Ne Isotopes Research Articles

S-process nucleosynthesis in primordial AGB stars

Despite the lack of iron seeds and provided partial mixing of protons takes place below the convective envelope, it is shown that the production of s-process elements can be efficient in primordial AGB stars. The synthesis proceeds as follows: neutrons are mainly produced by the 13C(α,n) 16O reaction and captured by the abundant CNe isotopes to synthesized heavier species which will then serve as efficient seed elements for the s-process thanks to their larger neutron cross sections. When the bottleneck formed by the 33S(n,α) 30Si is passed, the synthesis of s-elements proceeds in a standard way. Under the assumption that protons can diffuse below the envelope, primordial AGB stars are expected to be s-process enriched and to exhibit strong signatures of Pb and Bi.

Meteorite Ablation Evaluated from Data on the Distribution of Cosmogenic Neon Isotopes

The type of the functional dependence of the ratio of the production rates of the cosmogenic isotopes 22Ne/21Nec on their location depth d (cm) in ordinary chondrites with a radius R ≤ 60 cm was determined on the basis of experimental data on the elemental production rates of cosmogenic Ne isotopes in chondrites (Leya et al., 2000a). The dependence found is of the type 22Ne/21Nec = Aexp(–Bd) + C, where the parameters A, B, and C are determined from the relationships: B = 0.560exp(–0.0105R) – 0.187, C = 0.170exp(–0.092R) + 1.083, and 0.170exp(–0.092R) + 1.144. These relationships were used to calculate the average weighted values of the 22Ne/21Nec ratio for the volume of the fallen meteorite depending on its given preatmospheric radius. The data obtained served as a basis for plotting a nomogram that makes it possible to estimate the mass lost during passage through the Earth's atmosphere (ablation quantity) from the mass of the fallen meteorite and the average value of the 22Ne/21Nec ratio measured in it. The average (median) value of ablation found for 262 chondrites was 91.5+2.1–2.6%. In addition to the earlier-established (Alexeev, 2001a; Alexeev, 2001b) peculiarities of H5-chondrites that distinguish them from H-chondrites of other petrologic types, H5-chondrites appeared to exhibit a higher degree of ablation. The observed effect and other distinctive features of H5-chondrites may be due to the specific evolution of the parent body of H-chondrites in the process of its disintegration, reaccumulation, and subsequent reworking of the surface layers.

Excess helium and neon in the southeast Pacific: Tracers for glacial meltwater

We analyze helium (He) and neon (Ne) isotope data sets from the southeast Pacific sector of the Southern Ocean collected in 1992 and 1994 and describe a new method to estimate glacial meltwater fluxes independent of previous approaches. The waters sampled during these cruises reveal large atmospheric He and Ne excess concentrations in the upper 500 m that are derived predominantly from the dissolution of air contained in glacial ice during melting underneath and at the front of floating ice shelves at elevated hydrostatic pressure. On the stations sampled, the meltwater fractions are largest near the northern and southern ice shelf fronts of George VI Ice Shelf (1.5–2.5%), consistent with prior inferences from δ18O. The inventories of 4He and Ne with local glacial meltwater origin south of 70°S between 210°E and 295°E are (2.5 ± 0.7)1011 cm3STP 4He and (7.9 ± 2.7)1011 cm3STP Ne, respectively. The associated meltwater volumes are 430 ± 120 Gt for 4He and 400 ± 140 Gt for Ne. For an estimated seawater residence time of 2 years on the continental shelf, the amounts of glacial meltwater translate into bulk melting rates of 215 ± 170 Gt yr−1 for 4He and 200 ± 170 Gt yr−1 for Ne. The values derived from 4He are expected to be larger than those obtained from Ne because of the terrigenic He component, which accounts for 10–15% of the He excess in this region. Our regional melting rate of 200–215 Gt yr−1 is comparable to a prior estimate but subject to uncertainties resulting from the station distribution, the air‐sea gas exchange, and the estimate of the net transport through the study region.

