16O Reaction Research Articles

A new approach to monitor $$^{13}\hbox {C}$$-targets degradation in situ for $$^{13}\hbox {C}(\alpha ,\hbox {n})^{16}\hbox {O}$$ cross-section measurements at LUNA

Direct measurements of reaction cross-sections at astrophysical energies often require the use of solid targets able to withstand high ion beam currents for extended periods of time. Thus, monitoring target thickness, isotopic composition, and target stoichiometry during data taking is critical to account for possible target modifications and to reduce uncertainties in the final cross-section results. A common technique used for these purposes is the Nuclear Resonant Reaction Analysis (NRRA), which however requires that a narrow resonance be available inside the dynamic range of the accelerator used. In cases when this is not possible, as for example the 13C(alpha,n)16O reaction recently studied at low energies at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy, alternative approaches must be found. Here, we present a new application of the shape analysis of primary gamma rays emitted by the 13C(p,g)14N radiative capture reaction. This approach was used to monitor 13C target degradation {\em in situ} during the 13C(alpha,n)16O data taking campaign. The results obtained are in agreement with evaluations subsequently performed at Atomki (Hungary) using the NRRA method. A preliminary application for the extraction of the 13C(alpha,n)16O reaction cross-section at one beam energy is also reported.

Testing ab initio nuclear structure in neutron-rich nuclei: Lifetime measurements of second 2+ state in C16 and O20

To test the predictive power of ab initio nuclear structure theory, the lifetime of the second 2+ state in neutron-rich 20O, tau(2+_2 ) = 150(+80-30) fs, and an estimate for the lifetime of the second 2+ state in 16C have been obtained, for the first time. The results were achieved via a novel Monte Carlo technique that allowed us to measure nuclear state lifetimes in the tens-to-hundreds femtoseconds range, by analyzing the Doppler-shifted gamma-transition line shapes of products of low-energy transfer and deep-inelastic processes in the reaction 18O (7.0 MeV/u) + 181Ta. The requested sensitivity could only be reached owing to the excellent performances of the AGATA gamma-tracking array, coupled to the PARIS scintillator array and to the VAMOS++ magnetic spectrometer. The experimental lifetimes agree with predictions of ab initio calculations using two- and three-nucleon interactions, obtained with the valence-space in-medium similarity renormalization group for 20O, and with the no-core shell model for 16C. The present measurement shows the power of electromagnetic observables, determined with high-precision gamma spectroscopy, to assess the quality of first-principles nuclear structure calculations, complementing common benchmarks based on nuclear energies. The proposed experimental approach will be essential for short lifetimes measurements in unexplored regions of the nuclear chart, including r-process nuclei, when intense ISOL-type beams become available.

Missing red supergiants and carbon burning

ABSTRACT Recent studies on direct imaging of Type II core-collapse supernova progenitors indicate a possible threshold around MZAMS ∼ 16–20 M⊙, where red supergiants (RSG) with larger birth masses do not appear to result in supernova explosions and instead implode directly into a black hole. In this study, we argue that it is not a coincidence that this threshold closely matches the critical transition of central carbon burning in massive stars from the convective to radiative regime. In lighter stars, carbon burns convectively in the centre and result in compact final pre-supernova cores that are likely to result in explosions, while in heavier stars after the transition, it burns as a radiative flame and the stellar cores become significantly harder to explode. Using the $\rm {\small {kepler}}$ code we demonstrate the sensitivity of this transition to the rate of 12C(α, γ)16O reaction and the overshoot mixing efficiency, and we argue that the upper mass limit of exploding RSG could be employed to constrain uncertain input physics of massive stellar evolution calculations. The initial mass corresponding to the central carbon burning transition range from 14 to 26 M⊙ in recently published models from various groups and codes, and only a few are in agreement with the estimates inferred from direct imaging studies.

