16O Reaction Research Articles

Contribution from Mechanisms of Direct Nucleon Transfer to Proton Spectra in the 15N (α, p)18O Reaction

The differential cross section for the (α, p)18O reaction at an α-particle energy of 30 MeV is analyzed in the context of direct reaction mechanisms and the compound nucleus model. Spectroscopic amplitudes are calculated at the vertices of the diagrams illustrating triton and 14С cluster transfer reactions, including cases of mixed configurations of 1p–2d shells. The OP parameters of α–15N and p–18O interaction are found that provide the best agreement between the calculated and experimental data at an energy of ~30 MeV. It is shown that the main contribution to the production of protons comes from direct triton cluster transfer. The role of the heavy 14С cluster transfer mechanism and the compound nucleus mechanism is notable in the region of angles greater than 90°.

The Importance of the 13C(α,n)16O Reaction in Asymptotic Giant Branch Stars

Abstract Low-mass asymptotic giant branch stars are among the most important polluters of the interstellar medium. In their interiors, the main component (A ≳ 90) of the slow neutron capture process (the s-process) is synthesized, the most important neutron source being the 13C(α,n)16O reaction. In this paper, we review its current experimental status, discussing possible future synergies between some experiments currently focused on the determination of its rate. Moreover, in order to determine the level of precision needed to fully characterize this reaction, we present a theoretical sensitivity study, carried out with the FUNS evolutionary stellar code and the NEWTON post-process code. We modify the rate up to a factor of 2 with respect to a reference case. We find that variations of the 13C(α,n)16O rate do not appreciably affect s-process distributions for masses above 3 M ⊙ at any metallicity. Apart from a few isotopes, in fact, the differences are always below 5%. The situation is completely different if some 13C burns in a convective environment: this occurs in FUNS models with M < 3 M ⊙ at solar-like metallicities. In this case, a change of the 13C(α,n)16O reaction rate leads to nonnegligible variations of the element surface distribution (10% on average), with larger peaks for some elements (such as rubidium) and neutron-rich isotopes (such as 86Kr and 96Zr). Larger variations are found in low-mass, low-metallicity models if protons are mixed and burned at very high temperatures. In this case, the surface abundances of the heavier elements may vary by more than a factor of 50.

Coupled-channel calculation for cross section of fusion and barrier distribution of O 16 , 17 , 18 + 16 O ${}^{16,17,18}\mbox{O} + {}^{16}\mbox{O}$ reactions

In this work, the effect of multi-phonon excitation on heavy-ion fusion reactions has been studied and fusion barrier distributions of energy intervals near and below the Coulomb barrier have been studied for 16,17,18O + 16O reactions. The structure and deformation of nuclear projectiles have been studied. Given the adaptation of computations to experimental data, our calculations predict the behavior of reactions in intervals of energy in which experimental measurements are not available. In addition the S-factor for these reactions has been calculated. The results showed that the structure and deformation of a nuclear projectile are important factors. The S-factor, obtained in the coupled-channel calculations for the ${}^{16}\mbox{O} + {}^{16}\mbox{O}$ , ${}^{17}\mbox{O} +{}^{16}\mbox{O}$ and ${}^{18}\mbox{O} +{}^{16}\mbox{O}$ reactions, showed good agreement with the experimental data and had a maximum value at an energy near 5, 4.5 and 4 MeV, respectively.

A STUDY ON 4 REACTIONS FORMING 46Ti*

The NUCL-EX collaboration is carrying out an extensive research program on preequilibrium emission of light charged particles from hot nuclei. The ultimate goal is to study how cluster structures affect nuclear reactions [1,2,3,4]. Indeed, a strong correlation between nuclear structure and reaction dynamics emerges when some nucleons or clusters of nucleons are emitted or captured [5]. At this purpose, the four reactions 16O+30Si, 16O+30Si, 18O+28Si and 19F +27Al have been measured at about 120 MeV projectile energy. Experimental data were collected at Legnaro National Laboratories, using the GARFIELD+RCo array, fully equipped with digital electronics [6]. Following an initial identification of particles and the energy calibration procedures, the complete analysis is being performed on an event-by-event basis. Experimental data are then compared to the theoretical predictions where events are generated by numerical codes based on pre-equilibrium and statistical models and then filtered through a software replica of the setup. Differences between the experimental data and the predicted data put into evidence effects related to the entrance channel and to the cluster nature of the colliding ions. After a general introduction on the experimental campaign, this contribution will focus on the preliminary results obtained so far.