Mean field calculation of Ne, Mg and Si nuclei at N=20 with the separable monopole interaction

A new model has been developed to study nuclei using many-body perturbation theory with a density dependent separable monopole nucleon–nucleon interaction (SMO) [Phys. Rev. C 63 (2001) 054309; P.D. Stevenson, D.Ph. Thesis, Oxford 1999; Phys. Rev. C 65 (2002) 064312]. The model is in the zeroth order an alternative to mean-field models based on more conventional effective nucleon–nucleon interactions (e.g., Skyrme, Gogny). Results on shapes and related ground state observables are reported for even–even Ne, Mg, and Si nuclei with N∼20. The model predicts the weakening of the expected N=20 shell closure in Ne isotopes and its disappearance in Mg nuclei in full agreement with experimental results.

Target–ion-source system for RNB facility at KEK Tanashi

For the radioactive nuclear beam facility at KEK Tanashi based on an isotope separator on-line (ISOL), two different types of ion sources are currently employed: a single stage 6.4 GHz ECR and a surface ionization-type ion sources. Several proton-rich radioactive nuclear beams were developed for astrophysical interests. LiF powder target in a water-cooled Cu container is used for the production of Ne isotopes and CaF 2 mixed with fine graphite powder for Na isotopes. The ion beams are bunched at the exit of the ion source in order to reduce the beam loss in post acceleration.

Main channels of the decay of the giant dipole resonance in the 20,22Ne nuclei and isospin splitting of the giant dipole resonance in the 22Ne nucleus

Data published in the literature on various photonuclear reactions for the 20,22Ne isotopes and for their natural mixture are analyzed with the aim of exploring special features of the decay of giant-dipole-resonance states in these two isotopes. With the aid of data on the abundances of the isotopes and on the energy reaction thresholds, the cross sections for the reactions 20,22Ne[(γ, n)+(γ, np)] and 20,22Ne[(γ, p)+(γ, np)] are broken down into the contributions from the one-nucleon reactions (γ, n) and (γ, p) and the contributions from the reactions (γ, np). The cross sections for the reactions 20,22Ne(γ, n)19,21Ne and 20,22Ne(γ, p)19,21F in the energy range Eγ=16.0–28.0 MeV and the cross sections for the reactions 20,22Ne(γ, np)18,20F in the energy range Eγ=23.3–28.0 MeV are estimated. The behavior of the cross-section ratio r=σ(γ, p)/σ(γ, n) for the 22Ne nucleus as a function of energy is analyzed, and the isospin components of the giant dipole resonance in the 22Ne nucleus are identified. The contributions of the isospin components of the giant dipole resonance in the 22Ne nucleus to the cross sections for various photonuclear reactions are determined on the basis of an analysis of the diagram of the excitation and decay of pure isospin states in the 22Ne nucleus and in nuclei neighboring it, which are members of the corresponding isospin multiplets. The isospin splitting of the giant dipole resonance and the ratio of the intensities of the isospin components are determined to be ΔE=4.57±0.69 MeV and R=0.24±0.04, respectively.

An intense, slow and cold beam of metastable Ne(3s) 3 P 2 atoms

We employ laser cooling to intensify and cool an atomic beam of metastable Ne(3s) atoms. Using several collimators, a slower and a compressor we achieve a ^{20}Ne^* flux of 6 10^{10} atoms/s in an 0.7 mm diameter beam traveling at 100 m/s, and having longitudinal and transverse temperatures of 25mK and 300microK, respectively. This constitutes the highest flux in a concentrated beam achieved to date with metastable rare gas atoms. We characterize the action of the various cooling stages in terms of their influence on the flux, diameter and divergence of the atomic beam. The brightness and brilliance achieved are 2.1 10^{21} s^{-1} m^{-2} sr^{-1} and 5.0 10^{22} s^{-1} m^{-2} sr^{-1}, respectively, comparable to the highest values reported for alkali-metal beams. Bright beams of the ^{21}Ne and ^{22}Ne isotopes have also been created.

Deformation and Clustering in Even-Z Nuclei Up to Mg Studied Using AMD with the Gogny Force

Employing the Gogny force as an effective force, we study the ground state properties of light nuclei using antisymmetrized molecular dynamics (AMD). In a previous paper, we discussed the nuclear binding energies and nuclear radii of He, Be, C, O, Ne and Mg isotopes. In this paper, we mainly consider the deformation properties and the clustering nature of these isotopes. By comparing the calculated results with the AMD results by use of the Skyrme-III (SIII) force, we investigated the differences and similarities between the SIII force and the Gogny force. We find that the Gogny force yields rather better binding energy and larger deformation than the SIII force. We carry out the parity-projected calculations. Parity projection enhances the parity-violating deformation and the cluster structure of certain nuclei. Shape of the deformation energy surface is also changed by parity projection. This causes a competition between the mean-field-like structure and the cluster-like structure. A modified version of AMD, which employs deformed Gaussian wave packets instead of spherical ones, is shown to give large quadrupole moments in the case of Mg isotopes.