Preliminary results for the 19F(ρ,α)16O reaction cross section measured at INFN-LNS

The 19F(ρ,α)16O reaction is an important fluorine destruction chan- nel in the proton-rich outer layers of asymptotic giant branch (AGB) stars and it might also play a role in hydrogen-deficient post-AGB star nucleosynthesis. At present, theoretical models overproduce F abundances in AGB stars with re-spect to the observed values, thus calling for further investigation of the nuclear reaction rates involved in the production and destruction of fluorine. In the last years, new direct and indirect measurements improved significantly the knowl- edge of 19F(ρ,α)16O cross section at deeply sub-Coulomb energies (below 0.8 MeV). However, those data are larger by a factor of 1.4 with respect the previ- ous data reported in the NACRE compilation in the energy region 0.6-0.8 MeV. Using the Large High resolution Array of Silicons for Astrophysics (LHASA), we performed a new direct measurement of the 19F(ρ,α)16O. The goal of this experiment is to reduce the uncertainties in the nuclear reaction rate of the 19F(ρ,α)16O reaction. Here, experimental details, the calibration procedure and angular distributions are presented.

Systematic study of incomplete fusion reactions: Role of various entrance channel parameters

The evaporation residues, populated through complete and incomplete fusion processes in the reaction of 18O+ 165Ho, have been analyzed via excitation function measurements at projectile energies ≈ 4-7 MeV/nucleon. The cross-sections measured experimentally have been compared with the predictions of the compound nucleus model code PACE-4 calculations which only considers complete fusion (CF) reaction cross-sections. The experimental cross-section of the reaction residues populated through xn and pxn channels matches well with the theoretical model code PACE-4 predictions. On the other hand, α-emitting channels show an enhancement in the measured cross-section over PACE-4 calculations which reveals the occurrence of incomplete fusion (ICF) at the studied energy range. The relative percentage of incomplete fusion has been calculated from the experimental data and its dependence on various entrance channel parameters like projectile energy, mass-asymmetry, α-Q value and Coulomb factor (ZPZT) has been studied. The strength of incomplete fusion function obtained in the 18O+ 165Ho interaction has been compared with the previously studied systems. Results of the present study indicate that 18O (two neutron excess) projectile shows more incomplete fusion contribution as compared to 12C,13C and 16O projectiles due to its relatively small negative α-Q value.

Resonant reactions of astrophysical interest studied by means of the Trojan Horse Method. Two case studies

Resonant reactions in astrophysics play and important role as un- expected resonances may enhance the astrophysical factor with respect to the direct reaction contribution, altering the predicted nucleosynthesis scenarios by changing, for instance, the expected nucleosynthesis path. They also are of great interest in nuclear structure studies, since the determination of energies, spin-parities and partial widths sheds light on the occurrence of cluster structures, for instance. However, nuclear reactions in most astrophysical environments usually take place at energies below about 1 MeV, leading to an exponential de- crease of the cross sections due to the effect of the penetration of the Coulomb barrier. Also, at energies so low to be comparable with those associated to elec- tronic degrees of freedom, the effect of atomic and/or molecular clouds cannot be neglected, resulting in a shielding of nuclear charges and in an enhancement of the cross sections with respect to the case of bare nuclei (the so called elec- tron screening effect). Owing to vanishingly small cross sections and ambigui- ties in the extrapolation due to the electron screening, supplying accurate cross sections for astrophysical modeling is extremely challenging. Indirect methods have been introduced to explore the energy range of astrophysical interest with no need of extrapolation, even guided by theoretical arguments. In particular, the Trojan Horse Method makes use of quasi-free reactions with three particles in the exit channel,a+A ^ c + C + s,to deduce the cross section of the reaction of astrophysical interest,a + x ^ c + C,under the hypothesis thatAshows a strongx+scluster structure. Even if measurements are carried out above astro- physical energies to be free from Coulomb suppression and electron screening, the range of astrophysical interest can be covered thanks to the x - s intercluster motion and binding energy. In these proceedings we will show the application of the THM, in the case of resonant reactions, using the generalised R-matrix approach introduced by A.M. Mukhamedzhanov. We will discuss the possibil- ity to extract resonance parameters from the Trojan Horse data and perform a full spectroscopic study of low-energy and even sub-threshold resonances. In particular, we will focus on the19F(p, a)16O and the13C(a, n)16O reactions, of particular importance in the case of asymptotic giant branch stars and in the synthesis of heavy elements by means of the s-process.

