Ne Isotopes Research Articles

Kaprada L(5/6) chondrite: Chemistry, petrography, noble gases and nuclear tracks

A single stone weighing about 1.6 kg fell in Kaprada village of south Gujarat, India in October, 2004. It has been studied for mineralogy, petrography, chemical and isotopic composition and cosmogenic effects. The olivine is 23.7% fayalite. The petrography, bulk chemistry and oxygen isotopic composition indicate that it belongs to L(5/6) group of chondrites. The cosmic ray exposure age of the meteorite is estimated to be 11.4 Ma based on He, Ne and Ar isotopes. A trapped nitrogen amount of 0.25 ppm with δ 15N=4.7±0.3‰ is typical of ordinary chondrites of higher metamorphic grade. The gas retention ages, based on U/Th– 4He and K– 40Ar are calculated to be 2.6 and 4.1 Ga, respectively. Cosmic ray track data indicate that the pre-atmospheric radius of the meteorite was about 7 cm, and about 75% mass was ablated during its journey through the Earth's atmosphere.

Ne ISOTOPES IN INDIVIDUAL PRESOLAR GRAPHITE GRAINS FROM THE MURCHISON METEORITE TOGETHER WITH He, C, O, Mg-Al ISOTOPIC ANALYSES AS TRACERS OF THEIR ORIGINS

Ne isotopes measured in individual presolar graphite grains, solid samples of extinct stars preserved in primitive meteorites, provide information on the type of stellar sources of the grains and on nucleosynthetic mixing and ion-trapping processes which were operating. We present Ne and He isotope analyses of single presolar graphite grains from the KFB1 density fraction extracted from the carbonaceous chondrite Murchison. In addition, we measured isotopes of C, O, and Mg-Al with the NanoSIMS ion microprobe to better constrain the origin of the grains. Eleven out of 51 presolar graphite grains contain nucleosynthetic {sup 22}Ne above our detection limit. This fraction of {sup 22}Ne-rich grains is similar to the one reported by Nichols et al. although we have a lower {sup 22}Ne detection limit. We detected rare He-shell {sup 20}Ne in one {sup 22}Ne-rich grain and obtained the {sup 20}Ne/{sup 22}Ne ratio (0.03 {+-} 0.02) of the He-shell of an Asymptotic Giant Branch (AGB) star with 1.5-2 M {sub sun} and subsolar metallicity. We also detected {sup 4}He in this grain, while in the other grains, which originally acquired He, He-loss seems to be significant. We found unequivocal evidence for radiogenic {sup 22}Ne (Ne-R) in another graphite grain, which likelymore » condensed in a core-collapse supernova and which incorporated live radioactive {sup 22}Na (t {sub 1/2} = 2.6 yr). For the other grains, a clear assignment to a stellar source is more difficult to make. Putative stellar sources are supernovae, AGB stars, born-again AGB stars, J-type carbon stars, and CO novae.« less

Shell-model studies of the N = 14 and 16 shell closures in neutron-rich nuclei

Shell-model studies on the N = 14 and 16 shell closures in neutron-rich Be, C, O and Ne isotopes are presented. We calculate, with the WBT interaction, the excited states in these nuclei. The calculations agree with recent experiment data. Excited energies and B(E2) values are displayed to discuss the shell closures. Our results support the N = 16 shell closure in these isotopes, while indicating a disappearance of N = 14 shell closure in Be and C isotopes.

PROPERTIES OF NUCLEI AND HYPERNUCLEI WITH THE DEFORMED SKYRME HARTREE-FOCK MODEL

The isotope dependence of the deformation in carbon and neon isotopes is studied in deformed Skyrme Hartree–Fock model. We find that the deformations of C and Ne isotopes have a strong isotope dependence as a manisfestation of spontaneous breaking effect in nuclear physics. By extending the deformed Skyrme Hartree–Fock model, we studied the properties of Λ hypernuclei in a broad mass region. The results suggest that the core nuclei and the corresponding hypernuclei have similar deformations with the same sign. Some light single-Λ hypernuclei are substantially deformed and the Λ binding energy is modified by the deformation.