Deep Earth rare gases: initial inventories, capture from the solar nebula, and losses during Moon formation

The implications of mantle rare gas characteristics for both the acquisition of rare gases from the solar nebula and subsequent losses to space are examined. There is at least one deep mantle reservoir rich in 3He and Ne that was trapped early in Earth history, with minimum concentrations obtained by closed system calculations. Ne isotopes indicate the presence of a component that has a solar composition. Xe isotopes indicate that extensive late losses occurred from the mantle as well as from the atmosphere. Calculations based on a simple two-stage evolution provide times of losses of up to ∼100 Ma after the formation of the solar system from both the mantle and the atmosphere. These losses appear to have depleted the rare gases by ≥97%; therefore, there originally was at least two orders of magnitude more rare gases than now present. Mechanisms for the capture of rare gases soon after the start of the solar system into the deep Earth (or Earth-forming materials) must provide these high initial concentrations, presumably in the high-energy environment of planetary accretion where strong degassing of solids might be expected to have occurred. A mechanism that satisfies these requirements is the dissolution in a magma ocean of rare gases from a dense primary atmosphere. A massive atmosphere of solar composition would have been captured if the Earth had formed prior to dispersal of the solar nebula. The underlying mantle would have melted due to the energy of accretion and the blanketing effect of this atmosphere. Rare gases would then have entered the molten Earth by dissolution at the surface and downward advection. For typical solubility coefficients, a total pressure of ∼100 atm and surface temperatures of >∼2500°C are required to dissolve sufficient rare gases to account for the initial lower mantle concentrations. While Xe in the mantle is isotopically exchanging with the primary atmosphere, it will be buffered to a solar composition; therefore, somewhat less Xe must be trapped prior to the late loss event for longer periods of exchange. As solidification of the mantle proceeded outward during cooling, the distributions of retained rare gases would have been determined by the history of surface pressure and temperature during the coupled cooling of the Earth and atmosphere. The giant impact proposed for Moon formation may have been responsible for the inferred substantial and late gas losses from the deep mantle as well as from the atmosphere. Constraints on the timing of Moon formation derived from Hf–W systematics and simulations of the giant impact are consistent with the Xe isotope constraints for gas loss.

Noble gas study of the Reunion hotspot: evidence for distinct less-degassed mantle sources

We present an extensive He, Ne and Ar isotope data set from the Reunion hotspot that demonstrates the presence of a homogeneous plume source that has unique isotopic characteristics. 3He/ 4He ratios of the two volcanoes on Reunion (Piton de la Fournaise, <0.53 Ma; Piton des Neiges, 2–0.44 Ma) are uniform 12–13.5 Ra regardless of 4He concentration and sample age. The shield-building Older Series of Mauritius (5–8 Ma) has a constant 3He/ 4He ratio around 11.5 Ra. The similarity of 3He/ 4He and Sr–Nd isotope ratios among them demonstrate that the volcanoes have had a common homogeneous source related to the mantle plume activity over a period of 8 Myr. 20Ne/ 22Ne and 21Ne/ 22Ne of these volcanoes define a linear trend on a Ne three-isotope diagram with a slope between the Loihi and MORB correlation lines. There is a clear correlation between 20Ne/ 22Ne and 40Ar/ 36Ar. In contrast, Intermediate (2–3 Ma) and Younger Series (<1 Ma) of Mauritius and Rodrigues (1.5 Ma) have 3He/ 4He ratios similar to MORB and Sr and Nd isotope ratios closer to MORB than lavas from Reunion and Older Series of Mauritius. These Intermediate and Younger Series lavas therefore record a late stage thermal rejuvenation beneath Mauritius derived from a source that is unrelated to the mantle plume. The isotopic characteristics of the source of the Reunion magmatism are relatively low 3He/ 4He (13 Ra), an intermediate slope in a Ne three-isotope diagram and relatively radiogenic Sr isotope ratios. These source characteristics cannot be explained by either crustal contamination or MORB source mixing with Loihi-type primitive mantle. Thus He–Ne–Ar–Sr–Nd isotopes demonstrate that this plume source is distinct from the source of other large plumes (Loihi and Iceland), clearly showing that the mantle contains several relatively less-degassed reservoirs and not a single primitive source. Two possible models can account for the different isotopic signature of Reunion and Hawaii hotspots; (1) the Reunion source contains more recycled material than Loihi source and (2) the Reunion source experienced stronger degassing/differentiation than the Loihi source in the early stage of mantle evolution. In both cases a convection mode in the mantle is required that isolates and preserves several less-degassed reservoirs in the convectively stirred lower mantle.