Theoretical cross sections in the 12C(α,γ)16O reaction

The 12C(α,γ)16O reaction seems to be explained by the simple reaction mechanism of the direct-capture potential model. The theoretical cross section at Ec.m. = 0.3 MeV is found to be enhanced with the E2 transition. This is caused by the tail of the subthreshold state (Ex = 6.92 MeV). The total S-factor is consistent with the previous studies. The photodisintegration of 16O is also found to be dominated by the E2 excitation in the vicinity of the α-particle threshold.

Reprint of: A supersonic jet target for the cross section measurement of the 12C(α, γ)16O reaction with the recoil mass separator ERNA

Reprint of: A supersonic jet target for the cross section measurement of the 12C(α, γ)16O reaction with the recoil mass separator ERNA

Thermonuclear 19F(p, )16O reaction rate**Supported by National Natural Science Foundation of China (11490562, 11490560, 11675229) and National Key Research and Development Program of China (2016YFA0400503)

The thermonuclear 19F(p, )16O reaction rate in the temperature region 0.007–10 GK has been derived by re-evaluating the available experimental data, together with the low-energy theoretical R-matrix extrapolations. Our new rate deviates by up to about 30% compared to the previous results, although all rates are consistent within the uncertainties. At very low temperature (e.g. 0.01 GK) our reaction rate is about 20% lower than the most recently published rate, because of a difference in the low energy extrapolated S-factor and a more accurate estimate of the reduced mass used in the calculation of the reaction rate. At temperatures above ∼1 GK, our rate is lower, for instance, by about 20% around 1.75 GK, because we have re-evaluated the previous data (Isoya et al., Nucl. Phys. 7, 116 (1958)) in a meticulous way. The present interpretation is supported by the direct experimental data. The uncertainties of the present evaluated rate are estimated to be about 20% in the temperature region below 0.2 GK, and are mainly caused by the lack of low-energy experimental data and the large uncertainties in the existing data. Asymptotic giant branch (AGB) stars evolve at temperatures below 0.2 GK, where the 19F(p, )16O reaction may play a very important role. However, the current accuracy of the reaction rate is insufficient to help to describe, in a careful way, the fluorine over-abundances observed in AGB stars. Precise cross section (or S factor) data in the low energy region are therefore needed for astrophysical nucleosynthesis studies.

New direct investigation of the 19F(p,α0)16O down to 0.2 MeV

We discuss new results concerning the investigation of the 19F(p,α0)16O reaction at very low energies (0.6-0.2 MeV). This reaction is important both in Nuclear Astrophysics, because it closes the CNOF cycle and it is an important fluorine destruction channel in hydrogen rich stellar environments, and in Nuclear Structure, because it allows to study the spectroscopy of the self-conjugated 20Ne compound nucleus. Despite its importance, very few direct data have been published at low energies in the literature. Our work allowed to deduce the S-factor down to ≈ 0.2 MeV and to point out the role played by broad resonances at low energies. The experimental S-factor is ≈1.5-2 times larger than non-resonant extrapolations reported by nacre with potentially important astrophysical consequences.

Study of fission using multi-nucleon transfer reactions

Multi-nucleon transfer channels of the reactions of 18O+232Th, 18O+238U, 18O+248Cm were used to measure fission-fragment mass distribution for various nuclides and their excitation energy dependence. Predominantly asymmetric fission is observed at low excitation energies for all the studied cases, with an increase of the symmetric fission towards high excitation energies. Experimental data are compared with predictions of the fluctuation-dissipation model, where effects of multi-chance fission (neutron evaporation prior to fission) was introduced. It was shown that a reliable understanding of the observed fission fragment mass distributions can be obtained only invoking multi-chance fissions.

New perspectives for undoped CaF2 scintillator as a threshold activation neutron detector

