16O Reaction Research Articles

Probing α-cluster distribution via α-transfer reaction

The α-transfer reaction 16O(6Li, d)20Ne is investigated in order to extract the α- clustering probability of 20Ne. The relative wave function for the α-16O system is calculated by a microscopic cluster model. We show the angular distribution of the transfer cross section is a good probe to see the radial dependence of the α-clustering probability in the surface region.

Evaluation of the implementation of the R-matrix formalism with reference to the astrophysically important 18F(p,α)15O reaction

Background. The R-Matrix formalism is a crucial tool in the study of nuclear astrophysics reactions, and many codes have been written to implement the relevant mathematics. One such code makes use of Visual Basic macros. A further open-source code, AZURE, written in the FORTRAN programming language is available from the JINA collaboration and a C++ version, AZURE2, has recently become available.Purpose The detailed mathematics and extensive programming required to implement broadly applicable R-Matrix codes make comparisons between different codes highly desirable in order to check for errors. This paper presents a comparison of the three codes based around data and recent results of the astrophysically important 18F(p,α)15O reaction.Methods Using the same analysis techniques as in the work of Mountford et al. parameters are extracted from the two JINA codes, and the resulting cross-sections are compared. This includes both refitting data with each code and making low-energy extrapolations.Results All extracted parameters are shown to be broadly consistent between the three codes and the resulting calculations are in good agreement barring a known low-energy problem in the original AZURE code.Conclusion The three codes are shown to be broadly consistent with each other and equally valid in the study of astrophysical reactions, although one must be careful when considering low lying, narrow resonances which can be problematic when integrating.

Astrophysical S-factor of the 12C(α, γ) 16O reaction at solar energies

The astrophysical S-factor of the 4He+12C radiative capture is calculated in the potential model at the energy range 0.1–2.0 MeV. Radiative capture 12C(α, γ) 16O is extremely relevant for the fate of massive stars and determines if the remnant of a supernova explosion becomes a black hole or a neutron star. Because this reaction occurs at low energies, the experimental measurements are very difficult and perhaps impossible. In this paper, radiative capture of the 12C(α, γ) 16O reaction at very low energies is taken as a case study. In comparison with other theoretical methods and available experimental data, good agreement is achieved for the astrophysical S-factor of this process.

Selectivity of the 12C(18O, 16O)14C reaction

A study about the 12C(18O, 16O)14C two-neutron transfer reaction was performed at the Catania INFN-LNS laboratory at 84 MeV incident energy. The 16O ejectiles produced in the reactions were momentum analyzed and identified by the MAGNEX spectrometer. The Q-value spectrum of 14C shows several known bound and resonant states, in particular states with 2p-4h configuration respect to the 16O core. The integrated cross sections show an enhanced yield for the two-neutron transfer compared to the one-neutron transfer. These results are some experimental evidences that the (18O, 16O) reaction proceeds mainly by the direct transfer of the neutron pair, instead of a second order process.

Transfer to the continuum of 14C via (18O, 16O) reaction

A study of the continuum of 14C populated by the 12C(18O, 16O)14C reaction at 84 MeV incident energy has been performed. The ejectiles have been momentum analyzed at forward angles by the MAGNEX magnetic spectrometer. The energy spectra were obtained up to about 20 MeV excitation energy. The scattering of two neutrons independently removed from the projectile as it passes the target nucleus has been described by means of an optical potential with a semi-classical approximation for the relative motion. The calculations describe a significant part of the continuum observed in the energy spectra. In particular, a resonance of the residual nucleus dominates the region near the two-neutron emission threshold.

Delta-isobar configurations in 12C and 16O nuclei

The differential yield of the reaction 16O(γ, π + p) at the maximum photon energy of 450 MeV was measured in the region of high momentum transfers to the residual nuclear system. The experimental results obtained for the 16O nucleus and the cross section measured earlier for the reaction 12C(γ, π + p) were analyzed on the basis of a model that takes into account the admixture of isobar states in the nuclear wave function. The probabilities for the delta-isobar configurations in the ground states of the carbon and oxygen nuclei per nucleon were estimated empirically at 0.012 ± 0.003 ± 0.002 and 0.019 ± 0.003 ± 0.003, respectively.

