Ne Reaction Research Articles

The Importance of15O(α,γ)19Ne to X‐Ray Bursts and Superbursts

One of the two breakout reactions from the hot CNO (HCNO) cycle is 15O(α,γ)19Ne, which at low temperatures depends strongly on the resonance strength of the 4.033 MeV state in 19Ne. An experimental upper limit has been placed on its strength, but the lower limit on the resonance strength and therefore the astrophysical reaction rate is unconstrained experimentally. However, this breakout reaction is crucial to the thermonuclear runaway that causes type I X-ray bursts on accreting neutron stars. In this paper we exploit astronomical observations in an attempt to constrain the relevant nuclear physics and deduce a lower limit on the reaction rate. Our sensitivity study implies that if the rate were sufficiently small, accreting material would burn stably without bursts. The existence of type I X-ray bursts and superbursts consequently suggests a lower limit on the 15O(α,γ)19Ne reaction rate at low temperatures.

Dynamics of ionization of H2 by Ne*(P3) investigated by electron spectroscopy

The Penning ionization reaction Ne*(2p(5)3s 3P)+H2-->[NeH2]+ +e- has been studied in crossed supersonic molecular beams with electron-energy analysis at four collision energies E = 1.83, 2.50, 3.16, and 3.89 kcal/mol. The electron kinetic-energy spectra, which directly reflect the ionizing transition region, show resolved peaks assignable to v' = 0-4 of H2+. The vibrational populations deviate systematically from Franck-Condon behavior, suggesting that the discrete-continuum coupling increases with H2 bond stretching. Each peak displays both increasing breadth and increasing blueshift with increasing E, and the blueshift also increases with increasing v'. The first two properties are consistent with a predominantly repulsive excited-state potential-energy surface, while the last is speculated to be a reflection of the rHH dependence of the ionic surface. Quantum scattering calculations based on ab initio potential surfaces for the excited and ionic states in spherical and infinite-order-sudden rigid rotor approximations are in semiquantitative agreement with the measurements. Discrepancies suggest changes in the imaginary, absorptive part of the excited surface, which probably can be best effected by multiproperty fitting calculations.

On the Physics of Type I X-Ray Bursts on Accreting Neutron Stars at High Accretion Rates

We investigate the effect of the hot CNO cycle breakout reaction 15O(alpha,gamma)19Ne on the occurrence of type I X-ray bursts on accreting neutron stars. For f_rp <~ 0.1, where f_rp is a dimensionless factor by which we multiply the 15O(alpha,gamma)19Ne reaction rate of Caughlan & Fowler (1988), our model predicts that bursts should occur only for accretion rates below a critical value of approximately 0.3 times the Eddington limit. This agrees with observations. For larger values of f_rp, including the standard choice f_rp = 1, the model switches to a new regime in which bursts occur all the way up to roughly the Eddington limit. Since the latter regime disagrees with observations, we suggest that the true 15O(alpha,gamma)19Ne reaction rate is lower than usually assumed.

Relation between the 16O(α,γ)20Ne reaction and its reverse 20Ne(γ,α)16O reaction in stars and in the laboratory

The astrophysical reaction rates of the 16O(a,g)20Ne capture reaction and its inverse 20Ne(g,a)16O photodisintegration reaction are given by the sum of several narrow resonances and a small direct capture contribution at low temperatures. Although the thermal population of low lying excited states in 16O and 20Ne is extremely small, the first excited state in 20Ne plays a non-negligible role for the photodisintegration rate. Consequences for experiments with so-called quasi-thermal photon energy distributions are discussed.

Explosive hydrogen burning of 23Na in classical novae

It is frequently assumed that matter in the mass range A ⩾ 20 is processed through the NeNa-cycle during hydrogen-burning nucleosynthesis. The existence of such a reaction cycle implies that the 23Na(p,α)20Ne reaction is more likely to occur than the competing 23Na(p,γ)24Mg reaction. Recently evaluated thermonuclear rates for both reactions carry large uncertainties and allow for both possibilities, a closed and an open NeNa-cycle. We measured the 23Na(p,γ)24Mg reaction at the Laboratory for Experimental Nuclear Astrophysics (LENA). The present experimental results reduce the 23Na+p reaction rate uncertainties significantly. We demonstrate that a closed NeNa-cycle does not exist at elevated stellar temperatures of T = 0.2–0.4 GK. The implications for the nucleosynthesis in classical novae are discussed.

