The O-17(p, a)N-14 and O-17(p,gamma) F-18 reactions are of paramount importance for the nucleosynthesis in a number of stellar sites, including red giants (RGs), asymptotic giant branch (AGB) stars, massive stars, and classical novae. In particular, they govern the destruction of O-17 and the formation of the short-lived radioisotope F-18, which is of special interest for gamma-ray astronomy. At temperatures typical of the above-mentioned astrophysical scenario, T = 0.01-0.1 GK for RG, AGB, and massive stars and T = 0.1-0.4 GK for a classical nova explosion, the O-17(p, alpha) (14)Nreaction cross section is dominated by two resonances: one at about E-R(cm) = 65 keV above the F-18 proton threshold energy, corresponding to the EX = 5.673 MeV level in F-18, and another one at E-R(cm) = 183 keV (EX = 5.786 MeV). We report on the indirect study of the O-17(p, alpha) N-14 reaction via the Trojan Horse method by applying the approach recently developed for extracting the strength of narrow resonance at ultralow energies. The mean value of the strengths obtained in the two measurements was calculated and compared with the direct data available in literature. This value was used as input parameter for reaction-rate determination and its comparison with the result of the direct measurement is also discussed in the light of the electron screening effect.

B. Bucher, 2, ∗ X. D. Tang, † X. Fang, A. Heger, ‡ S. Almaraz-Calderon, § A. Alongi, A. D. Ayangeakaa, ¶ M. Beard, A. Best, ∗∗ J. Browne, C. Cahillane, M. Couder, R. J. deBoer, A. Kontos, L. Lamm, †† Y. J. Li, A. Long, W. Lu, S. Lyons, M. Notani, D. Patel, N. Paul, M. Pignatari, 7, ‡ A. Roberts, D. Robertson, K. Smith, E. Stech, R. Talwar, W. P. Tan, M. Wiescher, and S. E. Woosley Institute for Structure and Nuclear Astrophysics, Joint Institute for Nuclear Astrophysics, University of Notre Dame, Notre Dame, Indiana 46556, USA Lawrence Livermore National Laboratory, Livermore, California 94550, USA Institute of Modern Physics, Chinese Academy of Science, Lanzhou, Gansu 730000, P.R. China Monash Center for Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800, Australia China Institute of Atomic Energy, Beijing 102413, P.R. China Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Konkoly Thege Miklos ut 15-17, H-1121 Budapest, Hungary Department of Physics, University of Basel, Basel, CH-4056, Switzerland Department of Astronomy and Astrophysics, University of California, Santa Cruz, California 95064, USA (Dated: July 15, 2015)

Neutrons produced by the carbon fusion reaction (12)C((12)C,n)(23)Mg play an important role in stellar nucleosynthesis. However, past studies have shown large discrepancies between experimental data and theory, leading to an uncertain cross section extrapolation at astrophysical energies. We present the first direct measurement that extends deep into the astrophysical energy range along with a new and improved extrapolation technique based on experimental data from the mirror reaction (12)C((12)C,p)(23)Na. The new reaction rate has been determined with a well-defined uncertainty that exceeds the precision required by astrophysics models. Using our constrained rate, we find that (12)C((12)C,n)(23)Mg is crucial to the production of Na and Al in pop-III pair instability supernovae. It also plays a nonnegligible role in the production of weak s-process elements, as well as in the production of the important galactic γ-ray emitter (60)Fe.

The 12C+12C reaction is one of the single most important nuclear reactions in astrophysics. It strongly influences late evolution of massive stars as well as the dynamics of type Ia supernovae and x-ray superbursts. An accurate estimation of the cross section at relevant astrophysical energies is extremely important for modeling these systems. However, the situation is complicated by the unpredictable resonance structure observed at higher energies. Two recent studies at Notre Dame have produced results which help reduce the uncertainty associated with this reaction. The first uses correlations with the isotope fusion systems, 12C+13C and 13C+13C, to establish an upper limit on the resonance strengths in 12C+12C. The other focuses on the specific channel 12C+12C→23Mg+n and its low-energy measurement and extrapolation which is relevant to s-process nucleosynthesis. The results from each provide important constraints for astrophysical models.

