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Spatially-correlated overabundances of $^{15}$N and $^{18}$O observed in some low-density graphite meteoritic grains have been connected to nucleosynthesis taking place in the helium-burning shell during core-collapse supernovae. Two of the reactions which have been identified as important to the final abundances of $^{15}$N and $^{18}$O are $^{18}$F($n,\alpha$)$^{15}$N and $^{18}$F($n,p$)$^{18}$O. The relative strengths of the $^{18}$F($n,\alpha$)$^{15}$N and $^{18}$F($n,p$)$^{18}$O reactions depend on the relative $\alpha_0$ and $p_0$ decays from states above the neutron threshold in $^{19}$F in addition to other properties. Experimental data on the charged-particle decays from these highly excited states are lacking or inconsistent. Two experiments were performed using proton inelastic scattering from LiF targets and magnetic spectrographs. The first experiment used the high-resolution Q3D spectrograph at Munich to constrain properties of levels in $^{19}$F. A second experiment using the Orsay Split-Pole spectrograph and an array of silicon detectors was performed in order to measure the charged-particle decays of neutron-unbound levels in $^{19}$F. A number of levels in $^{19}$F have been identified along with their corresponding charged-particle decays. The first state above the neutron threshold which has an observed proton-decay branch to the ground state of $^{18}$O lies 68 keV above the neutron threshold while the $\alpha$-particle decays from the neutron-unbound levels are generally observed to be much stronger than the proton decays. Neutron-unbound levels in $^{19}$F are observed to decay predominantly by $\alpha$-particle emission, supporting the role of $^{18}$F($n,\alpha$)$^{15}$N in the production of $^{15}$N in the helium-burning shell of supernovae. Improved resonant-scattering reaction data are required in order to be able to determine the reaction rates accurately.

Classical novae result from thermonuclear explosions producing several $\gamma$-ray emitters which are prime targets for satellites observing in the MeV range. The early 511 keV gamma-ray emission depends critically on the $^{18}$F(p,$\alpha$)$^{15}$O reaction rate which, despite many experimental and theoretical efforts, still remains uncertain. One of the main uncertainties in the $^{18}$F(p,$\alpha$)$^{15}$O reaction rate is the contribution in the Gamow window of interference between sub-threshold $^{19}$Ne states and known broad states at higher energies. Therefore the goal of this work is to clarify the existence and the nature of these sub-threshold states. States in the $^{19}$Ne compound nucleus were studied at the Tandem-ALTO facility using the $^{19}$F($^3$He,t)$^{19}$Ne charge exchange reaction. Tritons were detected with an Enge Split-pole spectrometer while decaying protons or $\alpha$-particles from unbound $^{19}$Ne states were collected, in coincidence, with a double-sided silicon strip detector array. Angular correlations were extracted and constraints on the spin and parity of decaying states established. The coincidence yield at $E_x$ = 6.29 MeV was observed to be high spin, supporting the conclusion that it is indeed a doublet consisting of high spin and low spin components. Evidence for a broad, low spin state was observed around 6 MeV. Branching ratios were extracted for several states above the proton threshold and were found to be consistent with the literature. R-matrix calculations show the relative contribution of sub-threshold states to the astrophysically important energy region above the proton threshold. The levels schemes of $^{19}$Ne and $^{19}$F are still not sufficiently well known and further studies of the analogue assignments are needed. The tentative broad state at 6 MeV may only play a role if the reduced proton width is large.

International audience; Classical novae result from thermonuclear explosions producing several $\gamma$-ray emitters which are prime targets for satellites observing in the MeV range. The early 511 keV gamma-ray emission depends critically on the $^{18}$F(p,$\alpha$)$^{15}$O reaction rate which, despite many experimental and theoretical efforts, still remains uncertain. One of the main uncertainties in the $^{18}$F(p,$\alpha$)$^{15}$O reaction rate is the contribution in the Gamow window of interference between sub-threshold $^{19}$Ne states and known broad states at higher energies. Therefore the goal of this work is to clarify the existence and the nature of these sub-threshold states. States in the $^{19}$Ne compound nucleus were studied at the Tandem-ALTO facility using the $^{19}$F($^3$He,t)$^{19}$Ne charge exchange reaction. Tritons were detected with an Enge Split-pole spectrometer while decaying protons or $\alpha$-particles from unbound $^{19}$Ne states were collected, in coincidence, with a double-sided silicon strip detector array. Angular correlations were extracted and constraints on the spin and parity of decaying states established. The coincidence yield at $E_x$ = 6.29 MeV was observed to be high spin, supporting the conclusion that it is indeed a doublet consisting of high spin and low spin components. Evidence for a broad, low spin state was observed around 6 MeV. Branching ratios were extracted for several states above the proton threshold and were found to be consistent with the literature. R-matrix calculations show the relative contribution of sub-threshold states to the astrophysically important energy region above the proton threshold. The levels schemes of $^{19}$Ne and $^{19}$F are still not sufficiently well known and further studies of the analogue assignments are needed. The tentative broad state at 6 MeV may only play a role if the reduced proton width is large.