Possible PET isotope production using linear deuteron accelerators

PET isotopes like 11C and 18F may be produced in deuteron-induced reactions using linear deuteron accelerators for energies 2–2.5 MeV. It is shown that the yield of PET isotopes in reactions 10B(d,n) 11C and 20Ne(d,a) 18F may be higher than the yield of the same isotopes in proton-induced reactions at higher energies, where protons are accelerated by a cyclotron. The lower cost and smaller size of linear accelerators make deuteron-induced reactions very promising for industrial PET production.

The reaction rate for the destruction of fluorine in AGB stars

The 19F(α, p)22Ne reaction is considered to be the main source of fluorine depletion during the Asymptotic Giant Branch phase in stars. The reaction rate still retains large uncertainties due to the lack of experimental studies available. The yields for both the ground p0 and first excited state p1 exit channels of 22Ne have been measured with the University of Notre Dame KN van de Graaff accelerator. Ten resonances were found in the energy range E c m = 990 – 1570 keV and their strengths have been determined. A comparison between the rate calculated from the experimental data and other standard rates is provided.

New Limits for the 18F( p, α) 15O Rate in Novae

The degree to which the (p, γ) and (p,α) reactions destroy 18F at temperatures ∼ 1 − 4 × 10 8 K is important for understanding the synthesis of nuclei in nova explosions and for using the long-lived radionuclide 18F, a target of gamma ray astronomy, as a diagnostic of nova mechanisms. The reactions are dominated by low-lying proton resonances near the 18F+p threshold ( E x = 6.411 MeV in 19Ne). To gain further information about these resonances, we have used the d(18F,p)19F neutron transfer reaction to selectively populate corresponding mirror states in 19F. The results would suggest 18F(p,γ)19Ne and 18F(p,α)15O reaction rates that are 2-3 times lower than reported previously.

New 19Ne Level Observed with a Thick Target 18F( p, p) 18F Measurement

The rates of the 18 F( p , α ) 15 O and 18 F( p , γ ) 19 Ne reactions in astrophysical environments depend on the properties of 19 Ne levels above the 18 F + p threshold. There are at least 8 levels in the mirror nucleus 19 F for which analogs have not been observed in 19 Ne in the excitation energy range E x = 6.4 – 7.6 MeV . These levels may significantly enhance the 18 F + p reaction rates, and thus we have made a search for these levels by measuring the 1 H( 18 F, p ) 18 F excitation function over the energy range E c . m . = 0.3 - 1.3 MeV . We have identified and measured the properties of a newly observed level at E x = 7.420 ± 0.014 MeV , which is most likely the mirror to the J π = 7 2 + 19 F level at 7.56 MeV. We have additionally found a significant discrepancy with a recent compilation for the properties of a 19 Ne state at E x = 7.5 MeV and set upper limits on the proton widths of missing levels.

The effect of the19F(α, p)22Ne reaction rate uncertainty on the yield of fluorine from Wolf-Rayet stars

In the light of recent recalculations of the \el{19}{F}(alpha,p)\el{22}{Ne} reaction rate we present results of the expected yield of \el{19}{F} from Wolf-Rayet (WR) stars. We have computed models using the upper and lower limits for the rate in addition to the recommended rate and hence we constrain the uncertainty in the yield with respect to this reaction. We find a yield of 3.1x10^{-4}M_sol of \el{19}{F} with our recommended rate and a difference of a factor of two between the yields computed with the upper and lower limits. In comparison with previous work we find a difference in the yield of approximately a factor of 4, connected to a different choice of mass loss. Model uncertainties must be carefully evaluated in order to obtain a reliable estimate of the yield of fluorine from WR stars together with its uncertainties.

High pressure investigations of positive ion–molecule reactions in a mixture of C 3H 6 and Ne

Dissociative charge-exchange reactions of Ne + ions produced from neon with neutral propene have been studied in a quadrupole mass spectrometer with a high pressure ion source. The measurements were performed in a mixture of 1% C 3H 6+99% Ne within the pressure range 1.33–39.99 Pa. The fractional abundance I i / ∑ I i of prominent ions: C 2H 3 +, C 3H 3 +, C 3H 5 +, C 3H 7 +, C 4H 5 +, C 4H 7 + and neon ions have been determined as a function of gas pressure in the ion source and against the repeller electrode potential.