Systematic study of deformation effects on fusion cross-sections using various proximity potentials

Abstract The influence of static quadrupole and hexadecapole (positive & negative) deformation of targets are studied using eleven different versions of nuclear potentials. The height and position of the interaction barrier for the reactions induced by spherical projectile (16O) on the deformed targets such as 166Er, 154Sm and 176Yb have been estimated. It is found that the nucleus-nucleus potential strongly depends on the value of the deformation parameters and orientation of the target. The experimental fusion cross-section of the reactions 16O + 176Yb, 16O +166Er and 16O +154Sm are investigated by applyingWong’s formula using various parameterizations of the proximity potential as well as an assessment of the results of a multi-dimensional barrier penetration model (BPM). The fusion cross-sections by Prox 77, Prox 88, Prox 00, Prox 00DP, Denisov DP, Bass 80, CW 76 and AW 95 potentials are found to be better than the rest in comparison to experimental data.

Nuclear structure and the fate of core collapse (Type II) supernova

For a long time Gerry Brown and his collaborator Hans Bethe considered the question of the final fate of a core collapse (Type II) supernova. Recalling ideas from nuclear structure on Kaon condensate and a soft equation of state of the dense nuclear matter they concluded that progenitor stars with mass as low as 17–18M⊙ (including supernova 1987A) could collapse to a small mass black hole with a mass just beyond 1.5M⊙, the upper bound they derive for a neutron star. We discuss another nuclear structure effect that determines the carbon to oxygen ratio (C/O) at the end of helium burning. This ratio also determines the fate of a Type II supernova with a carbon rich progenitor star producing a neutron star and oxygen rich collapsing to a black hole. While the C/O ratio is one of the most important nuclear inputs to stellar evolution it is still not known with sufficient accuracy. We discuss future efforts to measure with gamma-beam and TPC detector of the C12(α,γ)O16 reaction that determines the C/O ratio in stellar helium burning.

GALACTIC CHEMICAL EVOLUTION AND SOLARs-PROCESS ABUNDANCES: DEPENDENCE ON THE13C-POCKET STRUCTURE

We study the s-process abundances (A > 90) at the epoch of the solar-system formation. AGB yields are computed with an updated neutron capture network and updated initial solar abundances. We confirm our previous results obtained with a Galactic Chemical Evolution (GCE) model: (i) as suggested by the s-process spread observed in disk stars and in presolar meteoritic SiC grains, a weighted average of s-process strengths is needed to reproduce the solar s-distribution of isotopes with A > 130; (ii) an additional contribution (of about 25%) is required in order to represent the solar s-process abundances of isotopes from A = 90 to 130. Furthermore, we investigate the effect of different internal structures of the 13C-pocket, which may affect the efficiency of the 13C(a, n)16O reaction, the major neutron source of the s-process. First, keeping the same 13C profile adopted so far, we modify by a factor of two the mass involved in the pocket; second, we assume a flat 13C profile in the pocket, and we test again the effects of the variation of the mass of the pocket. We find that GCE s-predictions at the epoch of the solar-system formation marginally depend on the size and shape of the 13C-pocket once a different weighted range of 13C-pocket strengths is assumed. We ascertain that, independently of the internal structure of the 13C-pocket, the missing solar-system s-process contribution in the range from A = 90 to 130 remains essentially the same.

Potential of a laser-driven source of characteristic γ-rays for fast detection and identification of nuclear materials

Active interrogation of special nuclear materials (SNM) represents the most promising detection technique in combating their illicit trafficking and diverted use. Among the many interrogation approaches under investigation, the one based on photoinduced fission relies on large prompt and delayed radiation signatures. Pulsed induction (tens of ns) of photofission, in particular, enables to distinguish between prompt and delayed radiation emission, providing additional signatures to identify fissile materials. As the probing radiation, 6- to 7-MeV characteristic γ-rays produced by the 19F(p,αγ)16O reaction are of particular interest; in fact, they have energy sufficient to induce fission in SNM, but insufficient to create significant radioactivity via photonuclear activation, especially in lighter elements. In this study, the feasibility of a pulsed source of characteristic γ-rays based on the interaction of laser-accelerated protons with a fluorine-rich target is assessed. For practical applications, proton energies above 2 MeV, a bunch population of the order of 1014 particles or higher, and a laser repetition rate of the order of 10 Hz are of interest. The development of ultrahigh-power table-top lasers could greatly contribute to the deployment of this kind of interrogation source for a variety of operations.