Backstreaming effects on probing postsaddle dissipation with light particle multiplicity

Nuclear dissipation retards fission. Using the stochastic Langevin model, we study pre- and post-saddle emitted neutrons, protons and α particles of heavy 245Cf nuclei as a function of the postsaddle dissipation strength (β) in the absence and presence of backstreaming around the saddle point. It is shown that the backstreaming effects (BEs) cause the fissioning system to return back inside saddle, which increases presaddle light particles but decreases postsaddle emission. Moreover, the BEs on pre- and post-saddle evaporation are found to become greater with increasing β and excitation energy. These findings demonstrate a significant role of BEs in tightly constraining the postsaddle dissipation strength with light particle multiplicity. Furthermore, we analyze the measured prescission neutron number in the reaction 16O (Elab=104 MeV) +232Th ⟶248Cf, and find that, after accounting for the BEs, the best-fit value of the β required to satisfactorily describe the experimental data changes from 9.6 ×1021 s−1 to 6×1021 s−1, showing a strong influence of backstreaming on accurately probing the postsaddle dissipation strength.

Discovery of s-process enhanced stars in the LAMOST survey

ABSTRACT Here we present the discovery of 895 s-process-rich candidates from 454 180 giant stars observed by the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) using a data-driven approach. This sample constitutes the largest number of s-process enhanced stars ever discovered. Our sample includes 187 s-process-rich candidates that are enhanced in both barium and strontium, 49 stars with significant barium enhancement only and 659 stars that show only a strontium enhancement. Most of the stars in our sample are in the range of effective temperature and log g typical of red giant branch (RGB) populations, which is consistent with our observational selection bias towards finding RGB stars. We estimate that only a small fraction (∼0.5 per cent) of binary configurations are favourable for s-process enriched stars. The majority of our s-process-rich candidates (95 per cent) show strong carbon enhancements, whereas only five candidates (<3 per cent) show evidence of sodium enhancement. Our kinematic analysis reveals that 97 per cent of our sample are disc stars, with the other 3 per cent showing velocities consistent with the Galactic halo. The scaleheight of the disc is estimated to be $z_{\rm h}=0.634 \pm {0.063}\, \mathrm{kpc}$, comparable with values in the literature. A comparison with yields from asymptotic giant branch (AGB) models suggests that the main neutron source responsible for the Ba and Sr enhancements is the 13C(α,n)16O reaction. We conclude that s-process-rich candidates may have received their overabundances via mass transfer from a previous AGB companion with an initial mass in the range $1\!-\!3\, \mathrm{M}_{\odot }$.

Energy Correlations of α-Particles in the 8Be-Nucleus Ground-State Formation Channel of the 12C(γ,3α) and 16O(γ,4α) Reactions

The method of diffusion chamber in the magnetic field making use of a bremsstrahlung beam with a maximum photon energy of 150 MeV is applied to study the 12C(y,3a) and 16O(y,4a) reactions. A resonance identified as the ground state of 8Be nucleus is found in the distribution of events over the energy of the relative motion of two a-particles. The partial cross-sections of the 8Be nucleus formation channels are measured. It is shown that the mechanism of interaction between a y-quantum and a virtual a-particle pair takes place in this case.