New constraints on theF18(p,α)O15rate in novae from the(d,p)reaction

The degree to which the (p,gamma) and (p,alpha) reactions destroy 18F at temperatures 1-4x10^8 K is important for understanding the synthesis of nuclei in nova explosions and for using the long-lived radionuclide 18F, a target of gamma-ray astronomy, as a diagnostic of nova mechanisms. The reactions are dominated by low-lying proton resonances near the 18F+p threshold (E_x=6.411 MeV in 19Ne). To gain further information about these resonances, we have used a radioactive 18F beam from the Holifield Radioactive Ion Beam Facility to selectively populate corresponding mirror states in 19F via the inverse d(18F,p)19F neutron transfer reaction. Neutron spectroscopic factors were measured for states in 19F in the excitation energy range 0-9 MeV. Widths for corresponding proton resonances in 19Ne were calculated using a Woods-Saxon potential. The results imply significantly lower 18F(p,gamma)19Ne and 18F(p,alpha)15O reaction rates than reported previously, thereby increasing the prospect of observing the 511-keV annihilation radiation associated with the decay of 18F in the ashes ejected from novae.

Low energy charge-transfer collisions of rare gas dications: Ne 2+ + Ne and Kr 2+ + Kr

We report measurements of the double charge transfer reactions Ne 2+ + Ne → Ne + Ne 2+ and Kr 2+ + Kr → Kr + Kr 2+ at hyperthermal energies. At collision energies below 0.8 eV, the experimental results are well reproduced by the capture (Langevin) cross section multiplied by the factor 1/2. For the single charge transfer channel, Ne 2+ + Ne → Ne + + Ne +, our experiment suggests that the process has a very small cross section.

Studies of the 18F( p, α) 15O reaction rate with a 18F beam at the HRIBF

The rates of the 18 F( p , α ) 15 O and 18 F( p , γ) 19 Ne reactions at T = 0.1 – 0.4 GK are necessary to understand the emission of positron-annihilation gamma rays expected from novae. Observations of such gamma rays by space-based telescopes would provide direct tests of nova models. These rates, however, are uncertain because of the unknown level structure of 19 Ne above the 18 F+ p threshold. Missing levels in 19 Ne were searched for by stopping a 24-MeV 18 F beam in a thick polypropylene target, and scattered protons were detected in a silicon-strip detector. The 18 F( p , p ) 18 F excitation function has been measured from E c.m. = 0.3 - 1.3 MeV covering most energies of astrophysical interest, and a significant discrepancy was observed with a recent compilation of 19 Ne resonance parameters.

Reaction Rate Uncertainties and the Production of19F in Asymptotic Giant Branch Stars

We present nucleosynthesis calculations and the resulting 19F stellar yields for a large set of models with different masses and metallicity. During the asymptotic giant branch (AGB) phase, 19F is produced as a consequence of nucleosynthesis occurring during the convective thermal pulses and also during the interpulse periods if protons from the envelope are partially mixed in the top layers of the He intershell (partial mixing zone). We find that the production of fluorine depends on the temperature of the convective pulses, the amount of primary 12C mixed into the envelope by third dredge-up, and the extent of the partial mixing zone. Then we perform a detailed analysis of the reaction rates involved in the production of 19F and the effects of their uncertainties. We find that the major uncertainties are associated with the 14C(α, γ)18O and 19F(α, p)22Ne reaction rates. For these two reactions we present new estimates of the rates and their uncertainties. In both cases the revised rates are lower than previous estimates. The effect of the inclusion of the partial mixing zone on the production of fluorine strongly depends on the very uncertain 14C(α, γ)18O reaction rate. The importance of the partial mixing zone is reduced when using our estimate for this rate. Overall, rate uncertainties result in uncertainties in the fluorine production of about 50% in stellar models with mass ≃3 M☉ and of about a factor of 7 in stellar models of mass ≃5 M☉. This larger effect at high masses is due to the high uncertainties of the 19F(α, p)22Ne reaction rate. Taking into account both the uncertainties related to the partial mixing zone and those related to nuclear reactions, the highest values of 19F enhancements observed in AGB stars are not matched by the models. This is a problem that will have to be revised by providing a better understanding of the formation and nucleosynthesis in the partial mixing zone, as well as in relation to reducing the uncertainties of the 14C(α, γ)18O reaction rate. At the same time, the possible effect of cool bottom processing at the base of the convective envelope should be included in the computation of AGB nucleosynthesis. This process could, in principle, help to match the highest 19F abundances observed by decreasing the C/O ratio at the surface of the star, while leaving the 19F abundance unchanged.