Low-lying dipole resonance in neutron-rich Ne isotopes

Microscopic structure of the low-lying isovector dipole excitation mode in neutron-rich $^{26,28,30}\mathrm{Ne}$ is investigated by performing deformed quasiparticle-random-phase-approximation (QRPA) calculations. The particle-hole residual interaction is derived from a Skyrme force through a Landau-Migdal approximation. We obtain the low-lying resonance in $^{26}\mathrm{Ne}$ at around 8.6 MeV. It is found that the isovector dipole strength at ${E}_{x}<10$ MeV exhausts about 6.0% of the classical Thomas-Reiche-Kuhn dipole sum rule. This excitation mode is composed of several QRPA eigenmodes, one is generated by a $\ensuremath{\nu}(2{s}_{1/2}^{\ensuremath{-}1}2{p}_{3/2})$ transition dominantly and the other mostly by a $\ensuremath{\nu}(2{s}_{1/2}^{\ensuremath{-}1}2{p}_{1/2})$ transition. The neutron excitations take place outside of the nuclear surface reflecting the spatially extended structure of the $2{s}_{1/2}$ wave function. In $^{30}\mathrm{Ne}$, the deformation splitting of the giant resonance is large, and the low-lying resonance overlaps with the giant resonance.

Plume–ridge interaction revisited: Evidence for melt mixing from He, Ne and Ar isotope and abundance systematics

Submarine volcanic glass He, Ne and Ar data from the Pacific–Antarctic-Rise at its intersection with the Foundation Seamount Chain have been obtained to further elucidate one of the main outstanding problems in plume–ridge interaction studies: the physical state in which mantle flow and mixing occur in those settings. Mixing of plume mantle and mid-ocean ridge basalt source material may either be in the form of two solids, of solid-melt or of two melt phases only. The positive correlation of 4He/ 40Ar ⁎ with 206Pb/ 204Pb as well as the negative correlation of 4He/ 40Ar ⁎ with 40Ar ⁎ and 4He concentrations, combined with a lack of correlation between 4He/ 40Ar ⁎ and ridge depth or MgO concentration, indicate that the enriched mantle plume material has been extensively degassed prior to mixing with the mid-ocean ridge source material. Degassing of the plume source and mid-ocean ridge source material prior to mixing is also suggested by the He versus Pb isotope systematics. However, the occurrence of degassing processes requires the existence of melts, implying that mixing between the plume and the mid-ocean ridge material occurs in the physical form of melts.

Solar and Solar-Wind Composition Results from the Genesis Mission

The Genesis mission returned samples of solar wind to Earth in September 2004 for ground-based analyses of solar-wind composition, particularly for isotope ratios. Substrates, consisting mostly of high-purity semiconductor materials, were exposed to the solar wind at L1 from December 2001 to April 2004. In addition to a bulk sample of the solar wind, separate samples of coronal hole (CH), interstream (IS), and coronal mass ejection material were obtained. Although many substrates were broken upon landing due to the failure to deploy the parachute, a number of results have been obtained, and most of the primary science objectives will likely be met. These objectives include He, Ne, Ar, Kr, and Xe isotope ratios in the bulk solar wind and in different solar-wind regimes, and 15N/14N and 18O/17O/16O to high precision. The greatest successes to date have been with the noble gases. Light noble gases from bulk solar wind and separate solar-wind regime samples have now been analyzed. Helium results show clear evidence of isotopic fractionation between CH and IS samples, consistent with simplistic Coulomb drag theory predictions of fractionation between the photosphere and different solar-wind regimes, though fractionation by wave heating is also a possible explanation. Neon results from closed system stepped etching of bulk metallic glass have revealed the nature of isotopic fractionation as a function of depth, which in lunar samples have for years deceptively suggested the presence of an additional, energetic component in solar wind trapped in lunar grains and meteorites. Isotope ratios of the heavy noble gases, nitrogen, and oxygen are in the process of being measured.

Noble gas compositions of the lithospheric mantle below the Chyulu Hills volcanic field, Kenya

We performed a noble gas isotope study on mantle xenoliths from the Quaternary volcanic field of Chyulu Hills, S Kenya, c. 100 km east of the Kenya rift graben. 4He/ 3He ratios were in the range of 100 000–120 000 and indistinguishable from other subcontinental lithospheric mantle compositions worldwide and similar to most results of the East African rift system. 21Ne/ 22Ne ratios corrected for atmospheric contributions displayed less nucleogenic values compared to mid ocean ridge basalts. This requires addition of Ne from a more primitive mantle source, that could be assigned to the postulated Kenyan mantle ‘plume’ or alternatively might be attributed to the ‘high 3He/ 4He’ Afar hotspot c. 2000 km distant. Whereas He and Ne isotopes reflect a rather homogeneous source composition, we observe a variable air corrected Ar isotopic composition. This might mirror additional contributions of atmospheric fluids strongly enriched in Ar. Nevertheless, in the least contaminated xenolith we were able to calculate a maximum 40Ar/ 36Ar ratio of 34,800 ± 500 assuming a mantle 20Ne/ 22Ne = 12.5. This is regarded as best approximation of the real 40Ar/ 36Ar ratio of the investigated mantle source. The same sample showed the highest observed 129Xe/ 130Xe and 136Xe/ 130Xe ratios of 7.01 ± 0.12 and 2.37 ± 0.05, respectively. The correlation of 129Xe/ 130Xe– 136Xe/ 130Xe ratios is in broad agreement with mantle Xe in OIB and MORB sources. Within uncertainties a minor contribution of crustally derived xenon is possible, though not compelling.

