Ne Reaction Research Articles

REACTION RATE AND COMPOSITION DEPENDENCE OF THE STABILITY OF THERMONUCLEAR BURNING ON ACCRETING NEUTRON STARS

The stability of thermonuclear burning of hydrogen and helium accreted onto neutron stars is strongly dependent on the mass accretion rate. The burning behavior is observed to change from Type I X-ray bursts to stable burning, with oscillatory burning occurring at the transition. Simulations predict the transition at a ten times higher mass accretion rate than observed. Using numerical models we investigate how the transition depends on the hydrogen, helium, and CNO mass fractions of the accreted material, as well as on the nuclear reaction rates of triple alpha and the hot-CNO breakout reactions 15O(a,g)19Ne and 18Ne(a,p)21Na. For a lower hydrogen content the transition is at higher accretion rates. Furthermore, most experimentally allowed reaction rate variations change the transition accretion rate by at most 10%. A factor ten decrease of the 15O(a,g)19Ne rate, however, produces an increase of the transition accretion rate of 35%. None of our models reproduce the transition at the observed rate, and depending on the true 15O(a,g)19Ne reaction rate, the actual discrepancy may be substantially larger. We find that the width of the interval of accretion rates with marginally stable burning depends strongly on both composition and reaction rates. Furthermore, close to the stability transition, our models predict that X-ray bursts have extended tails where freshly accreted fuel prolongs nuclear burning.

Neon diffusion in olivine and quartz

Lattice diffusion of neon in single, gem-quality crystals of quartz, forsterite, and natural iron-bearing olivine was determined from experiments using two different methods to introduce Ne. In the first set of experiments, 22Ne was implanted into polished samples, which were then annealed for various times and temperatures in 1-atm furnaces. In the second group of experiments, referred to as soaking experiments, polished slabs of crystals were placed in open containers and inserted into cold-seal type pressure vessels where they were exposed to Ne gas at 340–1340bars Ne pressure and elevated temperatures. Nuclear reaction analysis, using the reactions 22Ne(p,γ)23Na and 20Ne(α,γ)24Mg for the implantation and soaking experiments, respectively, was used to directly measure Ne distributions in the samples following diffusion anneals. The following Arrhenius relations were obtained for Ne diffusion parallel to c:Forsterite:DFo=1.4x10‐13exp‐131±6kJmol‐1/RTm2s‐1Quartz:DQtz=1.6x10‐14exp‐115±4kJmol‐1/RTm2s‐1There are no significant differences in Ne diffusion along the investigated crystallographic axes in either quartz or olivine, and Ne diffusivities for synthetic forsterite and natural Fe-bearing olivine are similar. In addition, diffusivities determined from the soaking experiments and the implantation experiments are similar, indicating little effect on diffusion of any lattice damage that might be induced by the 22Ne ion implantation. The lack of damage is also confirmed by imaging with high-resolution electron microscopy.Ne diffusivities in quartz measured in this study are about 10 orders of magnitude slower than those determined from the bulk-release experiments of Shuster and Farley (2005). In the case of olivine, Ne diffusivities in olivine measured in the present study are 1–3 orders of magnitude slower than diffusivities determined by Futagami et al. (1993) and 2–5 orders of magnitude slower than Ne diffusivities determined by Gourbet et al. (2012) over the temperature range of overlap of the respective studies. In addition, activation energies for diffusion differ widely, with the value from of Futagami et al. (1993) significantly lower (87kJ/mol) than that measured in the present work, and the preferred values of Gourbet et al. (2012) (360–370kJ/mol) significantly higher than our findings. These differences may reflect the influence of factors that may disproportionately affect measurements of Ne diffusion determined through bulk-outgassing methods compared with direct profiling techniques measuring lattice diffusion such as those employed in this study.Our data indicate that both quartz and olivine may be very retentive of Ne at temperature conditions typical near the surface of the earth, with 100μm radius quartz and olivine grains experiencing less than 1% Ne loss over times on order of the age of the earth at 100 and 150°C, respectively. Under mantle conditions, effective diffusion distances for Ne in olivine would be about 1m and 0.2m over 1billion years at 1550 and 1100°C, respectively. In contrast to Ne, helium is much more mobile, with effective diffusion distances of 1500m over 1billion years at 1500°C, compared with 0.8m for Ne under the same time–temperature conditions.