In this paper we present the prompt photofission neutron detection performance of undoped CaF2 scintillator using Threshold Activation Detection (TAD). The study is carried out in the frame of C-BORD Horizon 2020 project, during which an efficient toolbox for high volume freight non-intrusive inspection (NII) is under development. Technologies for radiation monitoring are the part of the project. Particularly, detection of various radiological threats on country borders plays an important significant role in Homeland Security applications. Detection of illegal transfer of Special Nuclear Material (SNM) - 235U, 233U and 239Pu - is particular due to the potential use for production of nuclear weapon as well as radiological dispersal device (RDD) V known also as a “dirty bomb”. This technique relies on activation of 19F nuclei in the scintillator medium by fast neutrons and registration of high-energy β particles and γ-rays from the decay of reaction products. The radiation from SNM is detected after irradiation in order to avoid detector blinding. Despite the low 19F(n,α)16N or 19F(n,p)19O reaction cross-section, the method could be a good solution for detection of shielded nuclear material. Results obtained with the CaF2 detector were compared with the previous study done for BaF2 and 3He detector. These experimental results were obtained using 252Cf source and 9 MeV Varian Linatron M9 linear accelerator (LINAC). Finally, performance of the prompt neutron detection system based on CaF2 will be validated at Rotterdam Seaport during field trails in 2018.

Formation of ozone by solid state reactions.

We studied the isotopic composition of ozone formed at low (3-10 K) temperature via O + O2 solid state reactions using a partially dissociated 16O/16O2 : 18O/18O2 = 1 : 1 mixture. The ozone ice has an isotopic abundance that differs from the statistical one and from gas phase studies. Ozone formation is influenced by the competition of the production of O2 (O + O or O + O3) vs. O3 (O + O2) and by the energy released in the O + O reaction. The exothermicity of the O + O reaction helps to overcome the barrier of the O + O2 reaction. Heating the ozone ice past 50 K brings about a transformation from amorphous to crystalline ice. The formation of ozone on water ice yields a blue shift of IR bands, and the yield of formed O3 increases up to the sample temperature of 100 K. When 18O/18O2 is deposited on H216O ice, formation of 18O18O16O is detected. We propose that the exothermicity of the reaction 18O + 18O drives water dissociation (16O + H2) followed by ozone formation (16O + 18O2 → 16O18O18O).

Evidence for O15+α resonance structures in Ne19 via direct measurement

States in the astrophysically important Ne19 nucleus have been populated directly using the O15(α,α)O15 reaction with a radioactive O15 beam. Due to the challenges involved, this represents the first such O15 measurement with high resolution and the first dedicated to exploring the cluster structure. This measurement comprises three experiments: the first to develop the O15 radioactive beam, the second the resonant scattering study, and the third to measure the precise energy loss of O ions in He4 to reduce systematic uncertainties. The results show evidence for O15+α cluster structure which could explain earlier results pointing to the increased α branching ratio in the astrophysical reaction rate for high-spin near-threshold resonances. Absolute differential cross sections have been measured. Additionally, spins, parities, and partial decay widths, essential for nuclear reaction-rate calculations, have been extracted using an R-matrix analysis.Received 25 July 2016Revised 3 August 2017DOI:https://doi.org/10.1103/PhysRevC.96.044317©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAlpha decayCluster modelsDirect reactionsH & He induced nuclear reactionsNuclear spin & parityNuclear structure & decaysUnstable nuclei induced nuclear reactionsProperties6 ≤ A ≤ 19Nuclear Physics

The 95Zr(n, γ)96Zr Cross Section from the Surrogate Ratio Method and Its Effect on s-process Nucleosynthesis

Abstract The 95Zr(n, γ)96Zr reaction cross section is crucial in the modeling of s-process nucleosynthesis in asymptotic giant branch stars because it controls the operation of the branching point at the unstable 95Zr and the subsequent production of 96Zr. We have carried out the measurement of the 94Zr(18O, 16O) and 90Zr(18O, 16O) reactions and obtained the γ-decay probability ratio of 96Zr* and 92Zr* to determine the 95Zr(n, γ)96Zr reaction cross sections with the surrogate ratio method. Our deduced Maxwellian-averaged cross section of 66 ± 16 mb at 30 keV is close to the value recommended by Bao et al., but 30% and more than a factor of two larger than the values proposed by Toukan & Käppeler and Lugaro et al., respectively, and routinely used in s-process models. We tested the new rate in stellar models with masses between 2 and 6 M ⊙ and metallicities of 0.014 and 0.03. The largest changes—up to 80% variations in 96Zr—are seen in models of mass 3–4 M ⊙, where the 22Ne neutron source is mildly activated. The new rate can still provide a match to data from meteoritic stardust silicon carbide grains, provided that the maximum mass of the parent stars is below 4 M ⊙, for a metallicity of 0.03.