Advances in cross section measurements at low energies

A systematic study was made to measure many cross sections for relevant neutron-induced reactions. This study was motivated by the need to better understand cosmic ray interactions with extraterrestrial materials. The major constituents of meteorites and lunar rocks include oxygen, silicon, and aluminum. The primary aim was to measure cross sections for neutron-induced reactions producing long-lived radionuclides (e.g. 14C) and stable isotopes (e.g. Ne isotopes) but the irradiation conditions allowed cross sections for many reactions producing relatively short-lived radionuclides (e.g. 22Na) to be well measured. Monitor foils used in the irradiations included C, Al, and Au. Quasi-monoenergetic neutron beams were produced by bombarding Be targets with 80, 120 and 160 MeV proton beams at iThemba LABS, South Africa. Two identical target stacks were irradiated in beam lines at 0° and 16° to the incident proton beam direction. The yield at an unique neutron energy was obtained by subtracting the yield measured at 16° (after suitable normalization) from that measured at 0°. The cross sections for the following reactions: 27Al(n,x)22,24Na; natC(n,x)7Be; 197Au(n,x)194,196Au; SiO2(n,x)22Na and natSi(n,x)24Na are reported.

He, Ne and Ar isotope compositions of fluid inclusions in hydrothermal sulfides from the TAG hydrothermal field Mid-Atlantic Ridge

Helium, rieon and argon isotope compositions of fluid inclusions have been measured in hydrothermal sulfide samples from the TAG hydrothermal field at the Mid-Atlantic Ridge. Fluid-inclusion He-3/He-4 ratios are 2.2-13.3 times the air value (Ra), and with a mean of 7.2 Ra. Comparison with the local vent fluids (He-3/He-4=7.5-8.2 Ra) and mid-ocean ridge basalt values (He-3/He-4=6-11 Ra) shows that the variation range of He-3/He-4 ratios from sulfide-hosted fluid inclusions is significantly large. Values for Ne-20/Ne-22 are from 10.2 to 11.4, which are significantly higher than the atmospheric ratio (9.8). And fluid-inclusion Ar-40/Ar-36 ratios range from 287 to 359, which are close to the atmospheric values (295.5). These results indicate that the noble gases of fluid inclusions in hydrothermal sulfides are a mixture of mantle- and seawater-derived noble gases; the partial mantle-derived components of trapped hydrothermal fluids may be from the lower mantle; the helium of fluid inclusions is mainly from upper mantle; and the Ne and Ar components are mainly from seawater.

Neutralization of low-energy Ne + ions scattered from metal surfaces: study by mass-resolved ion-scattering spectrometry

An updated dual-isotope method by mass-resolved ion-scattering spectrometry using simultaneously two isotopes of Ne + as primary-ion beam was developed for the study of neutralization phenomena occurring with low-energy (0.5–1.5 keV) ions scattered from the surface of noble/transition metals (Cu, Ag, Au and Pt). Data for the characteristic velocity ν c were estimated, namely 2.05×10 5 m s −1 (Cu), 2.15×10 5 m s −1 (Ag), 1.90×10 5 m s −1 (Au) and 1.67×10 5 m s −1 (Pt), and no monotone dependence of the ion-survival probability P + on the reciprocal velocity was evaluated. The empirical formula for P +( 1 ν ), containing an additional term dependent on the reciprocal velocity, was suggested. The presented results have confirmed the anomalous dependence of P +( 1 ν ) already observed in earlier low energy ion scattering studies.