Coriolis coupling effects in exact quantum scattering study of the isotopic reaction dynamics of [formula omitted

To study the Coriolis coupling (CC) effects on the reactions Ne+D2+/T2+, time-dependent quantum wave-packet scattering calculations are performed based on a new three-dimensional potential energy surface constructed by Lv et al. The CC and centrifugal sudden (CS) approximation reaction probabilities, the total reaction cross sections, and the rate constants are calculated over a collision energy range of 0.5–1.5eV. The CC reaction thresholds of both title reactions are lower than that of the CS reaction thresholds. Comparisons between the CC and CS cross sections for initial quantum numbers v=0 and j=0 reveal that neglecting the CC effect can significantly underestimate the reaction cross section. The thermal rate constants demonstrate that the mass difference induces the faster ion-molecular rotation of the D2+, which causes the reaction Ne+D2+ to occur easier than the reaction Ne+T2+. Initial vibrational excitation of the D2+/T2+ enhances the reactivity considerably.

Quantum Dynamical Study of the He + NeH+ Reaction on a New Analytical Potential Energy Surface

An analytical potential energy surface (PES) for the ground state of the [HeHNe](+) system has been constructed from a set of 19,605 ab initio data points, obtained from coupled cluster singles and doubles with perturbative triples correction calculations and the aug-cc-pVQZ basis set. The PES is based on the many-body expansion form proposed by Aguado and Paniagua (J. Chem. Phys. 1992, 96, 1265), and it has a root-mean-square error of 0.03 kcal/mol. The minimum energy pathways (MEPs) for different Ne-H-He angles are calculated, and it is found that the MEP for 180° (linear) goes through the deepest potential energy well. Preliminary quantum dynamical studies are performed for the He + NeH(+) (v = 0-2, j = 0-3) → HeH(+) + Ne reaction in the 0.0-0.5 eV collision energy range. Quantum calculations are carried out using a time-dependent wave packet method within the centrifugal sudden approximation. Reaction probabilities exhibit strong oscillatory behavior arising because of the metastable [HeHNe](+). Vibrational excitation has been found to enhance the reaction cross sections.

A compact Ge-BGO coincidence array for ultra-sensitive in-beam gamma spectroscopy

Ultra-weak resonances in nuclear reactions impose a challenge on experimentalists. The present work describes the setup used in the first direct measurement of extremely weak low energy resonances of the 18O(α, γ)22Ne reaction. A Clover HPGe detector in coincidence with specified energy windows in BGO counters covering a large solid angle, and combined with large plastic veto counters led to a significantly improved sensitivity, thus allowing for the clear identification of specific γ-transitions. Different modes of operation have been tested for optimizing the final experimental setup.

The Chemical Evolution of Heavy Elements in Globular Clusters

AbstractWe present preliminary results from a chemical evolution model that tracks the composition of heavy elements beyond iron in a globular cluster. The heavy elements can be used as tracers of the nucleosynthetic events that defined the formation and evolution of star clusters in the early Universe. In particular, the chemical evolution model focuses on the hypothesis that rapidly-rotating massive stars produced the heavy elements via the slow neutron-capture process and seeded the proto-cluster while the stars we see today were still forming.We compare our model with heavy element abundances in M4 and M5, and M22. Our results are strongly dependent on the highly uncertain rate of the 17O(α,γ)21Ne reaction, which determines the strength of 16O as a neutron poison. We find that the [Pb/Ba] ratio is too low to match the empirical value, which might suggest that a contribution from AGB stars is required.

Isγ-Ray Emission from Novae Affected by Interference Effects in theF18(p,α)O15Reaction?

The (18)F(p,α)(15)O reaction rate is crucial for constraining model predictions of the γ-ray observable radioisotope (18)F produced in novae. The determination of this rate is challenging due to particular features of the level scheme of the compound nucleus, (19)Ne, which result in interference effects potentially playing a significant role. The dominant uncertainty in this rate arises from interference between J(π)=3/2(+) states near the proton threshold (S(p)=6.411 MeV) and a broad J(π)=3/2(+) state at 665 keV above threshold. This unknown interference term results in up to a factor of 40 uncertainty in the astrophysical S-factor at nova temperatures. Here we report a new measurement of states in this energy region using the (19)F((3)He,t)(19)Ne reaction. In stark contrast to previous assumptions we find at least 3 resonances between the proton threshold and E(cm)=50 keV, all with different angular distributions. None of these are consistent with J(π)=3/2(+) angular distributions. We find that the main uncertainty now arises from the unknown proton width of the 48 keV resonance, not from possible interference effects. Hydrodynamic nova model calculations performed indicate that this unknown width affects (18)F production by at least a factor of two in the model considered.

