16O Cross Section Research Articles

Differential cross section measurement of 16O(d,p0,1) reactions at energies and angles relevant to NRA

Nuclear Reaction Analysis (NRA) and more specifically d-NRA is a suitable technique for 16O detection on or near surface of a compound containing heavy elements. The main problem of d-NRA is the limited number of NRA differential cross section data sets and their differences. The differential cross sections of 16O(d,p0,1) reactions were measured in the present work for Ed,lab=700–1800keV, at four scattering angles, namely at 90°, 135°, 150° and 165° which are suitable for NRA. Differential cross sections were collected with an energy step of ∼10keV. However, detailed measurements were carried out with an energy step of ∼2keV around resonances. The results are compared with previous studies and the similarities and differences are discussed.

Determination of molecular stopping cross section of 12C, 16O, 28Si, 35Cl, 58Ni, 79Br, and 127I in silicon nitride

Silicon nitride is a technologically important material in a range of applications due to a combination of important properties. Ion beam analysis techniques, and in particular, heavy ion elastic recoil detection analysis can be used to determine the stoichiometry of silicon nitride films, which often deviates from the ideal Si3N4, as well as the content of impurities such as hydrogen, even in the presence of other materials or in a matrix containing heavier elements. Accurate quantification of IBA results depends on the basic data used in the data analysis. Quantitative depth profiling relies on the knowledge of the stopping power cross sections of the materials studied for the ions involved, which in the case of HI-ERDA is both the primary beam, and the recoiled species. We measured the stopping cross section of 12C, 16O, 28Si, 35Cl, 58Ni, 79Br, and 127I in a well-characterised silicon nitride membrane. The measurements were made by independent groups utilising different experimental setups and methods. In some cases there is extensive overlap of the energy range in different experiments, allowing a comparison of the different results. The four independent data sets reported in this work are in excellent agreement with each other, in the cases where similar energy ranges were measured. On the other hand, the data are in most cases higher than calculations made with the interpolative schemes SRIM and MSTAR together with the Bragg rule. Better agreement is found with MSTAR in some of the cases studied. This work is a significant extension of the heavy ion stopping power data base for silicon nitride.

Predominance of single vacancy production in the formation of Oxygen dication

SynopsisAbsolute cross sections for O2++ production from O2 fragmentation by 30-400 eV electron impact were measured using the DETOF technique. This data is compared to the molecular orbitals ionization cross sections of O2, which were calculated using a simple and general procedure, and the fragmentation fractions arising from each molecular orbital were obtained. It was seen from the behaviour of the cross sections in this energy range that O2++ results mainly from single vacancy production followed by post-collisional Auger-like deexcitation.

Measurement of fusion cross sections for 16O+16O

In earlier works, the fusion cross section for the 16O+16O reaction has been measured using different techniques. In the present work, we have obtained an experimental excitation function for 16O+16O using γ-ray spectroscopy. The measurements were performed at center-of-mass energies between 8.28 and 12.25 MeV. The theoretical predictions obtained with a coupled-channel model are consistent with the experimental data. From our analyses, the extrapolated S-factor value at 6.6 MeV, corresponding to the Gamow peak energy for core oxygen burning conditions, is about 3.6 × 1025 MeV barn.

Covariance of Neutron Cross Sections for 16O through R-matrix Analysis

Through the R-matrix analysis, neutron cross sections as well as the covariance are estimated for 16O in the resolved resonance range. Although we consider the current results are still preliminary, we present the summary of the cross section analysis and the results of data uncertainty/covariance, including those for the differential cross sections. It is found that the values obtained highlight consequences of nature in the theory as well as knowledge from measurements, which gives a realistic quantification of evaluated nuclear data covariances.

Negatively and positively charged oxygen ion formation in collisions of positively charged projectiles with O molecules in the energy range 50–350 keV

Absolute cross sections for positive and negative ion fragments formed from O2 molecules following collisions with H+, H2+, He+, and Ar+ projectiles in the energy range 50–350 keV are presented. Cross sections for negatively charged secondary ions are much smaller ( cm2) compared to those for positively charged fragments. The non-dissociative formation of negatively charged O2− ions results from an electron transfer collision with the projectile. The impact energy dependence shows that the collision cross section for O2− formation strongly depends on the type of projectile involved.

Low-temperature collisional quenching of NO A(2)Σ(+)(v' = 0) by NO(X(2)Π) and O2 between 34 and 109 K.