The reaction 17O(n, α)14C was studied using virtual neutrons coming from the quasi-free deuteron break-up in the three body reaction 17O+d → α+14C+p. This technique, called virtual neutron method, extends the Trojan Horse method to neutron-induced reactions allowing to study the reaction cross section avoiding the suppression effects coming from the penetrability of the centrifugal barrier. For incident neutron energies from thermal up to a few hundred keV, direct experiments have shown the population of two out of three expected excited states at energies 8213 keV and 8282 keV and the influence of the sub-threshold level at 8038 keV. In the present experiment the 18O excited state at E* = 8.125 MeV, missing in the direct measurement, is observed. The angular distributions of the populated resonances have been measured for the first time. The results unambiguously indicate the ability of the method to overcome the centrifugal barrier suppression effect and to pick out the contribution of the bare nuclear interaction.

Fusion cross sections were measured for the exotic proton-halo nucleus B incident on a Ni target at several energies near the Coulomb barrier. This is the first experiment to report on the fusion of a proton-halo nucleus. The resulting excitation function shows a striking enhancement with respect to expectations for normal projectiles. Evidence is presented that the sum of the fusion and breakup yields saturates the total reaction cross section. PACS numbers: 25.60.-t, 25.60.Pj , 25.70.-z

The reaction O-17(n, alpha)C-14 was studied at energies from E-c.m. = 0 to E-c.m. = 350 keV by using the quasifree deuteron breakup in the three-body reaction O-17 + d -> alpha + C-14 + p, which extended the Trojan Horse indirect method (THM) to neutron-induced reactions. It is found that the O-18 excited state at E* = 8.125 +/- 0.002 MeV, observed in THM experiments, is absent in the direct measurement because of its high centrifugal barrier. The angular distributions of the populated resonances have been measured by using this method. The results unambiguously indicate the ability of the THM to overcome the centrifugal barrier suppression effect and to pick out the contribution of the bare nuclear interaction. DOI: 10.1103/PhysRevC.87.012801

Evaporation protons from the fusion of the ${}^{3}\mathrm{He}{+}^{58}$Ni system were measured at three energies near but above the Coulomb barrier. The respective elastic scattering angular distribution was also determined for one of these energies. The fusion data, after being properly reduced to eliminate size and charge effects, are compared to the recently measured fusion excitation function for the proton-halo system ${}^{8}\mathrm{B}{+}^{58}$Ni. As a reference, fusion data for the ${}^{16}\mathrm{O}{+}^{58}$Ni system are also presented. With respect to this reference, the fusion cross sections for the proton-rich systems show an enhancement likely related to static effects. An excitation function for the respective total-reaction cross section was extracted from the measured elastic data along with additional data from the literature. Surprisingly, this excitation function follows the trend expected for weakly bound systems.

The fusion cross section for ${}^{12}$C+${}^{13}$C has been measured down to ${E}_{\mathrm{c}.\mathrm{m}.}=2.6$ MeV, at which the cross section is of the order of 20 nb. By comparing the cross sections for the three carbon isotope systems, ${}^{12}$C+${}^{12}$C, ${}^{12}$C+${}^{13}$C, and ${}^{13}$C+${}^{13}$C, it is found that the cross sections for ${}^{12}$C+${}^{13}$C and ${}^{13}$C+${}^{13}$C provide an upper limit for the fusion cross section of ${}^{12}$C+${}^{12}$C over a wide energy range. After calibrating the effective nuclear potential for ${}^{12}$C+${}^{12}$C using the ${}^{12}$C+${}^{13}$C and ${}^{13}$C+${}^{13}$C fusion cross sections, it is found that a coupled-channels calculation with the ingoing wave boundary condition (IWBC) is capable of predicting the major peak cross sections in ${}^{12}$C+${}^{12}$C. A qualitative explanation for this upper limit is provided by the Nogami-Imanishi model and by level density differences among the compound nuclei. It is found that the strong resonance found at 2.14 MeV in ${}^{12}$C+${}^{12}$C exceeds this upper limit by a factor of more than 20. The preliminary result from the most recent measurement shows a much smaller cross section at this energy, which agrees with our predicted upper limit.