Effect of Woods-Saxon Potential on the 16O+20Ne Reaction Cross Section

The excitation function and angular distributions for the 16O+20Ne system have been explained using the distorted wave Born approximation (DWBA) calculations. The real and imaginary Woods-Saxon optical potentials are assumed to be energy-dependent. The gross resonant structures observed in the 20Ne(16O,16O)20Ne excitation function are well described by the present DWBA calculations. Although the elastic and elastic-transfer analyses introduce a qualitative description of the experimental data, the coherent sum of the two reaction processes exhibit a much better result for both forward and large-angle data.

The [formula omitted] reaction at stellar energies

The 17 F( p,γ) 18 Ne radiative capture reaction is investigated with a two-cluster generator-coordinate method. A multi-channel description of the 18Ne wave functions as a superposition of the 17 F+ p and 14 O+α channels is used. The 17F and 14O wave functions are defined in the shell model involving s, p and sd orbitals. The model provides a good overall description of the 18Ne spectroscopy at energies typical of nova and x-ray burst processes. In particular, the 3 + 1 state, an s-wave resonance dominating the reaction rate at temperatures above 5×10 8 K, is well described. We find a proton width Γ p =21.1 keV, in good agreement with the experimental value Γ p =18.0±2 stat±1 sys keV. Its gamma width, unknown experimentally, is Γ γ =33 meV, similar to values deduced from the shell model, and used in astrophysics ( Γ γ =30±20 meV). The non-resonant ( E1) as well as the resonant contributions ( M1, E2) of the astrophysical S-factor are determined. At zero energy we find S(0)=3.5 keV-b. The corresponding 17 F( p,γ) 18 Ne reaction rate is calculated, and compared with previous approaches.

A New17F(p, γ)18Ne Reaction Rate and Its Implications for Nova Nucleosynthesis

Proton capture by 17F plays an important role in the synthesis of nuclei in nova explosions. A revised rate for this reaction, based on a measurement of the 1H(17F, p)17F excitation function using a radioactive 17F beam at Oak Ridge National Laboratory's Holifield Radioactive Ion Beam Facility, is used to calculate the nucleosynthesis in nova outbursts on the surfaces of 1.25 and 1.35 M☉ ONeMg white dwarfs and a 1.00 M☉ CO white dwarf. We find that the new 17F (p, γ)18Ne reaction rate changes the abundances of some nuclides (e.g., 17O) synthesized in the hottest zones of an explosion on a 1.35 M☉ white dwarf by more than a factor of 104 compared to calculations using some previous estimates for this reaction rate, and by more than a factor of 3 when the entire exploding envelope is considered. In a 1.25 M☉ white dwarf nova explosion, this new rate changes the abundances of some nuclides synthesized in the hottest zones by more than a factor of 600, and by more than a factor of 2 when the entire exploding envelope is considered. Calculations for the 1.00 M☉ white dwarf nova show that this new rate changes the abundance of 18Ne by 21% but has negligible effect on all other nuclides. Comparison of model predictions with observations is also discussed.

Polar recombination as the main process explaining the M‐effect

AbstractA full explanation of the M‐effect, namely the reduction of the neon emission spectra practically to only one line of wavelength λ1 = 585.3 nm, when the filling gas is a mixture Ne + Ar + H2 or Ne + Xe + H2 is presented. The M‐effect is explained by ion‐ion recombination according to the reaction Ne+ H– = Ne*(2p1)+H, which feeds selectively the level 2p1 of neon. All observed behavior of the M‐effect is explained by polar recombination. Extension of the M‐effect to other gas mixtures containing at least one electronegative component is also underlined.

Nuclear astrophysics at ISAC with DRAGON: Initial studies

The new DRAGON recoil separator facility, designed and built to measure directly the rates of radiative proton and alpha capture reactions important for nuclear astrophysics, is now in operation at the TRIUMF-ISAC radioactive beams facility in Vancouver, Canada. Experiments have been conducted for the first time on the 21Na(p,γ) 22Mg reaction. The evolution of nova explosions, and particularly their 22Na abundance, depends sensitively on this reaction rate. Commissioning studies using the well-known stable beam reactions 21Ne(p,γ) 22Na, 20Ne(p,γ) 21Na, and 24Mg(p,γ) 25Al have shown that the recoil separator performs within its design specifications both in suppression power and acceptance. The first radioactive beam studies were done using a beam of 5 × 10 8 21Na atoms/s. Yield measurements recording simultaneously singles and coincident heavy-ion and gamma signals were performed, scanning in energy over the known resonance reported previously in 22Mg at E cm = 212 keV, and in addition, over a strong resonance observed at E cm ≈822 keV.