Background: It has been recently suggested that hydrogen ingestion into the helium shell of massive stars could lead to high C13 and N15 excesses when the shock of a core-collapse supernova passes through its helium shell. This prediction questions the origin of extremely high C13 and N15 abundances observed in rare presolar SiC grains which is usually attributed to classical novae. In this context the N13(α,p)O16 reaction plays an important role since it is in competition with N13β+ decay to C13. Purpose: The N13(α,p)O16 reaction rate used in stellar evolution calculations comes from the Caughlan and Fowler compilation with very scarce information on the origin of this rate and with no associated uncertainty. The goal of this work is to provide a recommended N13(α,p)O16 reaction rate, based on available experimental data, with a meaningful statistical uncertainty. Method: Unbound nuclear states in the F17 compound nucleus were studied using the spectroscopic information of the analog states in O17 nucleus that were measured at the Tandem-Alto facility using the C13(Li7,t)O17 α-particle-transfer reaction. The α-particle spectroscopic factors were derived using a finite-range distorted-wave Born approximation analysis. This spectroscopic information was used to calculate a recommended N13(α,p)O16 reaction rate with meaningful uncertainty using a Monte Carlo approach. Results: The N13(α,p)O16 reaction rate from the present work is found to be within a factor of two of the previous evaluation in the temperature range of interest, with a typical uncertainty of a factor ≈2-3. The source of this uncertainty has been identified to come from the three main contributing resonances at Erc.m.=221, 741, and 959 keV. This new error estimation translates to an overall uncertainty in the C13 production of a factor of 50 when using the lower and upper reaction rates in the conditions relevant for the N13(α,p)O16 activation. Conclusions: The main source of uncertainty on the re-evaluated N13(α,p)O16 reaction rate currently comes from the uncertain α-particle width of relevant F17 states.

Carbon burning is a critical phase for nucleosynthesis in massive stars. The conditions for igniting this burning stage, and the subsequent isotope composition of the resulting ashes, depend strongly on the reaction rate for C12+C12 fusion at very low energies. Results for the cross sections for this reaction are influenced by various backgrounds encountered in measurements at such energies. In this paper, we report on a new measurement of C12+C12 fusion cross sections where these backgrounds have been minimized. It is found that the astrophysical S factor exhibits a maximum around Ecm=3.5-4.0 MeV, which leads to a reduction of the previously predicted astrophysical reaction rate.

Neutron-capture reactions on $^{18}$F in the helium-burning shell play an important role in the production of $^{15}$N during core-collapse supernovae. The competition between the $^{18}$F($n,p/\alpha$)$^{18}$O/$^{15}$N reactions controls the amount of $^{15}$N produced. The strengths of these reactions depend on the decay branching ratios of states in $^{19}$F above the neutron threshold. We report on an experiment investigating the decay branching ratios of these states in order to better constrain the strengths of the reactions., Comment: Submitted as part of the Proceedings of NIC 2018

International audience; Carbon burning is a critical phase for nucleosynthesis in massive stars. The conditions for igniting this burning stage, and the subsequent isotope composition of the resulting ashes, depend strongly on the reaction rate for C12+C12 fusion at very low energies. Results for the cross sections for this reaction are influenced by various backgrounds encountered in measurements at such energies. In this paper, we report on a new measurement of C12+C12 fusion cross sections where these backgrounds have been minimized. It is found that the astrophysical S factor exhibits a maximum around Ecm=3.5–4.0 MeV, which leads to a reduction of the previously predicted astrophysical reaction rate.