15 O(α,γ) is the critical breakout reaction from the hot CNO cycles, which triggers the thermonu-

We have measured the fusion cross sections of the 12C(13C, p)24Na reaction through off-line measurement of the beta-decay of 24Na using the beta-gamma coincidence method. Our new measurements in the energy range of Ec.m. = 2.6-3.0 MeV do not show an obvious S-factor maximum but a plateau. Comparison between this work and various models is presented.

The breakout reaction O-15(alpha,gamma)Ne-19 from the hot CNO cycle is critical to understanding explosive astrophysical phenomena such as x-ray bursts. In spite of considerable past experimental effort via indirect methods, this reaction rate remained mostly uncertain until our recent measurement [Phys. Rev. Lett. 98, 242503 (2007)] of the eluding alpha-decay branching ratios of the near-threshold states in Ne-19, particularly the critical level at 4.03 MeV. In this paper, we present more details of and deeper insights into the measurement and the uncertainties of the experimental results. The alpha-unbound states in Ne-19 were populated via the reaction F-19(He-3,t), and the alpha-decay branch was observed from t-alpha coincidences using a low-energy particle detection array and the TWINSOL facility at the University of Notre Dame. In particular, the measured branching ratio of the 4.03-MeV state is 2.9 +/- 2.1x10(-4). In combination with previous measurements of the lifetimes of these states, a new experimental reaction rate of O-15(alpha,gamma)Ne-19 is proposed and discussed in the astrophysical scenario. Further experimental investigations are necessary to reduce the remaining uncertainties.

At the high temperature and density conditions of hot or explosive helium burning, the F-18(alpha,p)Ne-21 reaction may compete successfully with the F-18(beta(+)nu) decay. This suggests Ne-21(alpha,n) as an alternative neutron source in the r-process. We have determined the total cross section of the F-18(alpha,p)Ne-21 reaction by studying the time-reverse reaction Ne-21(p,alpha)F-18. Using the activation technique, the total reaction yield was measured in the proton beam energy range of 2.3-4.0 MeV, which corresponds to energies of 0.5-2.1 MeV in the F-18+alpha system. The resulting yield curve was analyzed in terms of the thick target formalism and the R-matrix theory. The reaction rate was deduced experimentally for the first time for the temperature of 0.1 < T-9 < 1. The experimental reaction rate was compared with Hauser-Feshbach predictions. The astrophysical implications of the new rate are discussed.

Background: Experimental information on the structure of low-lying levels with isospin 3/2 in systems with 13 nucleons can be used to evaluate the quality of modern theoretical predictions for the light exotic systems. This level structure is poorly known at present. Purpose: Search for T=3/2 states in C-13 was performed in this work. Method: These states were observed in the resonance elastic scattering of radioactive beam B-12 on protons using the thick target inverse kinematics technique. Results: Six new states in C-13 were identified as T=3/2 states. Tentative spin-parity assignments are suggested. Comparison to the previous knowledge of the level structure of T=3/2 A=13 system and to shell-model predictions is given. Conclusion: The elastic scattering of neutron-rich beams on proton target is a convenient tool for isobaric analog states search. The observed B-12+p excitation function is determined by the T=3/2 states and no features associated with T=1/2 states were found. Although the level scheme of T=3/2 states in A=13 systems is still far from being complete the emerging picture shows significant disagreements with predictions of contemporary shell models.

The Magnet for Astrophysical Nucleosynthesis studies Through Isobar Separation (MANTIS) system is the new Accelerator Mass Spectrometry (AMS) set-up created during recent upgrades of the Browne–Buechner spectrograph at the University of Notre Dame. Commissioning measurements performed on the separation of 58Fe–58Ni isobars at 114MeV out of the FN tandem accelerator have shown clear separation, opening the door for a number of future measurements in nuclear astrophysics. The separation of mass-58 isobars has made this system the first in the world to utilise a Browne–Buechner spectrograph in gas-filled mode for AMS measurements with a special focus on nuclear astrophysics.