Analysis of intermediate and light mass fragments from composite systems 26–29Al⁎ formed in [formula omitted] reactions

A comparative analysis of intermediate mass fragments (IMFs) and light particles (LPs) [equivalently, evaporation residues (ERs)] in the decay of 26,27,28,29Al⁎ has been carried out for reactions O16+10,11B and O18+10,11B at the energy range 1 MeV≤E/A≤4 MeV, within dynamical cluster decay model (DCM) of collective clusterisation approach. We find that, for IMFs having Z=3,4,5 and 6, the range of mass minima are ∑i=58Ai, ∑i=89Ai, ∑i=913Ai and ∑i=1115Ai, respectively. With increase in the mass of compound nuclei, Z=6 becomes more prominent as compared to its neighboring fragments which is also observed in the experimental data. The LPs cross sections (σLPs) in the decay of 26–29Al⁎ are also estimated. We find that the value of ∑P0 of LPs for 26,27Al⁎ is less as compared to that for 28,29Al⁎, contrary to the value of ∑P. Eventually, the cross sections for LPs are having higher contribution in the decay of 28,29Al⁎ in agreement with the experimental data. As expected for light mass nuclei, the σLPs for 26,27Al⁎ find better comparison with experimental data for spherical considerations rather than β2 deformations. We also conform Bohr's hypothesis via the entrance channel independence for compound nucleus 28Al⁎ formed through O18+10B and F19+9Be channels at an excitation energy ECN⁎∼44 MeV, by having a unique choice of neck length parameter ΔR for both the reactions.

Differential cross section for the 16O(t, p)18O reaction and determination of the size of the two-neutron periphery in the 18O nucleus

Within the theoretical formalism that combines a four-body problem with themultiparticle shell model, it is shown that the cross section for the dineuteron-stripping mechanism is consistent with the experimental angular distribution of protons from the 16O(t, p)18O reaction. This makes it possible to find the wave function for the relative motion of the dineutron and 16O and to obtain thereby the probability density W(r) for the dineutron in 18O, the nn–16O interaction potential, and the root-mean-square distance 〈L〉 nn between the dineutron and 16O. The respective calculations reveal that, at r ≈ 8 fm, the dineutron probability density and a rather deep nn–16O potential become negligible, which leads to the absence of a dineuntron periphery in 18O. It seems that one can explain this fact by a rather large value (12.19 MeV) of the dineutron binding energy in this nucleus. Thus, the 18O nucleus is quite compact an object, despite the excess of two neutrons, and has a neutron skin rather than a periphery.

New Improved Indirect Measurement of the 19F(p, α)16O Reaction at Energies of Astrophysical Relevance

Abstract Fluorine abundance determination is of great importance in stellar physics to understand s-elements production and mixing processes in asymptotic giant branch (AGB) stars. Up to now, theoretical models overproduce F abundances in AGB stars with respect to the observed values, thus calling for further investigation of the reactions involving fluorine. In particular, the 19F(p, α)16O reaction is the main destruction channel of fluorine at the bottom of the convective envelope in AGB stars, an H-rich environment where it can experience temperatures high enough to determine its destruction, owing to additional mixing processes. In this paper the Trojan horse method (THM) was used to extract the 19F(p, α 0)16O S-factor in the energy range of astrophysical interest (E cm ≈ 0–1 MeV). This is the most relevant channel at the low temperatures (few 107 K) characterizing the bottom of the convective envelope, according to current knowledge. A previous indirect experiment using the THM has observed three resonances in the energy regions below E cm ≈ 450 keV. These energies correspond to typical AGB temperatures, thus implying a significant increase in the reaction rate. Statistics are scarce for performing an accurate separation between resonances, preventing one from drawing a quantitative conclusion about their total widths and spin parities. Before THM measurement, only extrapolations were available below about 500 keV, showing a non-resonant behavior that sharply contradicts the trend of the astrophysical factor at higher energies. A new experiment has been performed to verify the measured TH astrophysical factor and to perform more accurate spectroscopy of the involved resonances.

A supersonic jet target for the cross section measurement of the 12C(α, γ)16O reaction with the recoil mass separator ERNA

12C(α, γ)16O cross section plays a key-role in the stellar evolution and nucleosynthesis of massive stars. Hence, it must be determined with the precision of about 10% at the relevant Gamow energy of 300keV. The ERNA (European Recoil mass separator for Nuclear Astrophysics) collaboration measured, for the first time, the total cross section of 12C(α, γ)16O by means of the direct detection of the 16O ions produced in the reaction down to an energy of Ecm=1.9MeV. To extend the measurement at lower energy, it is necessary to limit the extension of the He gas target. This can be achieved using a supersonic jet, where the oblique shock waves and expansion fans formed at its boundaries confine the gas, which can be efficiently collected using a catcher. A test version of such a system has been designed, constructed and experimentally characterized as a bench mark for a full numerical simulation using FV (Finite Volume) methods. The results of the commissioning of the jet test version and the design of the new system that will be used in combination with ERNA are presented and discussed.