On the recent parametric determination of an asteroseismological model for the DBV star KIC 08626021

Context.Asteroseismology of white dwarf stars is a powerful tool that allows us to reveal the hidden chemical structure of white dwarfs and infer details about their present and past evolution by comparing the observed periods with those obtained from appropriate stellar models. A recent asteroseismological study has reproduced the period spectrum of the helium-rich pulsating white dwarf KIC 08626021 with the unprecedented precision of (Pobs − Pmodel)/Pmodel < 10−8. The chemical structure derived from that asteroseismological analysis is notably different from that expected for a white dwarf according to currently accepted formation channels. It therefore poses a challenge to the theory of stellar evolution.Aims.We explore the relevant micro- and macrophysics processes that act during the formation and evolution of KIC 08626021 and might lead to a chemical structure similar to that found through asteroseismology. We quantify to which extent it is necessary to modify the physical processes that shape the chemical structure in order to reproduce the most important features of the asteroseismic model.Methods.We modeled the previous evolution of KIC 08626021 by exploring specific changes in the12C(α, γ)16O reaction rate, screening processes, microscopic diffusion, and convective boundary mixing during core-He burning.Results.We find that in order to reproduce the core chemical profile derived for KIC 0862602, the12C+αnuclear reaction rate has to be increased by a factor of ∼10 during the helium-core burning, and reduced by a factor of ∼1000 during the following helium-shell burning as compared with the standard predictions for this rate. In addition, the main chemical structures derived for KIC 0862602, such as the very thin helium-pure envelope, the mass of the carbon-oxygen core, and the pure C buffer, cannot be reconciled with our current knowledge of white dwarf formation.Conclusion.We find that within our current understanding of white dwarf formation and evolution, it is difficult to reproduce the most important asteroseismologically derived features of the chemical structure of KIC 08626021.

Calibration of 3 MV Tandetron™ accelerator over nominal energy range

The method based on the measurement of proton beam energies using the non-resonant direct-capture reaction 16O(p,γ)17F* with both charged and neutral projectiles was applied to perform the energy calibration of the 3 MV Tandetron™ accelerator throughout the nominal energy range. The uncertainty of the beam energy was found to be well within the accuracy specified by the manufacturer. No deviations from the linearity were revealed.

Cross section of the reaction 18O(p,γ)19F at astrophysical energies: The 90 keV resonance and the direct capture component

The observation of oxygen isotopes in giant stars sheds light on mixing processes operating in their interiors. Due to the very strong correlation between nuclear burning and mixing processes it is very important to reduce the uncertainty on the cross sections of the nuclear reactions that are involved. In this paper we focus our attention on the reaction O18(p,γ)19F. While the O18(p,α)15N channel is thought to be dominant, the (p,γ) channel can still be an important component in stellar burning in giants, depending on the low energy cross section. So far only extrapolations from higher-energy measurements exist and recent estimates vary by orders of magnitude. These large uncertainties call for an experimental reinvestigation of this reaction. We present a direct measurement of the O18(p,γ)19F cross section using a high-efficiency 4π BGO summing detector at the Laboratory for Underground Nuclear Astrophysics (LUNA). The reaction cross section has been directly determined for the first time from 140 keV down to 85 keV and the different cross section components have been obtained individually. The previously highly uncertain strength of the 90 keV resonance was found to be 0.53 ± 0.07 neV, three orders of magnitude lower than an indirect estimate based on nuclear properties of the resonant state and a factor of 20 lower than a recently established upper limit, excluding the possibility that the 90 keV resonance can contribute significantly to the stellar reaction rate.

Extension of the Launay Quantum Reactive Scattering Code and Direct Computation of Time Delays.

Scattering computations, particularly within the realm of molecular physics, have seen an increase in study since the development of powerful quantum methods. These dynamical processes can be analyzed via (among other quantities) the duration of the collision process and the lifetime of the intermediate complex. We use the Smith matrix Q = -iℏS†dS/dE calculated from the scattering matrix S and its derivative with respect to the total energy. Its real part contains the state-to-state time delays, and its eigenvalues give the lifetimes of the metastable states [ Smith Phys. Rev. 1960 , 118 , 349 - 356 ]. We propose an extension of the Launay HYP3D code [ Launay and Le Dourneuf Chem. Phys. Lett. 1989 , 163 , 178 - 188 ] for molecular reactive scattering and give the full details of the mathematical elements needed to compute the Q matrix from the wave function without numerical differentiation of S. The log-derivative of the wave function and its energy derivative are propagated asymptotically with an extended Johnson-Manolopoulos integration [ Manolopoulos J. Chem. Phys. 1986 , 85 , 6425 - 6429 ], from which the Q matrix is calculated. As a first test of our new code, lifetimes of the metastable intermediate ozone complex O3* have been calculated for the oxygen exchange reaction 18O + 16O16O → 50O3* → 16O18O + 16O, related to the mass-independent fractionation problem of stratospheric ozone. These preliminary results reproduce previously published works limited to zero total angular momentum as benchmark.