HEAVY ELEMENTS IN GLOBULAR CLUSTERS: THE ROLE OF ASYMPTOTIC GIANT BRANCH STARS

Recent observations of heavy elements in Globular Clusters reveal intriguing deviations from the standard paradigm of the early galactic nucleosynthesis. If the r-process contamination is a common feature of halo stars, s-process enhancements are found in a few Globular Clusters only. We show that the combined pollution of AGB stars with mass ranging between 3 to 6 M$_\odot$ may account for most of the features of the s-process overabundance in M4 and M22. In these stars, the s process is a mixture of two different neutron-capture nucleosynthesis episodes. The first is due to the 13C(a,n)16O reaction and takes place during the interpulse periods. The second is due to the 22Ne(a,n)25Mg reaction and takes place in the convective zones generated by thermal pulses. The production of the heaviest s elements (from Ba to Pb) requires the first neutron burst, while the second produces large overabundances of light s (Sr, Y, Zr). The first mainly operates in the less-massive AGB stars, while the second dominates in the more-massive. From the heavy-s/light-s ratio, we derive that the pollution phase should last for $150\pm 50$ Myr, a period short enough compared to the formation timescale of the Globular Cluster system, but long enough to explain why the s-process pollution is observed in a few cases only. With few exceptions, our theoretical prediction provides a reasonable reproduction of the observed s-process abundances, from Sr to Hf. However, Ce is probably underproduced by our models, while Rb and Pb are overproduced. Possible solutions are discussed.

Investigation of deep implanted carbon and oxygen channeling profiles in [1 1 0] silicon, using d-NRA and SEM

Ion implantation is one of the most important techniques used in the silicon-based semiconductor industry. Using the ion axial channeling effect, which occurs when an ion beam is oriented along a crystallographic axis, it is theoretically possible to implant ions deeper in the crystal, in comparison with the ‘random’ ion beam-solid orientation, while – at the same time – minimizing the induced crystal lattice damage.In the present work, 4MeV 12C2+ and 5MeV 16O2+ ions were implanted in high-purity [110] Si crystal wafers at fluences of the order of ∼1017particles/cm2, in both the channeling and random orientations. The resulting profiles were measured using d-NRA, i.e. implementing the 12C(d,p0) and 16O(d,p0,α0) reactions respectively, at Ed,lab=1.2–1.4MeV. The results were validated using SEM (Scanning Electron Microscopy), while the extent of crystalline damage was monitored during the implantation via RBS/C (Rutherford Backscattering Spectrometry/Channeling) spectra. The resulting profiles seem to be in good agreement with those obtained in the past for fluorine and nitrogen ions implanted in silicon, and clearly demonstrate the capabilities of high-energy channeling implantations, as well as, the accuracy of d-NRA (Nuclear Reaction Analysis) profiling measurements.

Ambiguities in the rate of oxygen formation during stellar helium burning in the12C(α,γ) reaction

The rate of oxygen formation determines the C/O ratio during stellar helium burning. It is the single most important nuclear input of stellar evolution theory including the evolution of Type II and Type Ia supernova. Yet the low energy cross section of the fusion of 4He + 12C denoted as the 12C(a,g)16O reaction still remains uncertain after forty years of extensive work. We analyze and critically review the most recent measurements of complete angular distributions of the outgoing gamma-rays at very low energies (Ecm > 1.0 MeV). Our analysis of the angular distribution measured with the EUROGAM/GANDI arrays lead us to conclude considerably larger error bars than published hence they are excluded from the current sample of "world data". We show that the current sample of "world data" of the measured E2 cross section factors below 1.7 MeV cluster to two distinct groups that lead to two distinct extrapolations of SE2(300) ~ 60 or ~ 154 keVb. We point to a much neglected discrepancy between the measured E1-E2 phase difference (phi_12) and unitarity as required by the Watson theorem, suggesting systematic problem(s) of some of the measured gamma-ray angular distributions. The ambiguity of the extrapolated SE2(300) together with a previously observed ambiguity of SE1(300) represent the current state of the art of the field and they must be resolved by future measurements of complete and detailed angular distributions of the 12C(a,g) reaction at very low energies (Ecm < 1.0 MeV).