On the investigation of resonances above and below the threshold in nuclear reactions of astrophysical interest using the Trojan Horse Method.

The occurrence of resonances in reactions of astrophysical interest might significantly enhance the astrophysical factor with respect to the direct reaction contribution, divert nucleosynthesis path and change the energy production, with significant impact on astrophysics. Moreover, the determination of resonance parameters, that is, energy, spin-parity and partial widths, allows one to perform nuclear structure studies leading, for instance, to determine the cluster structure of the state under investigation. However, nuclear reactions in stars take place at energies well below ~ 1 MeV owing to the typical temperatures characterising these environments. Therefore, the Coulomb barrier exponentially suppressing the cross section and the electron screening effect, due to the shielding of nuclear charges by atomic electrons, make it very difficult to provide accurate astrophysical factors. The THM is an indirect method allowing to overcome such difficulties. It 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 that A shows a strong x + s cluster structure, right at astrophysical energies. By using a generalised R-matrix approach, the resonance parameters can be deduced from THM data allowing one to perform a full spectroscopic study of low-energy and sub-threshold resonances. In this work, we will discuss two examples of reactions of astrophysical interest, whose cross sections show a resonant behaviour: the19F(p, α)16O cross section that displays resonances at energies above the particle emission threshold and the13C(α, n)16O reaction, dominated by the −3 keV sub-threshold resonance due to the 6.356 MeV level in17O.

Observational Evidence for High Neutronization in Supernova Remnants: Implications for Type Ia Supernova Progenitors

Abstract The physical process whereby a carbon–oxygen white dwarf explodes as a Type Ia supernova (SN Ia) remains highly uncertain. The degree of neutronization in SN Ia ejecta holds clues to this process because it depends on the mass and the metallicity of the stellar progenitor, and on the thermodynamic history prior to the explosion. We report on a new method to determine ejecta neutronization using Ca and S lines in the X-ray spectra of Type Ia supernova remnants (SNRs). Applying this method to Suzaku data of Tycho, Kepler, 3C 397, and G337.2−0.7 in the Milky Way, and N103B in the Large Magellanic Cloud, we find that the neutronization of the ejecta in N103B is comparable to that of Tycho and Kepler, which suggests that progenitor metallicity is not the only source of neutronization in SNe Ia. We then use a grid of SN Ia explosion models to infer the metallicities of the stellar progenitors of our SNRs. The implied metallicities of 3C 397, G337.2−0.7, and N103B are major outliers compared to the local stellar metallicity distribution functions, indicating that progenitor metallicity can be ruled out as the origin of neutronization for these SNRs. Although the relationship between ejecta neutronization and equivalent progenitor metallicity is subject to uncertainties stemming from the 12C + 16O reaction rate, which affects the Ca/S mass ratio, our main results are not sensitive to these details.

Partial mixing and the formation of 13C pockets in AGB stars: effects on the s-process elements

The production of the elements heavier than iron via slow neutron captures (the s process) is a main feature of the contribution of asymptotic giant branch (AGB) stars of low mass (< 5 Msun) to the chemistry of the cosmos. However, our understanding of the main neutron source, the 13C(alpha,n)16O reaction, is still incomplete. It is commonly assumed that in AGB stars mixing beyond convective borders drives the formation of 13C pockets. However, there is no agreement on the nature of such mixing and free parameters are present. By means of a parametric model we investigate the impact of different mixing functions on the final s-process abundances in low-mass AGB models. Typically, changing the shape of the mixing function or the mass extent of the region affected by the mixing produce the same results. Variations in the relative abundance distribution of the three s-process peaks (Sr, Ba, and Pb) are generally within +/-0.2 dex, similar to the observational error bars. We conclude that other stellar uncertainties - the effect of rotation and of overshoot into the C-O core - play a more important role than the details of the mixing function. The exception is at low metallicity, where the Pb abundance is significantly affected. In relation to the composition observed in stardust SiC grains from AGB stars, the models are relatively close to the data only when assuming the most extreme variation in the mixing profile.