Cross section measurements for neutron-induced reactions off C, Al, SiO2, Si and Au producing relatively short-lived radionuclides at neutron energies between 70 and 160 MeV

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 160MeV 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.

Presolar He and Ne Isotopes in Single Circumstellar SiC Grains

Noble gas isotopes in presolar silicon carbide (SiC) dust grains from primitive meteorites provide, together with major element isotopic compositions, insight into the nucleosynthetic output of different types of evolved stars >4.5 Gyr ago. We report here new results from helium and neon isotopic analyses of single presolar SiC grains with sizes between 0.6 and 6.3 μm using an ultrahigh sensitivity mass spectrometer. These noble gas studies were complemented by an ion microprobe study (NanoSIMS) of Si, C, and N isotopic compositions of the same grains. About 40%, or 46 of the 110 grains analyzed, contain nucleosynthetic 22Ne and/or 4He from their parent stars above our mass spectrometer's detection limit. We discuss the possible stellar sources using isotopic ratios as constraints combined with new model predictions for low- to intermediate-mass (1.5, 2, 3, and 5 M☉) asymptotic giant branch (AGB) stars of different metallicities (1, 1/2, 1/3, and 1/6 Z☉). Most SiC grains are of the mainstream type and originated in low-mass AGB stars. We find a higher-than-expected percentage of A/B type grains, with some containing 22Ne and/or 4He. In addition, we find one noble gas-rich nova grain candidate, one supernova grain (X-type grain), and one 22Ne-rich X- or Z-type grain candidate.

Reaction rate uncertainties and the operation of the NeNa and MgAl chains during HBB in intermediate-mass AGB stars

Context.We test the effect of proton-capture reaction rate uncertainties on the abundances of the Ne, Na, Mg and Al isotopes processed by the NeNa and MgAl chains during hot bottom burning (HBB) in asymptotic giant branch (AGB) stars of intermediate mass between 4 and 6 and metallicities between and 0.02.

Noble gas and geochronology study of the Hana Ridge, Haleakala volcano, Hawaii; implications to the temporal change of magma source and the structural evolution of the submarine ridge

In order to constrain the magma sources and the structural evolution of Hana Ridge, noble gas isotope ratios and 40Ar/ 39Ar ages were determined for this submarine extension of the east rift zone of Haleakala volcano. 40Ar/ 39Ar ages of ten lavas from Hana Ridge are bimodally distributed. Three samples have ages of 1.8–1.9 Ma and seven samples have ages of 1.4–1.5 Ma. The observation of older lavas overlying younger ones in two locations suggests that growth of Hawaiian rift zones occurs endogenously by continuous intrusion of magmas and patchy resurfacing at outbreak points. The 0.5 Ma age range for submarine Hana Ridge lavas, coupled with previously published K/Ar ages of 0.97–1.1 Ma for subaerial Honomanu lavas on the Haleakala Volcano, indicates that Haleakala shield volcanism persisted for nearly 1 My. Furthermore, the new 40Ar/ 39Ar ages, when considered with isotopic compositions of the lavas, suggests that the source for Hana Ridge magmas gradually shifted over a period of 0.5–1.0 My. The majority of samples from the ridge have relatively uniform 3He/ 4He ratios between 18 and 22 Ra, which is higher than 3He/ 4He of subaerial Honomanu tholeiites. 20Ne/ 22Ne and 21Ne/ 22Ne ratios define linearly correlated trend that overlaps with the Loihi–Kilauea trend. He–Ne systematics of the Hana Ridge indicate that the magmas comprising most parts of the ridge were derived from a source with a primordial less-degassed mantle component. Since Hana Ridge lavas predate the subaerial Honomanu lavas, temporal decrease of 3He/ 4He ratios suggests that contribution of this primordial component had decreased in the magma source during establishment of Hana Ridge and Haleakala volcano. Pb–Sr isotopes demonstrate that Hana Ridge magmas are representative of the Kea component. However, such isotopic signatures associated with elevated high 3He/ 4He ratios precludes that the Kea component is a distinct endmember, such as recycled oceanic crust or lithospheric mantle. Alternatively, we propose that it is a common sub-component that is a mixture of Loihi endmember and recycled oceanic crust. Ne isotope ratios of the Hawaiian samples, including the Hana Ridge, show primordial signature irrespective of 3He/ 4He ratios. Such apparent decoupling of He and Ne isotopes may be also attributed to mixing of the Loihi component and recycled component in the mantle plume, either of which components needs to be elementally fractionated prior to mixing.