19F(p, α)16O and 19F(α,p)22Ne Reaction Rate Measured via THM and Fluorine Nucleosynthesis in AGB stars

Asymptotic Giant Branch (AGB) stars have been proven to be sites of F production through spectroscopy observations by several authors, but it is not clear whether these stars might account for the total fluorine abundance of the Galaxy. Recently the two main channels for 19F destruction in AGB stars, namely the 19F(α,p)22Ne and 19F(p,α)16O reactions, have been studied via the Trojan Horse Method in the energy range of interest for astrophysics. In both cases experimental results have shown the presence of resonant structures below 500 keV, hinting to an enhancements in efficiency of fluorine destruction by stellar H- and He- burning. In particular the 19F(p, α)16O reaction rate at T9 ≤ 0.2K turns out to be increased up to a factor of 1.7 while the 19F(p, α)16O is enhanced more than a factor of 4 at 0.1 ≤ T9 ≤ 0.25. We present here a re-analysis of the role of AGB stars as fluorine galactic source by comparing stellar observations with predictions of AGB nucleosynthesis (for stellar masses from 1.5 to 5M⊙) computed by employing in state-of-the-art models the THM reaction rates for 19F destruction.

THM applied to the investigation of explosive astrophysical scenarios

The Trojan Horse Method (THM) makes use of quasi-free reactions to deduce the cross section of nuclear reactions relevant for astrophysics at the energies of interest. Thanks to the suppression of the Coulomb barrier, the THM cross section does not exponentially vanishes at astrophysical energies. Here we will briefly summarise the fundamentals of the method, then we will discuss two applications of the method to reactions that have a pivotal role in the latest stages of stellar evolution, leading to explosive scenarios. In particular, we will focus on the indirect investigation of the 18F(p,α)15O reaction, which is the most important 18F destruction channel in novae, and the 12C + 12C reaction, which plays a critical role in astrophysics to understand stellar burning scenarios in carbon-rich environments, including supernovae.

Detection of alpha particles from 7Li(p,α)4He and 19F(p,α)16O reactions induced by laser-accelerated protons using CR-39 with potassium hydroxide-ethanol-water etching solution.

Alpha particles generated by 7Li(p,α)4He and 19F(p,α)16O reactions are selectively detected in the presence of abundant primary protons by reducing the proton sensitivity of CR-39 using a potassium hydroxide-ethanol-water (PEW) etching solution. These nuclear reactions are induced in a LiF crystal using the laser-accelerated protons (4 × 1011 protons/pulse with a maximum energy of 3.3 MeV) generated and accelerated by the interaction of a 40-fs laser pulse with a polyethylene thin film target at a peak intensity of 5 × 1019 W/cm2. Subsequent etching of the CR-39 in the PEW solution (KOH: 17 wt. %; C2H5OH: 25 wt. %; H2O: 58 wt. %) permits the selective detection of 4.0 MeV alpha particles, which is independently confirmed by an experiment using alpha particles from an 241Am source. The described method is expected to be useful for research into nuclear reactions in laser plasma.