We present measurements of collisional fluorescence quenching cross sections of NO(A(2)Σ(+), v' = 0) by NO(X(2)Π) and O2 between 34 and 109 K using a pulsed converging-diverging nozzle gas expansion, extending the temperature range of previous measurements. The thermally averaged fluorescence quenching cross sections for both species show a monotonic increase as temperature decreases in this temperature range, consistent with earlier observations. These new measurements, however, allow discrimination between predictions obtained by extrapolating fits of previous data using different functional forms that show discrepancies exceeding 120% for NO and 160% for O2 at 34 K. The measured self-quenching cross section is 52.9 Å(2) near 112 K and increases to 64.1 Å(2) at 35 K, whereas the O2 fluorescence quenching cross section is 42.9 Å(2) at 109 K and increases to 58.3 Å(2) at 34 K. Global fits of the quenching cross section temperature dependence show that, when including our current measurements, the low temperature behavior of the quenching cross sections for NO and O2 is better described by a parameterization that accounts for the long-range interactions leading to the collisional deactivation via an inverse power law model.

On the distribution of CO2 and CO in the mesosphere and lower thermosphere

AbstractWe have used the Whole Atmosphere Community Climate Model (WACCM) to calculate the distribution of CO2 and CO in the mesosphere and lower thermosphere (MLT), and we have compared the results with observations, mainly from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer and Michelson Interferometer for Passive Atmospheric Sounding satellite‐borne instruments. We find that WACCM can reproduce the observed distribution of CO2 in the MLT and the rapid falloff of CO2 above about 80 km. Analysis of the principal terms in the calculated budget of CO2 shows that its global‐mean vertical profile is determined mainly by the competition between molecular diffusive separation and eddy mixing by gravity waves. The model underestimates somewhat the mixing ratio of CO2 in the thermosphere compared to that in the observations, but we show that the discrepancy may be eliminated by a reasonable adjustment of the Prandtl number used to calculate the diffusivity due to gravity waves. Simulated CO is also consistent with observations, except that in the standard version of the model, its mixing ratio is uniformly lower than observed above about 100 km. We conclude that WACCM likely underestimates the rate of production of CO in the lower thermosphere from photolysis of CO2 at wavelengths < 121 nm, and we show that this stems from the use of a very large absorption cross section for O2 in the wavelength range 105–121 nm. When a smaller cross section is used, photolysis of CO2 increases by a factor of 2–3 at ~ 95–115 km and, as a result, CO mixing ratios become larger and agree much more closely with observations. We emphasize that the increase in CO2 photolysis implies only minor changes in the vertical profile of CO2 because photolytic loss is a minor term in the budget of CO2 in the MLT.

Reactivity Impact of 2H and 16O Elastic Scattering Nuclear Data on Critical Systems with Heavy Water

The accuracy of deuterium nuclear data is important for reactor physics simulations of heavy water (D2O) reactors. The elastic neutron scattering cross section data at thermal energies, σs,th, have been observed to have noticeable impact on the reactivity values in simulations of critical systems involving D2O. We discuss how the uncertainties in the thermal scattering cross sections of 2H(n,n)2H and 16O(n,n)16O propagate to the uncertainty of the calculated neutron multiplication factor, keff, in thermal critical assemblies with heavy water neutron moderator/reflector. The method of trial evaluated nuclear data files, in which specific cross sections are individually perturbed, is used to calculate the sensitivity coefficients of keff to the microscopic nuclear data, such as σs(E) characterized by σs,th. Large reactivity differences of up to ≃ 5–10 mk (500–1000 pcm) were observed using 2H and 16O data files with different elastic scattering data in MCNP5 simulations of the LANL HEU heavy-water solution thermal critical experiments included in the ICSBEP handbook.

Total cross sections for O2 and S2 by electron impact

We report total cross sections for electron collision with O2 and S2 molecules in the energy range 0.1–2000eV by combining two theoretical methods. Below the ionization threshold of the target, total cross section is calculated by the ab initio R-matrix method through Quantemol-N software. For intermediate and high energies, we have applied a Spherical complex optical potential formalism. Comparison is made with the available theoretical and experimental data and a reasonable agreement is observed. The high energy cross section for S2 molecule is reported for the first time.