Parity violating elastic electron scattering and neutron density distributions in the relativistic Hartree-Bogoliubov model

Parity-violating elastic electron scattering on neutron-rich nuclei is described in the framework of relativistic mean-field theory. Self-consistent ground state density distributions of Ne, Na, Ni, and Sn isotopes are calculated with the relativistic Hartree-Bogoliubov model, and the resulting neutron radii are compared with available experimental data. For the elastic scattering of 850 MeV electrons on these nuclei, the parity-violating asymmetry parameters are calculated using a relativistic optical model with inclusion of Coulomb distortion effects. The asymmetry parameters for chains of isotopes are compared, and their relation to the Fourier transforms of neutron densities is studied. It is shown that parity-violating asymmetries are sensitive not only to the formation of the neutron skin, but also to the shell effects of the neutron density distribution.

Rare gas systematics on the southernmost Mid‐Atlantic Ridge: Constraints on the lower mantle and the Dupal source

Concentrations and isotopic compositions of He, Ne, Ar, Kr, and Xe have been measured for mid‐ocean ridge basalt glasses from the Mid‐Atlantic Ridge Discovery section, centered at 47°30′S, thus extending the database for the 50°–53°S Shona section [Moreira et al, 1995]. The 44°–53°S part of the Mid‐Atlantic Ridge includes the Discovery and Shona bathymetrie and geochemical ridge anomalies [Douglass et al, 1999], which also appear clearly in the rare gas isotopic record. In addition to air, present at the surface or possibly mantle recycled, three source components are identified, upper mantle, primitive plume, and a Dupal‐related component. He and Ne isotopes indicate a very primitive source for both the Discovery and Shona plumes, which must originate in deep, poorly degassed mantle. Ne and Ar, corrected from air based on Ne systematics, reveal very consistent along‐strike He, Ne, and Ar isotopic patterns, also consistent with Xe data. These systematics provide evidence that plume argon has low 40Ar/36Ar and plume Xe low isotopic ratios relative to degassed mantle. A segment of the Discovery ridge anomaly shows a Dupal‐type, low 206Pb/204Pb component named LOMU (low μ, where μ = 238U/204Pb) by Douglass et al. [1999], and has radiogenic 4He/3He and 21Ne/22Ne, relatively elevated 20Ne/22Ne, mildly radiogenic 40Ar/36Ar, and low Xe isotopic ratios, possibly representing the Dupal rare gas signature. Interpretations of this component as either recycled oceanic crust, or delaminated subcontinental lithosphere are consistent with the rare gas systematics. In the former case, a maximum subduction age of 500 Ma can be calculated. In the latter case, the sublithospheric mantle should have a 40K/36Ar ratio 2–5 times lower than the convective mantle and a 238U/3He ratio 2–3 times higher.

The carbon Cepheid RT Trianguli Australis: additional evidence of triple- and CNO cycling

We have used echelle spectra of resolving power 35 000 to derive chemical abundances and the 12C/13C ratio in the 1.9-d carbon Cepheid RT TrA and the Cepheid U TrA, employed as a comparison star. We confirm that RT TrA is very metal-rich with [Fe/H]=+0.4. In addition, C and N are substantially in excess, and a small deficiency in O is present. We interpret these anomalies as resulting from the appearance on the stellar surface of material enriched in 12C by the 3-α process, followed by CNO cycling to convert 12C to 13C and 14N. In addition, some 16O has been processed to 14N. The partial processing of 16O to 14N indicates that substantial 17O may be present. Proton capture seems to have enhanced 23Na from the Ne isotopes.

Accurate isotope shift measurements on short-lived neon isotopes

We report on fast beam collinear laser spectroscopy measurements that demonstrate the feasibility of accurate isotope shift measurements for short lived Ne isotopes including the proton halo candidate 17Ne. Ultra high sensitivity is achieved by the application of an efficient resonance detection scheme based on collisional ionization and ion counting via the β decay. A direct measurement of the kinetic energy of the atomic Ne beam provides the required high accuracy.