The angular distributions of $^{8}\mathrm{Be}$ states in the excitation energy region ${E}_{x}\ensuremath{\sim}(16.5\text{--}18.2)$ MeV produced in the $^{7}\mathrm{Li}(^{3}\mathrm{He},d)^{8}\mathrm{Be}$ proton transfer reaction have been measured at the Orsay $14.8\ensuremath{-}\mathrm{MV}$ tandem accelerator for $^{3}\mathrm{He}^{2+}$ ion bombarding energy, ${E}_{\text{lab}}\phantom{\rule{4pt}{0ex}}=20$ MeV, and forward angular range, ${\ensuremath{\theta}}_{\text{lab}}=5{}^{\ensuremath{\circ}}\text{--}50{}^{\ensuremath{\circ}}$. A high energy resolution detection system composed of a split-pole magnetic spectrometer and a $\mathrm{\ensuremath{\Delta}}E\ensuremath{-}E$, position-sensitive drift chamber was used to record the energy spectra of outgoing deuterons. The measured cross section data for the direct reaction component have been separated from the compound nucleus one, then analyzed in the framework of the nonlocal, finite-range (FR)--distorted-wave Born approximation (DWBA) theory. New values of the ${C}^{2}S$ and (${S}_{p1/2},\phantom{\rule{4pt}{0ex}}{S}_{p3/2})$ proton absolute and partial spectroscopic factors and related ${\ensuremath{\gamma}}_{p}^{2}(a)$ proton reduced widths versus the $p+^{7}\mathrm{Li}$ channel radius have been extracted for the ${2}^{+}(16.626)$ and ${2}^{+}(16.922),T=0+1$ isospin-mixed loosely bound states of astrophysical interest and the ${1}^{+}(17.640)$, $T=1$ unbound state of $^{8}\mathrm{Be}$. They are compared to sparse earlier experimental values and to shell-model-predicted ones from the literature, and are discussed. In particular, the status of the spectroscopic information on the ${2}^{+}$ isospin-mixed doublet is reviewed and updated. The application in nuclear astrophysics of the DWBA derived results is emphasized.

International audience; The 12C+12C fusion reaction is one of the most important for nuclear astrophysics since it determines the carbon ignition in stellar environments. Two experiments which make use of the gamma-particle coincidence technique to measure the 12C+12C S-factors at deep sub barrier energies are discussed. Results are presented showing a decrease of the S-factor below Ec.m. = 3 MeV.

International audience; The angular distributions of Be8 states in the excitation energy region Ex∼(16.5–18.2) MeV produced in the Li7(He3,d)Be8 proton transfer reaction have been measured at the Orsay 14.8−MV tandem accelerator for He2+3 ion bombarding energy, Elab=20 MeV, and forward angular range, θlab=5∘–50∘. A high energy resolution detection system composed of a split-pole magnetic spectrometer and a ΔE−E, position-sensitive drift chamber was used to record the energy spectra of outgoing deuterons. The measured cross section data for the direct reaction component have been separated from the compound nucleus one, then analyzed in the framework of the nonlocal, finite-range (FR)–distorted-wave Born approximation (DWBA) theory. New values of the C2S and (Sp1/2,Sp3/2) proton absolute and partial spectroscopic factors and related γp2(a) proton reduced widths versus the p+Li7 channel radius have been extracted for the 2+(16.626) and 2+(16.922),T=0+1 isospin-mixed loosely bound states of astrophysical interest and the 1+(17.640), T=1 unbound state of Be8. They are compared to sparse earlier experimental values and to shell-model-predicted ones from the literature, and are discussed. In particular, the status of the spectroscopic information on the 2+ isospin-mixed doublet is reviewed and updated. The application in nuclear astrophysics of the DWBA derived results is emphasized.

Carbon burning plays a crucial role in stellar evolution, where this reaction is an important route for the production of heavier elements. A particle-γ coincidence technique that minimizes the backgrounds to which this reaction is subject and provides reliable cross sections has been used at the Argonne National Laboratory to measure fusion cross-sections at deep sub-barrier energies in the 12 C+12 C system. The corresponding excitation function has been extracted down to a cross section of about 6 nb. This indicates the existence of a broad S-factor maximum for this system. Experimental results are presented and discussed.

The C-12+C-12 fusion reaction is one of the most important for nuclear astrophysics since it determines the carbon ignition in stellar environments. Two experiments which make use of the gamma-particle coincidence technique to measure the C-12+C-12 S-factors at deep sub barrier energies are discussed. Results are presented showing a decrease of the S-factor below E-c.m. = 3 MeV.