The F-19(p,gamma)Ne-20 reaction represents the only breakout path for the carbon-nitrogen-oxygen cycle operating at temperatures below T=0.1 GK, an energy regime important for main-sequence hydrogen burning as well as hydrogen burning in asymptotic giant branch stars. Large experimental uncertainties exist due to unknown low energy direct and resonant reaction contributions that have been difficult to study because of the high gamma-ray background from the F-19(p,alpha(2)gamma) reaction. A new detection technique has been developed at the University of Notre Dame to measure the F-19(p,gamma) and F-19(p,alpha(i)gamma) reactions over an energy range of E-c.m.=200-760 keV. The analysis was carried out in a Breit-Wigner framework. This allowed a new determination of the resonance parameters as well as a first measurement of the signs of the interference terms. Partial widths and resonance strengths are reported for the resonances in this region.

The cross section of the reaction $^{112}\mathrm{Sn}$($\ensuremath{\alpha},\ensuremath{\gamma})^{116}\mathrm{Te}$ has been measured in the energy range of astrophysical interest for the $p$-process. Highly enriched self-supporting $^{112}\mathrm{Sn}$ foils were bombarded with \ensuremath{\alpha} beams in the effective center of mass energy range from 7.59 to 11.42 MeV at the Notre Dame FN Tandem Van de Graaff accelerator. The characteristic activity of $^{116}\mathrm{Te}$ was counted with a pair of large volume Ge clover detectors in close geometry to maximize the detection efficiency. The cross section of the concurrent ($\ensuremath{\alpha},p$) reaction has also been measured. The results are compared with statistical model predictions for different global \ensuremath{\alpha}-nucleus potentials.

15O(α ,γ) is the critical breakout reaction from the hot CNO cycles, w hich triggers the thermonuclear runaways or X-ray bursts occurring in accreting neutr on stars. Recent studies have shown that this reaction is critical for the burst amplitude and pe riodicity of X-ray bursters. However, a direct measurement of this reaction rate at astrophysical ly relevant temperatures is not feasible yet due to the lack of very high intensity radioactive 15O beams. There has been considerable effort in the past to investigate this reaction rate indirec tly by obtaining gamma and alpha decay widths of the alpha-unbound states in 19Ne. While this approach has been successful for investigating higher energy resonances, the critical level at 4. 03 MeV remains unknown. This leaves the reaction rate largely uncertain since previous attempt s have only provided limits on its gamma width and its alpha decay branching ratio. We present new exp erimental work conducted at the University of Notre Dame. Lifetimes of the 4.03 MeV state and other relevant states in 19Ne have been measured successfully using the 17O(3He,n-γ) reaction. We will also present the results of our recent measurement of the alpha-decay branching ratios . Alpha-unbound states in 19Ne were populated via the reaction 19F(3He,3H-α ) and triton-alpha coincidences were observed using a low energy particle detection Silicon array and the TWINSOL f acility. The first experimental reaction rate is proposed and its astrophysical implicatio ns will be discussed.

The Nuclear Structure Laboratory (NSL) at the University of Notre Dame installed its Browne–Buechner spectrograph in the early 1970s for highly accurate energy measurements of nuclear reactions. Current renovation and upgrading of this spectrograph will enable operation of the magnet in a gas-filled mode, in particular for the study of nuclear reactions with low cross-sections of interest in nuclear astrophysics. One of the principle issues shared by measurements of extremely low abundances in Accelerator Mass Spectrometry (AMS) and nuclear astrophysics is the discrimination between the nuclei of interest and often very intense isobaric background. Recently the AMS technique of the gas-filled magnet has very successfully been used at Argonne National Laboratory (ANL) to overcome this in the study of both environmental noble gas traces (39Ar) and the measurement of cross-sections of interest in stellar nucleosynthesis i.e. the 62Ni(n, γ)63Ni reaction. We hope to extend these techniques further to the observations of astrophysically important reactions such as 40Ca(α, γ)44Ti and 78Kr(α, γ)82Sr.

The cross section of the reaction Sn-112(alpha,gamma)Te-116 has been measured in the energy range of astrophysical interest for the p-process. Highly enriched self-supporting Sn-112 foils were bombarded with alpha beams in the effective center of mass energy range from 7.59 to 11.42 MeV at the Notre Dame FN Tandem Van de Graaff accelerator. The characteristic activity of Te-116 was counted with a pair of large volume Ge clover detectors in close geometry to maximize the detection efficiency. The cross section of the concurrent (alpha,p) reaction has also been measured. The results are compared with statistical model predictions for different global alpha-nucleus potentials.