Reactivity Impact of 16O Thermal Elastic-Scattering Nuclear Data for Some Numerical and Critical Benchmark Systems

The thermal neutron elastic-scattering cross-section data for 16O used in various modern evaluated nuclear data libraries were reviewed and found to be generally too high compared with the best available experimental measurements. Some of the proposed revisions to the ENDF/B-VII.0 16O data library and recent results from the TENDL system increase this discrepancy further. The reactivity impact of revising the 16O data downward to be consistent with the best measurements was tested using the JENDL-3.3 16O cross-section values and was found to be very small in MCNP5 simulations of the UO2 and reactor-recycle MOX fuel cases of the American Nuclear Society Doppler defect numerical benchmark. However, large reactivity differences of up to [approximately]14 mk (1400 pcm) were observed using 16O data files from several evaluated nuclear data libraries in MCNP5 simulations of the Los Alamos National Laboratory highly enriched uranium (HEU) heavy water solution thermal critical experiments, which were performed in the 1950s. The latter result suggests that new measurements using HEU in a heavy water-moderated critical facility, such as the ZED-2 zero-power reactor at the Chalk River Laboratories, might help to resolve the discrepancy between the 16O thermal elastic-scattering cross-section values and thereby reduce or better define its uncertainty, although additional assessment work would be needed to confirm this.

Sensitivity of stratospheric dynamics to uncertainty in O3production

Some key photochemical uncertainties that cannot be readily eliminated by current observations translate into a range of stratospheric O3abundances in the tens of percent. The uncertainty in O3production due to that in the cross sections for O2in the Hertzberg continuum is studied here with the NCAR Community Atmosphere Model, which allows for interactive climate and ozone chemistry. A min‐max range in the O2cross sections of 30%, consistent with current uncertainties, changes O3abundances in the lower tropical stratosphere by up to 30%, with a relatively smaller and opposite change above 30 hPa. Here we have systematically examined the changes in the time‐mean state, the seasonal cycle, and the interannual variability of the temperature and circulation associated with the ±30% change in O2cross sections. This study points to the important role of O3in the lower tropical stratosphere in determining the physical characteristics of the tropical tropopause layer. Reducing O2cross sections by 30% increases ozone abundances which warms the lower stratosphere (60°S −60°N; 2 K maximum at equator) and lowers the tropopause height by 100–200 m (30°S –30°N). The large‐scale warming leads to enhanced stratification near the tropopause which reduces upward wave propagation everywhere except for high latitudes. The lowermost tropical stratosphere is better ventilated during austral winter. The annual cycle of ozone is amplified. The interannual variability of the winter stratospheric polar vortices also increases, but the mechanism involves wave‐mean flow interaction, and the exact role of ozone in it needs further investigation.

Electron-impact dissociation and ionization of OH+ and OD+ ions

Absolute cross sections for electron impact ionization and dissociation of OH+ and OD+ leading to the formation of the OH2+, O+, O2+, O3+ and D+ ions have been measured by applying the animated electron-ion beam method in the energy range from the respective reaction thresholds up to 2.5 keV. The maximum of the single ionization cross section is found to be (0.95 ± 0.02) × 10-19 cm2 at 155 eV. The maximum total cross sections for O+ and D+ fragments production are observed to be (15.7 ± 0.2) × 10-17 cm2 at 95 eV and (10.8 ± 0.5) × 10-17 cm2 at 95 eV, respectively. The cross sections for O2+ and O3+ are much smaller, (5.37 ± 0.04) × 10-18 cm2 at 135 eV and (7.95 ± 0.23) × 10-20 cm2 at 315 eV, respectively. The collected data are analyzed in details in order to determine separately the contributions of dissociative excitation and of dissociative ionization to the O+ and D+ fragments production.

The ACOS CO<sub>2</sub> retrieval algorithm – Part II: Global X<sub>CO<sub>2</sub></sub> data characterization

Abstract. Here, we report preliminary estimates of the column averaged carbon dioxide (CO2) dry air mole fraction, XCO2, retrieved from spectra recorded over land by the Greenhouse gases Observing Satellite, GOSAT (nicknamed "Ibuki"), using retrieval methods originally developed for the NASA Orbiting Carbon Observatory (OCO) mission. After screening for clouds and other known error sources, these retrievals reproduce much of the expected structure in the global XCO2 field, including its variation with latitude and season. However, low yields of retrieved XCO2 over persistently cloudy areas and ice covered surfaces at high latitudes limit the coverage of some geographic regions, even on seasonal time scales. Comparisons of early GOSAT XCO2 retrievals with XCO2 estimates from the Total Carbon Column Observing Network (TCCON) revealed a global, −2% (7–8 parts per million, ppm, with respect to dry air) XCO2 bias and 2 to 3 times more variance in the GOSAT retrievals. About half of the global XCO2 bias is associated with a systematic, 1% overestimate in the retrieved air mass, first identified as a global +10 hPa bias in the retrieved surface pressure. This error has been attributed to errors in the O2 A-band absorption cross sections. Much of the remaining bias and spurious variance in the GOSAT XCO2 retrievals has been traced to uncertainties in the instrument's calibration, oversimplified methods for generating O2 and CO2 absorption cross sections, and other subtle errors in the implementation of the retrieval algorithm. Many of these deficiencies have been addressed in the most recent version (Build 2.9) of the retrieval algorithm, which produces negligible bias in XCO2 on global scales as well as a ~30% reduction in variance. Comparisons with TCCON measurements indicate that regional scale biases remain, but these could be reduced by applying empirical corrections like those described by Wunch et al. (2011b). We recommend that such corrections be applied before these data are used in source sink inversion studies to minimize spurious fluxes associated with known biases. These and other lessons learned from the analysis of GOSAT data are expected to accelerate the delivery of high quality data products from the Orbiting Carbon Observatory-2 (OCO-2), once that satellite is successfully launched and inserted into orbit.

