthesis depend on the rate of the {sup 3}He({alpha},{gamma}){sup 7}Be reaction. In an extension of a previous study showing cross section data at 127-167 keV center-of-mass energy, the present work reports on a measurement of the {sup 3}He({alpha},{gamma}){sup 7}Be cross section at 106 keV performed at Italy's Gran Sasso underground laboratory by the activation method. This energy is closer to the solar Gamow energy than ever reached before. The result is {sigma}=0.567{+-}0.029{sub stat}{+-}0.016{sub syst} nb. The data are compared with previous activation studies at high energy, and a recommended S(0) value for all {sup 3}He({alpha},{gamma}){sup 7}Be activation studies, including the present work, is given.