Performance Analysis of Two Quantum Reaction Dynamics Codes: Time-dependent and Time-lndependent Strategies

The computer simulation of reaction dynamics has nowadays reached a remarkable degree of accuracy. Triatomic elementary reactions are rigorously studied with great detail on a straightforward basis using a considerable variety of Quantum Dynamics computational tools available to the scientific community. In our contribution we compare the performance of two quantum scattering codes in the computation of reaction cross sections of a triatomic benchmark reaction such as the gas phase reaction Ne + H2+ → NeH++ H. The computational codes are selected as representative of time-dependent (Real Wave Packet [1]) and time-independent (ABC [2]) methodologies. The main conclusion to be drawn from our study is that both strategies are, to a great extent, not competing but rather complementary. While time-dependent calculations advantages with respect to the energy range that can be covered in a single simulation, time-independent approaches offer much more detailed information from each single energy calculation. Further details such as the calculation of reactivity at very low collision energies or the computational effort related to account for the Coriolis couplings are analyzed in this paper.

Theoretical study on stereodynamics of H + NeH+ (v = 0, j = 0) → H2 + + Ne reaction

Stereodynamics of reaction H + NeH+ (v = 0, j = 0) → H2+ + Ne is investigated by quasi-classical trajectory method using a new potential energy surface constructed by Lv et al. The distributions of P(θr), P(ϕr) and PDDCSs are calculated at four different collision energies. The rotational polarization of product H2+ presents different characters at different collision energies. The product rotational angular momentum vector j′ is not only aligned, but also oriented along the direction perpendicular to the scattering plane. With the increase of collision energy, the rotation of product molecule has a preference of changing from the “in-plane” mechanism to the “out-of-plane” mechanism. Although the title reaction is mainly dominated by the direct reaction mechanism, the indirect mechanism plays a role when the collision energies are low.

General overview of the strong projectile-target-core interaction in Ne (e, 2e) reaction

By measuring the momentum distributions of recoil ions in Ne (e, 2e) reaction, we showed the evidence of the strong projectile-target core interaction and confirmed the existence of a two-step collision mechanism, namely, the incident electron experiences an elastic scattering on the target core before or after the target ionization occurs.

Nuclear Physics Constraints on the Characteristics of Astrophysical Thermonuclear Flashes

We review the nuclear physics that is associated with the outbursts of Type Ia (thermonuclear) supernova explosions and with the thermonuclear runaway events that define the outbursts of both classical novae and recurrent novae. We describe how distinguishing characteristics of these two classes of astrophysical explosion are strongly dependent both upon fuel ignition in degenerate matter and upon the rates of critical charged-particle reaction rates and weak interaction rates. In this centennial celebration of the important contributions of Rutherford and his collaborators to our understanding of the structure of the nucleus of an atom, it is quite interesting to note the evolution of the α-particle scattering experiments described in Rutherford's seminal paper (Rutherford 1911) to current studies of α-particle induced reactions and their defining roles in studies of stellar, nova, and supernova nucleosynthesis. We identify and discuss for example: (1) the manner in which (α, p) reactions in proximity to the Z = N line carry the major flows from 12C and 16O to 56Ni in Type Ia supernovae; and (2) the critical role of the 15O(α, γ)19Ne reaction in possibly effecting "breakout" of the Hot CNO cycles at the highest temperatures achievable in Classical Novae. In this contribution, we first review the current status our understanding of Type Ia supernova events and then that of Classical Novae.

The Role of 12C(12C,n) in the Astrophysical S-Process

The elements between iron and strontium are largely produced by the weak s-process occurring in massive stars during the late stages of convective core He burning and the convective shell carbon burning with the primary source of neutrons coming from the reaction 22Ne(α,n)25Mg. However, shell carbon burning may produce hot enough temperatures to activate 12C(12C,n)23Mg as a significant neutron source. Few studies have been done on this reaction, and the extrapolation from experimental data down to the relevant astrophysical energies is uncertain. Recent studies performed at the Nuclear Science Laboratory of Notre Dame aim to improve the existing reaction data using new experimental techniques as well as provide a more reliable extrapolation to the low energies not accessible by experiment. Preliminary results will be presented and the astrophysical implications will be discussed.