Heterogeneous mantle argon isotope composition in the subcontinental lithospheric mantle beneath the Red Sea region

We performed argon measurements on rocks of the subcontinental lithospheric mantle (SCLM) from the Red Sea region applying stepwise crushing extractions, and argon analyses using high-resolution stepwise heating techniques, the latter combined with 40Ar/ 39Ar dating. Maximum 40Ar/ 36Ar ratios of individual samples range from 4190 ± 240 to 25,590 ± 620, the latter (sample SA 87-2/12) being the highest measured value of any mantle peridotite or pyroxenite sample so far. For five samples, reasonably defined binary mixing correlations between atmospheric and mantle-derived argon and neon resulted in calculated sample-specific mantle- 40Ar/ 36Ar ratios of 5040 ± 50, 7500 ± 70, 9510 ± 220, 29,600 ± 2700, and 39,000 ± 2500 at an assumed mantle- 20Ne/ 22Ne value of 12.5. These observations indicate a highly heterogeneous argon isotopic composition of the trapped mantle fluids. Furthermore, most samples exhibit mantle- 36Ar/ 22Ne ratios higher than considered to represent an unfractionated mantle source. These characteristics could be explained by mixing of plume-related (low 40Ar/ 36Ar) and lithospheric (high 40Ar/ 36Ar) melts or fluids following elemental fractionation between Ar and Ne. Alternatively, we propose the presence of a severely fractionated atmospheric component highly enriched in argon, admixed to the mantle fluids before trapped by the ultramafic rocks. This primary, seriously fractionated atmospheric component cannot be monitored by Ne isotopes. Incorporation of such a primary atmospheric component could occur either during magma ascent at relatively shallow, upper crustal levels, or by mobilization of atmospheric fluids in a subduction-related metasomatized lithospheric mantle during Pan African times (c. 600–800 Ma ago).

Cluster and shell correlations in Ne isotopes

Structure of Ne isotopes are investigated by a microscopic model, where thousands of Slater determinants are stochastically generated, and important ones are selected out of them. The importance of the α-cluster correlation is systematically shown, and also, the singleparticle parity of each nucleon is studied, and deviation from the pure shell-model-like picture is discussed.

Xenon isotope systematics, giant impacts, and mantle degassing on the early Earth

The relationships between the major terrestrial volatile reservoirs are explored by resolving the different components in the Xe isotope signatures displayed by Harding County and Caroline CO 2 well gases and mid-ocean ridge basalts (MORB). For the nonradiogenic isotopes, there is evidence for the presence of components enhanced in the light 124–128Xe/ 130Xe isotope ratios with respect to the terrestrial atmosphere. The observation of small but significant elevations of these ratios in the MORB and well gas reservoirs means that the nonradiogenic Xe in the atmosphere cannot be the primordial base composition in the mantle. The presence of solar-like components, for example U–Xe, solar wind Xe, or both, is required. For radiogenic Xe generated by decay of short-lived 129I and 244Pu, the 129Xe rad/ 136Xe 244 ratios are indistinguishable in MORB and the present atmosphere, but differ by approximately an order of magnitude between the MORB and well gas sources. Correspondence of these ratios in MORB and the atmosphere within the relatively small uncertainties found here significantly constrains possible mantle degassing scenarios. The widely held view that substantial early degassing of 129Xe rad and 136Xe 244 from the MORB reservoir to the atmosphere occurred and then ended while 129I was still alive is incompatible with equal ratios, and so is not a possible explanation for observed elevations of 129Xe/ 130Xe in MORB compared to the atmosphere. Detailed degassing chronologies constructed from the isotopic composition of MORB Xe are therefore questionable. If the present estimate for the uranium/iodine ratio in the bulk silicate Earth (BSE) is taken to apply to all interior volatile reservoirs, the differing 129Xe rad/ 136Xe 244 ratios in MORB and the well gases point to two episodes of major mantle degassing, presumably driven by giant impacts, respectively ∼ 20–50 Ma and ∼ 95–100 Ma after solar system origin assuming current values for initial 129I/ 127I and 244Pu/ 238U. The earlier time range, for degassing of the well gas source, spans Hf–W calculations for the timing of a moon-forming impact. The second, later impact further outgassed the upper mantle and MORB source. A single event that degassed both the MORB and gas well reservoirs at the time of the moon-forming collision would be compatible with their distinct 129Xe rad/ 136Xe 244 ratios only if the post-impact iodine abundance in the MORB reservoir was about an order of magnitude lower than current estimates. In either case, such late dates require large early losses of noble gases, so that initial inventories acquired throughout the Earth must have been substantially higher. The much larger 129Xe rad/ 136Xe 244 ratio in the well gases compared to MORB requires that these two Xe components evolve from separate interior reservoirs that have been effectively isolated from each other for most of the age of the planet, but are now seen within the upper mantle. These reservoirs have maintained distinct Xe isotope signatures despite having similar Ne isotope compositions that reflect similar degassing histories. This suggests that the light noble gas and radiogenic Xe isotopes are decoupled, with separate long-term storage of the latter. However, without data on the extent of heterogeneities within the upper mantle, this conclusion cannot be easily reconciled with geophysical observations without significant re-evaluation of present noble gas models. Nevertheless the analytic evidence that two different values of 129Xe rad/ 136Xe 244 exist in the Earth appears firm. If the uranium/iodine ratio is approximately uniform throughout the BSE, it follows that degassing events from separate reservoirs at different times are recorded in the currently available terrestrial Xe data.