Resonance strengths in the N14 ( p,γ)O15 astrophysical key reaction measured with activation

The 14N(p,gamma)15O reaction plays a vital role in various astrophysical scenarios. Its reaction rate must be accurately known in the present era of high precision astrophysics. The cross section of the reaction is often measured relative to a low energy resonance, the strength of which must therefore be determined precisely. The activation method, based on the measurement of 15O decay, has not been used in modern measurements of the 14N(p,gamma)15O reaction. The aim of the present work is to provide strength data for two resonances in the 14N(p,gamma)15O reaction using the activation method. The obtained values are largely independent from previous data measured by in-beam gamma-spectroscopy and are free from some of their systematic uncertainties. Solid state TiN targets were irradiated with a proton beam provided by the Tandetron accelerator of Atomki using a cyclic activation. The decay of the produced 15O isotopes was measured by detecting the 511 keV positron annihilation gamma-rays. The strength of the Ep = 278 keV resonance was measured to be 13.4 +- 0.8 meV while for the Ep = 1058 keV resonance the strength is 442 +- 27 meV. The obtained Ep = 278 keV resonance strength is in fair agreement with the values recommended by two recent works. On the other hand, the Ep = 1058 keV resonance strength is about 20% higher than the previous value. The discrepancy may be caused in part by a previously neglected finite target thickness correction. As only the low energy resonance is used as a normalization point for cross section measurements, the calculated astrophysical reaction rate of the 14N(p,gamma)15O reaction and therefore the astrophysical consequences are not changed by the present results.

The determination of the astrophysical S-factor of the direct 18O(p,$\gamma$)19F capture by the ANC method

The depletion of 18O via the $(\mathrm{p},\gamma)$ capture competes with the $(\mathrm{p}, \alpha)$ capture during the CNO cycles in AGB stars. Despite the fact that the $(\mathrm{p}, \alpha)$ capture is dominant, the $ (\mathrm{p}, \gamma)$ can play an important role in mixing stages of the AGB star evolution. Here, we attempt to determine the astrophysical S-factor of the direct part of the 18O(p,$\gamma$)19F capture by the indirect method of asymptotic normalization coefficients (ANC). We measure the differential cross section of the transfer reaction 18O(3He, d)19F at a 3He energy of 24.6 MeV. The measurement was performed on the cyclotron of the CANAM infrastructure of NPI in Řež, Czech Republic, with the gas target consisting of the high-purity 18O (99.9%). The reaction products were measured by eight $ \Delta E$-E telescopes composed of thin and thick silicon surface-barrier detectors. The parameters of the optical model for the input channel were deduced by means of the code ECIS and the analysis of transfer reactions to 12 levels of the 19F nucleus up to 8.014 MeV was made by the code FRESCO. The deduced ANCs were then used to specify the direct contribution to the 18O(p,$\gamma$)19F capture process and were compared with the mutually different results of two works.

DFMSPH19: A C-code for the double folding interaction potential of two spherical nuclei

DFMSPH19: A C-code for the double folding interaction potential of two spherical nuclei

Benchmarking the natSi(p,p0) and natO(d,d0) elastic scattering and the 16O(d,p0,1,α0) reactions for IBA purposes

The benchmarking experimental procedure in IBA (Ion Beam Analysis) is carefully designed in order to validate evaluated and experimental differential cross–section datasets of charged particles via the acquisition of EBS and NRA spectra from thick targets of known composition, followed by their simulations. In the present work, such benchmarking measurements have been performed at the laboratory of the Institute for Nuclear and Particle Physics “Demokritos”, for the elastic scattering of protons on natSi in the energy range of 1.1 – 3.5 MeV at four backward angles, at 140o, 150o, 160o and 170o. In addition, measurements were performed for the elastic scattering of deuterons on 16O in the energy range of 1.1 – 1.7 MeV at four backward angles, at 140o, 150o, 165o and 170o. More specifically, a thick non–polished Si target with Au evaporated on top and a Nb2O5 tablet were used. The spectra acquired were compared with simulated ones using the SIMNRA program along with the evaluated differential cross-section datasets from IBANDL. All the experimental parameters were thoroughly investigated. The obtained results, the observed discrepancies and the encountered problems during the benchmarking process are discussed and analyzed.