Analysis of the 19F(p, α0)16O reaction at low energies and the spectroscopy of 20Ne

The investigation of the 19F(p, α0) reaction at low bombarding energies allows the study of the spectroscopy of the 20Ne compound nucleus in an energy region where the existence of quartet excitations has been suggested in the literature. Moreover, this reaction plays a major role in the fourth branch of the CNO cycle since it is relevant for the correct description of the hydrogen burning of fluorine in stars. For these reasons, we decided to investigate the 19F(p, α0) reaction in the Ep ≃ 0.6–1 MeV energy range. The analysis of angular distributions and excitation functions allows one to improve the 20Ne spectroscopy in an excitation energy region where some ambiguities concerning Jπ assignments exist in the literature. In particular, the present data suggest a Jπ = 0+ assignment to the Ex = 13.642 MeV resonance. For this state, both partial and reduced widths for the α0 channel have been deduced. The trend of the astrophysical factor has been obtained from the integrated cross section. A comparison of the present results with data reported in the literature is also discussed.

ON THE MEASUREMENT OF THE13C(α,n)16OS-FACTOR AT NEGATIVE ENERGIES AND ITS INFLUENCE ON THEs-PROCESS

The 13C(?, n)16O reaction is the neutron source for the main component of the s-process, responsible for the production of most of the nuclei in the mass range 90 A 208. This reaction takes place inside the helium-burning shell of asymptotic giant branch stars, at temperatures 108?K, corresponding to an energy interval where the 13C(?, n)16O reaction is effective in the range of 140-230?keV. In this regime, the astrophysical S(E)-factor is dominated by the ?3?keV sub-threshold resonance due to the 6.356?MeV level in 17O, giving rise to a steep increase in the S-factor. Its contribution is still controversial as extrapolations, e.g., through the R-matrix and indirect techniques such as the asymptotic normalization coefficient (ANC), yield inconsistent results. The discrepancy amounts to a factor of three or more precisely at astrophysical energies. To provide a more accurate S-factor at these energies, we have applied the Trojan horse method (THM) to the 13C(6Li, n 16O)d quasi-free reaction. The ANC for the 6.356?MeV level has been deduced through the THM as well as the n-partial width, allowing us to attain unprecedented accuracy for the 13C(?, n)16O astrophysical factor. A larger ANC for the 6.356?MeV level is measured with respect to the ones in the literature, ?fm?1, yet in agreement with the preliminary result given in our preceding letter, indicating an increase of the 13C(?, n)16O reaction rate below about 8 ? 107?K if compared with the recommended values. At ~108?K, our reaction rate agrees with most of the results in the literature and the accuracy is greatly enhanced thanks to this innovative approach.

Re-examination of constraints on the Maxwell-Boltzmann distribution by Helioseismology

Nuclear reactions in stars occur between nuclei in the high-energy tail of the energy distribution and are sensitive to possible deviations from the standard equilibrium thermal-energy distribution, the well-known Maxwell-Boltzmann Distribution (MBD). Strong constraints on such deviations were made previously with the detailed helioseismic information of the solar structure. With a small deviation parameterized with a factor exp[−δ(E/kT)2], it was shown δ restricted between −0.005 and +0.002. These constraints have been carefully re-examined in the present work. We find that a normalization factor was missed in the previous modified MBD. In this work, the normalization factor c is calculated as a function of δ. It shows the factor c is almost unity within the range 0<δ≤0.002, which supports the previous conclusion. However, it demonstrates that δ cannot take a negative value from the normalization point of view. As a result, a stronger constraint on δ is defined as 0≤ δ ≤0.002. The astrophysical implication on the solar neutrino fluxes is simply discussed based on a positive δ value of 0.003. The reduction of the 7Be and 8B neutrino fluxes expected from the modified MBD can possibly shed alternative light on the solar neutrino problem. In addition, the resonant reaction rates for the 14N(p, γ)15O reaction are calculated with a standard MBD and a modified MBD, respectively. It shows that the rates are quite sensitive even to a very small δ. This work demonstrates the importance and necessity of experimental verification or testing of the well-known MBD at high temperatures.