(e,2e)processes on atomic targets (Ne, Ar) and molecular targets (CH4, N2, CO2): Role of electron-nucleus scattering

The momentum distributions of the recoil ions were measured for the single ionization of atomic targets (Ne, Ar) and of molecular targets (CH${}_{4}$, N${}_{2}$, CO${}_{2}$) at incident electron energies ranging from 80 to 220 eV by using the reaction microscope. It was found that there are a remarkable number of recoil ions with large longitudinal momenta, and the relative contribution of these ions for Ne ($e$, 2$e$) reactions decreases faster than that for Ar ($e$, 2$e$) reactions with the incident electron energy increasing. This observed feature can be qualitatively interpreted by the different effective potential of the target core experienced by the electrons. The appearance of the large longitudinal momentum recoil ions for CH${}_{4}$, N${}_{2}$, and CO${}_{2}$ ($e$, 2$e$) reactions indicates that the strong interaction between the electron and the component-atoms target core in the molecules plays a significant role in the single ionization of small molecular targets. Depending on the independent atom model, we can intuitively understand the characteristics of the momentum distributions of the recoil ions CH${}_{4}^{+}$, N${}_{2}^{+}$, and CO${}_{2}^{+}$, respectively.

Reaction dynamics and nuclear structure of moderately neutron-rich Ne isotopes by heavy-ion reactions

The heavy-ion reaction 22Ne+208Pb at 128 MeV beam energy has been studied using the PRISMA-CLARA experimental setup at Legnaro National Laboratories. Elastic, inelastic, and one-nucleon transfer differential cross sections are measured and global agreement is obtained with semiclassical and distorted-wave Born approximation (DWBA) calculations. In particular, the angular distribution of the 2+ state of 22Ne is analyzed by DWBA and a similar calculation is performed for the unstable 24Ne nucleus, using existing data from the reaction 24Ne+208Pb at 182 MeV (measured at SPIRAL with the VAMOS-EXOGAM setup). In both cases the DWBA model gives a good reproduction of the experiment, pointing to a strong reduction of the β2C charge deformation parameter in 24Ne. This follows the trend predicted for the evolution of the quadrupole deformation along the Ne isotopic chain.Received 8 May 2012DOI:https://doi.org/10.1103/PhysRevC.85.064621©2012 American Physical Society

Nuclear Astrophysics Experiments in Collaboration with Ruhr University

This paper reports on two nuclear astrophysics experiments performed in collaboration with Ruhr University. In a 12C+12C fusion reaction, the 12C(12C, α)20Ne and 12C(12C, p)23Na reactions were studied in the energy range of E = 2.10 MeV to 4.75 MeV using γ-ray spectroscopy. The deduced astrophysical S(E)* factor exhibited a new, strong resonance at E = 2.14 MeV, which lay at the high-energy tail of the Gamow peak. The resonance increased the reaction rate of the α-channel by a factor of five near T = 8 × 108 K. The electron screening in d(d, p)t was studied for a series of deuterated metal, insulator and semiconductor targets. Compared with the measurements performed with a gaseous D2 target, a large effect was observed in most metals, while a comparatively small effect was found in the insulators and semiconductors. Subsequently the temperature dependence of the electron screening in the d(d, p)t reaction was studied for the deuterated metals Pt and Co. Enhanced electron screening decreased with increasing temperature. These data represent the first observations of the temperature dependence of a nuclear cross section.