One-electron capture reactions between Kr2+ ions and molecules in the sub-thermal energy region

One-electron capture reactions between doubly charged krypton ions and molecules (H2, N2, O2, CO and NO) were studied using an injected-ion drift tube mass spectrometer. An energy range of 14–60 meV was studied in a center-of-mass system. The drift tube used in this experiment was cooled with liquid nitrogen, and the temperature of buffer gas, which was Kr, was measured to be about 90 K. Doubly charged ions produced in a conventional electron impact-type ion source may contain metastable states. In this work, the cross sections of ground state Kr2+ ions were separated from those of metastable states using variations in the ion mobilities. The cross sections of H2 targets strongly depended on the ion states. In contrast, no ion state dependence was observed for the cross sections of O2, CO or NO targets.

Activation cross section measurements of 16O(n,t) above 18.1 MeV up to 33.1 MeV

We have measured the cross sections of the 16O(n,t) reactions above 18.1 up to 33.1 MeV in an neutron activation method. H2O (water) as an 16O target was irradiated with semi-monoenergetic neutrons generated from the 9Be(p,n)9B reaction with 25–35 MeV protons. The neutron flux was obtained with the aid of previous study by Uwamino et al. (Nucl Instr Methods A 271:546, 1988). The tritium activities were measured by using the liquid scintillation counting (LSC) method. The present value for the cross section of 16O(n,t) reaction agrees with previous values measured by using the same LSC method at similar neutron energy ranging from 18.1 up to 33.1 MeV.

Folding Model Analysis to Calculate Differential Cross Sections for the (16O, 12C) System

Folding Model Analysis to Calculate Differential Cross Sections for the (16O, 12C) System

Interferences associated with electron emission from O2 by fast ions compared with H2 and N2

Oscillatory structures associated with electron emission from O2 by 3 MeV/u H2 and 1.9 MeV/u O5,82impact are analyzed and compared with results for H2 and N2. Measured molecular O2 cross sections normalized to theoretical molecular O2 cross sections indicate oscillations suggestive of second-order interferences. Contrary to previous findings for H2 targets, no evidence of first-order interferences was revealed. The results are in qualitative agreement with recent observations for 3 MeV/u H2 2 N2 collisions, where a strong suppression of the primary Young-type interferences was also reported.

Interference effects in electron emission spectra for 3 MeV/u H+ + O2 collisions

The present work extends the study of oscillatory structures associated with electron emission from diatomic molecular targets to 3 MeV/u H+ + O2. Electrons ejected from O2 were detected in the energy range 5 to 400 eV and for observation angles 30°, 60°, 90° and 150°. Experimental molecular O2 cross sections normalized to theoretical molecular O2 cross sections reveal oscillatory structures suggestive of secondary interferences as evidenced by their independence on the observation angle. Contrary to results for 1, 3 and 5 MeV/u H+ + H2, no obvious evidence for primary Young-type interferences was seen, a result similar to that found for H+ + N2 collisions.

Electron impact double ionization cross sections of light elements

A simple user-friendly semiempirical model is proposed to calculate electron impact double ionization cross sections of He, Li, Li+, B+, C+, C3+, O, O2+, O3+, Ne, Ne+, Ne2+, Na, Mg, Al3+, S, and Arq+ (q=0–7) targets for the incident electron energies from threshold to 106 eV. The contributions in the total double ionization cross sections from the direct double ionization and inner-shell ionization processes are taken into account on the basis of experimental data considered. The results of the present analysis are compared with the available experimental data and theoretical calculations. The model is found successful for the description of experimental cross sections. Since, this model may be a prudent selection to meet the demand level in plasma modeling due to its simple inherent structure.