Calculating Method and Characteristics of the Distribution of Neutron Exposures in the Radiative S-process Nucleosynthesis Model for AGB Stars

A study on the distribution of neutron exposures in a low-mass asymptotic giant branch (AGB) star is presented, according to the s-process nucleosynthesis model with the 12C(α, n)16O reaction occurred under radiative conditions in the interpulse phases. The model parameters, such as the over- lap factor r of two successive convective thermal pulses, the mass ratio q of the 13C shell with respect to the He intershell, and the effective mass of 13C in the 13C shell, vary with the pulse number. Considering these factors, a calculating method for the distribution of neutron exposures in the He intershell has been presented. This method has the features of simplicity and universality. Using this method, the exposure distribution for the stellar model of a star with the mass of 3 Mʘ and the solar metallicity has been calculated. The results suggest that under the reasonable assumption that the number density of neutrons is uniform in the 13C shell, the final exposure distribution approaches to an exponential distribution. For a stellar model with the definite initial mass and metallicity, there is a definite relation between the mean neutron exposure τ0 and the neutron exposure Δτ of each pulse, namely τ0 = 0.434λ(q1, q2, …, qmmax +1, …, r1, r2, …, rmmax +1)Δτ, where mmax is the total number of thermal pulses with the third dredge-up episode, and the proportional coeffcient λ(q1, q2, …, qmmax +1, …, r1, r2, …, rmmax +1) can be determined by an exponential curve fitting to the final exposure distribution. This new formula quantitatively unifies the classical model with the s-process nu- cleosynthesis model by means of neutron exposure distribution, and makes the classical model continue to offer guidance and constraints to the s-process nu- merical calculations of stellar models.

Structure of unbound neutron-rich9He studied using single-neutron transfer

The 8He(d,p) reaction was studied in inverse kinematics at 15.4A MeV using the MUST2 Si-CsI array in order to shed light on the level structure of 9He. The well known 16O(d,p)17O reaction, performed here in reverse kinematics, was used as a test to validate the experimental methods. The 9He missing mass spectrum was deduced from the kinetic energies and emission angles of the recoiling protons. Several structures were observed above the neutron-emission threshold and the angular distributions were used to deduce the multipolarity of the transitions. This work confirms that the ground state of 9He is located very close to the neutron threshold of 8He and supports the occurrence of parity inversion in 9He.

Probing effect of tensor interactions in 16O via (p, d) reaction

We have measured the 16O(p, d) reaction using 198-, 295- and 392-MeV proton beams to search for a direct evidence on an effect of the tensor interactions in light nucleus. Differential cross sections of the one-neutron transfer reaction populating the ground states and several low-lying excited states in 15O were measured. Comparing the ratios of the cross sections for each excited state to the one for the ground state over a wide range of momentum transfer, we found a marked enhancement of the ratio for the positive-parity state(s). The observation is consistent with large components of high-momentum neutrons in the initial ground-state configurations due to the tensor interactions.

Experimental study for the production cross sections of positron emitters induced from 12C and 16O nuclei by low-energy proton beams

In proton therapy, positron emitters are induced from 12C and 16O nuclei by protons on the beam path in the patient. Many studies for monitoring positron emitters with beam-induced PET technique have been performed by various groups to verify the proton beam range and the dose in the patient for quality assurance (QA). The QA methods proposed by some groups require accurate production cross sections of the positron emitters produced by protons, especially in the low-energy region. The aim of this study was to develop a method for measuring the production cross sections of positron emitters using standard equipment for proton therapy, and to measure the cross sections of positron emitters produced by low-energy protons and verify them in comparison with data of previous experiments. An 80-MeV proton beam was produced by a synchrotron, and the energy was degraded by polyethylene blocks to obtain various beam energies. The number of protons was estimated from the charge induced in a parallel-plate ionization chamber by protons. Low-energy protons of 14–70 MeV were used to bombard 12C-rich and 16O-rich target materials: namely, polyethylene and gelatinous water. The time-activity curve was then measured by a high-sensitivity PET scanner to extract the number of positron emitters produced in the target. The production cross sections for four reaction channels: 16O(p, pn)15O, 16O(p, 3p3n)11C, 16O(p, 2p2n)13N, and 12C(p, pn)11C were then measured. The cross sections for the 16O(p, pn)15O reaction channel were consistent with data of previous experiments within the uncertainties, while those of 12C(p, pn)11C were generally lower than data of previous experiments. These results suggested that further measurements of the production cross sections will be necessary.