Direct evidence for the onset of intruder configurations in neutron-rich Ne isotopes

Abstract We report on direct experimental evidence of the population of the 3 / 2 − intruder state in 27Ne in the knockout of a single neutron from the ground state of 28Ne. This low-lying negative parity state is consistent with a narrower shell gap for exotic nuclei with Z ≪ N and N ≈ 20 . Monte Carlo shell-model calculations with the modern SDPF-M interaction successfully describe neutron-rich nuclei in the vicinity of N = 20 where normal and intruder configurations coexist at low excitation energy. This observation demonstrates the importance of direct reactions for the study of exotic nuclei and the predictive power of these large-scale shell-model calculations.

A macroscopic–microscopic study of Ne and Mg nuclei around N = 20

A new set of Nilsson parameters, with the isospin-dependent spin–orbit potential (κ) and l2t term (μ), is proposed for neutron-rich nuclei of Ne and Mg isotopes. The ground-state deformations and B(E2) values of Ne and Mg nuclei around N = 20 are studied within the framework of the macroscopic–microscopic (MM) model with the new set of Nilsson parameters. The calculated results are compared with the experimental data. The experimental large deformations and B(E2) values are well reproduced by the new calculation of this paper. The calculated results are also compared with the recent results of mean-field models and shell models. Detailed discussions of the shell structure around N = 20 are given. It is predicted that the enhanced negative shell correction energies at deformed shape and the weak N = 20 magic shell closure lead to the large deformations for Ne and Mg nuclei around N = 20.

Shell-model-like Approach (SLAP) for the Nuclear Properties in Relativistic Mean Field Theory

A Shell-model-like approach suggested to treat the pairing correlations in relativistic mean field theory is introduced, in which the occupancies thus obtained have been iterated back into the densities. The formalism and numerical techniques are given in detail. As examples, the ground state properties and low-lying excited states for Ne isotopes are studied. The results thus obtained are compared with the data available. The binding energies, the odd–even staggering, as well as the tendency for the change of the shapes in Ne isotopes are correctly reproduced.

Development of a Miniature Double Focusing Mass Spectrograph Using a Focal Plane Detector

A miniature mass spectrograph was newly designed and constructed as a prototype model for future lunar or planetary explorations. The ion optical system was newly designed based on Mattauch-Herzog geometry. The mass spectrograph employs a focal plane detector consisting of a microchannel plate (MCP), a phosphor layer, a fiber-optic plate (FOP) and a charge-coupled device (CCD). The 2D and 1D spectra of the residual gases, krypton and neon were observed in preliminary experiments. The mass resolution of 130 was achieved experimentally, and this was in good agreement with the simulation based on the transfer matrix method. The experimental value of the detectable m/z range was also consistent with the calculated version. Moreover, stable isotopes of Kr and Ne were observed without saturation of the detector. A dynamic range of 300 was achieved.

Deformations and B(E2) Values of Ne and Mg Nucleiaround N = 20

The ground-state deformations and B(E2) values of Ne and Mg nucleiaround N = 20 are studied within the framework of the macroscopic–microscopic (MM) model with isospin-dependent Nilssonpotential. The calculated results are compared with those obtained fromstandard Nilsson parameters and with experimental ones. It is found thatthe calculations with new Nilsson parameters well reproduce theexperimental large deformations and B(E2) values for Ne and Mg nucleiaround N = 20. The N = 20 shell closure of Ne and Mg isotopes disappearsin the MM model and this agrees with experimental data.