International audience

Intense and purified radioactive beam of post-accelerated $^{14}$O was used to study the low-lying states in the unbound $^{15}$F nucleus. Exploiting resonant elastic scattering in inverse kinematics with a thick target, the second excited state, a resonance at E$\_R$=4.757(6)(10)~MeV with a width of $\Gamma$=36(5)(14)~keV was measured for the first time with high precision. The structure of this narrow above-barrier state in a nucleus located two neutrons beyond the proton drip line was investigated using the Gamow Shell Model in the coupled channel representation with a $^{12}$C core and three valence protons. It is found that it is an almost pure wave function of two quasi-bound protons in the $2s\_{1/2}$ shell., Comment: 8 pages, 2 figures, 1 table, Submitted to Phys. Lett. B

The full-energy peak efficiency calibration and the energy resolution measurements of the 2 in . × 2 in . LaBr 3 γ -ray detector are presented for γ -ray energies in the 700 keV–17.6 MeV range. Measurements were done using a combination of proton-capture nuclear reactions on Al 27 , Na 23 , K 39 , Li 7 and B 11 for high-energy γ -rays, and radioactive sources such as Co 60 and Eu 152 for the lowest energies. At high energies, two γ -rays in a cascade from proton resonance capture were employed using Al, Na 2 WO 4 , K 2 SO 4 and LiBO 2 targets. The obtained results were compared to the simulations performed using a GEANT4 code.

The early E ⩽ 511 keV gamma-ray emission from novae depends critically on the F 18 ( p , α ) O 15 reaction. Unfortunately the reaction rate of the F 18 ( p , α ) O 15 reaction is still largely uncertain due to the unknown strengths of low-lying proton resonances near the F 18 + p threshold which play an important role in the nova temperature regime. We report here our last results concerning the study of the d ( F 18 , p ) F 19 ( α ) N 15 transfer reaction. We show in particular that these two low-lying resonances cannot be neglected. These results are then used to perform a careful study of the remaining uncertainties associated to the F 18 ( p , α ) O 15 and F 18 ( p , γ ) Ne 19 reaction rates.

International audience; The cross section of the ^12C+^12C fusion reaction at low energies is of paramount importance for models of stellar nucleosynthesis in different astrophysical scenarios, such as Type Ia supernovae and Xray superbursts, where this reaction is a primary route for the production of heavier elements. In a series of experiments performed at Argonne National Laboratory, using Gammasphere and an array of Silicon detectors, measurements of the fusion cross section of ^12C+^12C were successfully carried out with the gamma and charged-particle coincidence technique in the center-of-mass energy range of 3-5 MeV. These were the first background-free fusion cross section measurements for ^12C+^12C at energies of astrophysical interest. Our results are consistent with previous measurements in the high-energy region; however, our lowest energy measurement indicates a fusion cross section slightly lower than those obtained with other techniques.

The cosmological 7Li problem arises from the signi cant discrepancy of about a factor 3 between the predicted primordial 7Li abundance and the observed one. The main process for the production of 7Li during Big-Bang nucleosynthesis is the decay of 7Be. Many key nuclear reactions involved in the production and destruction of 7Be were investigated in attempt to explain the 7Li de cit but none of them led to successful conclusions. However, some authors suggested recently the possibility that the destruction of 7Be by 3He and 4He may reconcile the predictions and observations if missing resonant states in the compound nuclei 10C and 11C exist. Hence, a search of these missing resonant states in 10C and 11C was investigated at the Orsay Tandem-Alto facility through 10B(3He,t)10C and 11B(3He,t)11C charge-exchange reactions respectively. After a short overview of the cosmological 7Li problem from a nuclear physics point of view, a description of the Orsay experiment will be given as well as the obtained results and their impact on the 7Li problem.

International audience; The cosmological 7Li problem arises from the significant discrepancy of about a factor 3 between the predictedprimordial 7Li abundance and the observed one [1]. The main process for the production of 7Li during Big-Bangnucleosynthesis is the decay of 7Be. Many key nuclear reactions involved in the production and destruction of 7Bewere investigated in attempt to explain the 7Li deficit but none of them led to successful conclusions. However,some authors suggested recently the possibility that the destruction of 7Be by 3He and 4He may reconcile thepredictions and observations if missing resonant states in the compound nuclei 10C and 11C exist [2]. Hence, a search of these missing resonant states in 10C and 11C was investigated at the Orsay Tandem-Alto facility through10B(3He,t)10C and 11B(3He,t)11C charge-exchange reactions respectively.After a short overview of the cosmological 7Li problem from a nuclear physics point of view, the Orsay experiment aswell as the obtained results and their impact on the 7Li problem [3] will be presented.