Large collection of astrophysicalSfactors and their compact representation

Numerous nuclear reactions in the crust of accreting neutron stars are strongly affected by a dense plasma environment. Simulations of superbursts, deep crustal heating, and other nuclear burning phenomena in neutron stars require astrophysical $S$ factors for these reactions (as a function of center-of-mass energy $E$ of colliding nuclei). A large database of $S$ factors is created for about 5000 nonresonant fusion reactions involving stable and unstable isotopes of Be, B, C, N, O, F, Ne, Na, Mg, and Si. It extends the previous database of about 1000 reactions involving isotopes of C, O, Ne, and Mg. The calculations are performed using the S\~ao Paulo potential and the barrier penetration formalism. All calculated $S$ data are parameterized by an analytic model for $S(E)$ proposed before [Phys. Rev. C 82, 044609 (2010)] and further elaborated here. For a given reaction, the present $S(E)$ model contains three parameters. These parameters are easily interpolated along reactions involving isotopes of the same elements with only seven input parameters, giving an ultracompact, accurate, simple, and uniform database. The $S(E)$ approximation can also be used to estimate theoretical uncertainties of $S(E)$ and nuclear reaction rates in dense matter, as illustrated for the case of the ${}^{34}$Ne${+}^{34}$Ne reaction in the inner crust of an accreting neutron star.

Measurement of the cross section for the reaction 20Ne(n,α)17O in the neutron-energy between 4 and 7 MeV

The cross section for the reaction 20Ne(n, α)17O was measured in the neutron-energy range 4–7 MeV. An ionization chamber equipped with a Frisch grid combined with a pulse-shape digitizer was used as a detector. Gaseous neon that served as a target on which the reaction being studied proceeded was added to the gas filling the ionization chamber. The partial cross sections for the α 0, α 1, α 2, and α 3 channels of the reaction 20Ne(n, α)17O were obtained for the first time.

Stereodynamics of the reactions Ne+H+2/Ne+D+2/Ne+T2

The vector correlations between products and reagents for the reactions Ne+H+2, Ne+D+2, and Ne+T+2 are calculated by means of the quasi-classical trajectory method on a new potential energy surface constructed by Lü et al. [J. Chem. Phys. 2010 132, 014303]. The polarization-dependent differential cross-sections (2π/σ)(dσ00/dωt), (2π/σ)(dσ20/dωt), (2π/σ)(dσ22+/dωt), and (2π/σ)(dσ21−/dωt), and the distributions of P(θr), P(φr), and P(θr, φr) are calculated. The isotopic effect, which is associated with the difference in mass factor among the three reactions, is revealed.

Nuclear astrophysics with neutrons

Neutrons play a crucial role in astrophysics during the heavy element nucleosynthesis. The largest fraction of isotopes heavier than iron is produced by neutron capture processes on short (r process) and long timescales (s process). During the ``slow neutron capture process'' (s process) heavier elements are produced by successive captures of in-situ produced neutrons from the reactions 13C(α,n)16O and 22Ne(α,n)25Mg (with densities of 106−1010 cm−3) in the interior of stars and following β-decays. With this scenario the reaction path runs along the valley of stability up to 209Bi and produces about 50% of the solar abundances of the heavy elements. Important nuclear physics parameters for s-process nucleosynthesis are neutron capture cross sections (for En = 0.3–300 keV, corresponding to stellar temperatures between kT= 8 and 90 keV) and β-decay half-lives. Neutron capture measurements can be performed via activation in a quasi-stellar neutron spectrum utilizing several (p,n) reactions, or by the time-of-flight technique. The ``rapid neutron capture process'' (r process) is responsible for the remaining 50% of the solar abundances. Here neutrons with densities of 1020−1030 cm−3 are captured on a very fast timescale (ms) during a Core Collapse Supernova in a region close to the forming neutron star. The r-process nuclei are thus very short-lived, neutron-rich isotopes up to the actinides, which can only be produced and investigated at large-scale radioactive-beam facilities. Here the most important nuclear physics parameters are masses, half-lives, and at later stages also β-delayed neutrons. This paper will summarize the role of neutrons in nuclear astrophysics and give a short overview about the related astrophysics programs at the GSI Helmholtz research center and the FRANZ facility in Germany.

Experimental studies of (n, α) reaction cross-sections performed at the Institute for Physics and Power Engineering

Studies of (n, α) reaction cross-sections on a number of isotopes over a wide range of neutron energies were performed systematically at the Institute for Physics and Power Engineering (IPPE) A new spectrometer that includes an ionization chamber and a pulse shape digitizer were used. The experimental technique and the results of measurements of the reaction cross-sections 16O(n, α)13C, 14N(n, α)11B, 20Ne(n, α)17O, 36Ar(n, α)33S, 40Ar(n, α)37S, 10B(n, α)7Li, and 50Cr(n, α)7Li, are described.