Noble gases in the Cameroon line and the He, Ne, and Ar isotopic compositions of high μ (HIMU) mantle

Ultramafic xenoliths, basaltic lavas, and CO2 gases from the Cameroon line volcanic chain provide the first characterization of combined He, Ne, and Ar isotopes in a high‐time‐integrated 238U/204Pb = μ (HIMU) magmatic system. Helium isotopic compositions typically range from 5.0 to 6.7 Ra, with an average of 6.3. These values are indistinguishable from the 3He/4He of other HIMU locales (Austral Islands, St. Helena). Neon isotopic compositions for xenoliths and CO2 gases are mid‐ocean ridge basalt (MORB)‐like, with a maximum 20Ne/22Ne of 11.87 and 21Ne/22Ne of 0.0508. Argon isotopic compositions in silicates range from atmospheric to 40Ar/36Ar = 4910±430 (crushing) and up to 16,300±1000 (single grain, laser step heating). The correlation between 20Ne/22Ne and 40Ar/36Ar in CO2 gases suggests a minimum 40Ar/36Ar = 1650±30 for the mantle‐derived component. Uniform 3He/4He in silicates and in CO2 fluids across both the continental and oceanic sectors of the Cameroon line argues strongly for a negligible lithospheric contribution to noble gas isotopic compositions. This inference is supported by high 238U/3He in lherzolites, indicating that noble gases in these samples must have been recently introduced (&lt;50,000 years ago) to the sample, most likely from the host magma. Ocean crust recycling models of mixing between MORB source regions and highly radiogenic slabs cannot produce the observed He and Ne isotopic compositions. Isolation and aging of MORB source mantle can generate the isotopic compositions but require extreme 3He/22Ne fractionation. Involvement of plume‐derived gases, consistent with the lithophile element isotopic compositions, alleviates the need for strong 3He/22Ne fractionation. Closed‐system aging of plume‐derived heterogeneities can reproduce the data with minimum 3He/22Ne fractionation at reasonable 238U/3He ratios. However, diffusive exchange of He and to a lesser extent Ne between aged MORB source and aged plume veins could explain the occurrence of low 3He/4He compositions in all HIMU centers and the apparent low time‐integrated 3He/22Ne of the Cameroon line.

Varying shell gap and deformation inN∼20unstable nuclei studied by the Monte Carlo shell model

The structure of neutron-rich nuclei in the $N\ensuremath{\sim}20$ region is studied by the Monte Carlo shell model based on the quantum Monte Carlo diagonalization method. We present a comprehensive description of even-even isotopes of O, Ne, Mg, and Si. It is demonstrated that, for different nuclei, various particle-hole excitations from the $\mathrm{sd}$ to $\mathrm{pf}$ shell are mixed in different ways, producing distinct effects sometimes, for instance, in ${}^{28}\mathrm{Ne}.$ The monopole interaction is examined and modified, resulting in the shell gap changing from nucleus to nucleus. The drip line of O isotopes is then reproduced. The interplay between the $T=0$ and $T=1$ monopole interactions is discussed from the viewpoint of the potential energy surface and the effective single-particle energy. The extension of the neutron drip line of Ne isotopes is explained, and the boundary of the ``island of inversion'' is shown to be rather indistinct.

Shell-model and Hartree-Fock calculations for even-mass O, Ne, and Mg nuclei

Shell-model and deformed Hartree-Fock plus BCS calculations are reported for even-even nuclei [sup 18[minus]30]O, [sup 18[minus]36]Ne, and [sup 20[minus]42]Mg; shell-model calculations additionally included [sup 38,40]Ne and [sup 44,46,48]Mg. Ground-state binding energies and 2[sub 1][sup +] quadrupole moments are calculated by both models. Shell-model calculations, aided by a new truncation method, include 2[sub 1][sup +] excitation energies and magnetic moments. Hartree-Fock calculations with SkI6, RATP, Z[sub [sigma]][sup [asterisk]], and SkX Skyrme forces include ground-state deformations and rms radii; SkI6 gives the best overall agreement with experiment. The two models are compared with each other and with experiment. Two-neutron separation energies, evidence for a neutron halo or skin in heavy O isotopes, and deformation of Ne and Mg isotopes are discussed. Both models indicate disappearance of the shell gap at N=28 (Mg), and the shell model does so additionally at N=20 (Ne and Mg). [copyright] [ital 1999] [ital The American Physical Society]