Nuclear Reaction 18O Research Articles

Carbon and noble gases in Archean chert

In order to study the origin and evolution of volatiles trapped in Archean and Early Proterozoic cherts, we have measured contents and isotopic compositions of carbon and noble gases using step-heating and mass spectrometry. The observed negative correlation between the δ 13C-value and nitrogen/carbon ratio in the samples suggests that trapped carbon in Archean cherts is a mixture of carbonate, sedimentary organic matter and an atmospheric component. Elemental abundance patterns of noble gases show a slight depletion of 20Ne and excess of 132Xe relative to air, which is explained by atmospheric noble gases dissolved in water at relatively low temperature. The 3 He 4 He ratios indicate an apparent crustal value of (1–3)·10 −3, consistent with the in situ radiogenic production ratio. Significant excess 21Ne, 131Xe and 132–136Xe as well as slight enrichment of 38Ar and 86Kr relative to atmosphere are found in the Onverwacht chert at the 800° and 1200°C steps. Similar but small isotope anomalies are detected in the Frere Formation chert at the 1800°C step. Excess 21Ne shows positive correlations with 38Ar, 86Kr and 132Xe- 136Xe enrichments while excess 131Xe has no relation with the other isotope anomalies. The 21Ne anomaly can be attributable to the nuclear reaction 18O(α,n) 21Ne. The enrichments of 86Kr and 132Xe- 136Xe are derived from spontaneous fission of 238U in the cherts. The slight excess 38Ar may be due to the 35Cl(α,p) 38Ar reaction based on the positive correlation between 21 Ne 22 Ne and 38 Ar 36 Ar ratios, but should be verified in future work.

Light-element analysis with the CENBG nuclear microprobe

In the field of light-element detection ( Z < 11), the capability of the nuclear muprobe to carry out nuclear reaction analysis offers significant advantages compared with other methods. But taking into account the low beam current available, reactions with large cross sections must be selected in order to achieve acquisition in a reasonable time. This restriction prevents the use of RBS analysis for detection of light elements in heavy matrices. Moreover, the 8 μm beryllium window of standard X-ray detectors does not permit the PIXE measurement of light elements. We present here some examples in the use of the resonant nuclear reactions 12C(p, p) 12C and 16O(α, α) 16O for carbon and oxygen determination in different materials. Results are discussed in terms of strength and limit of nuclear reactions compared with new possibilities of X-ray detectors following improvements in the transmission of low energy radiation through windows.

Modifications of the gallium arsenide crystal surface during annealing

Abstract Nuclear reaction analysis (NRA) and the channeling technique (RBSC) have been used to obtain information about the surface composition and the lattice distortion of gallium arsenide (GaAs) during annealing. The samples were annealed at temperatures up to 600°C under different gases (air, N2, CO2). Using the nuclear reactions 12C(d,p)13C and 16O(d,p)17O the quantity of carbon and oxygen at the surface of the GaAs was measured. By means of the channeling technique it is possible to study the lattice damage in the surface region. At the surface of GaAs an addition of oxygen and a reduction of carbon during annealing under air and CO2 is obtained. The results of the channeling data show a disorder in the surface region of samples with increasing concentration of oxygen.

Channeling study of high- Tc superconducting single crystal sublattices

Backscattering of H, 4He, 3He ions, X-ray radiation and the nuclear reactions 16O( 4He, 4He) 16O, 18O(p, α) 15N, 16O( 3He, 4He) 15O, 7Li(p, α) 4He, etc., in combination with orientation effects have been used to investigate the structure and properties of the single crystals Nd 2− x Ce x CuO 4(T'-phase) and La 2− x Sr x CuO 4 (T-phase). The possibility of a selective study of cation and onion sublattices, as well as oxygen O1, O2 positions is demonstrated. The dependence of the La 2− x Sr x CuO 4 structural perfection on the Sr content is established. The location of Li and B is determined. Studies were made of the diffusion mobility and adsorption properties of 18O in the YBa 2Cu 3O 7−y ceramics. The influence of crystal irradiation with H and 4He ions on the shapes of the angular dependences of the yields of nuclear reactions was studied. The effect of interest has been observed for the α-yield of the 16O( 4He, 4He) 16O reactions after 4He ion irradiation of Nd 2− x Ce x CuO 4, namely, the dip with X min(O) ≅ 0.18 transforms to a peak with X min(O) ≅ 1.5; the X min(Nd) value changes in this case from 0.024 ro 0.19.

Structure, Mechanical Properties, and Oxidation Behavior of Nanolayered MoSi2/SiC Coatings

ABSTRACTThe structure, hardness and elastic properties, and oxidation behavior of a nanolayered MoSi2/SiC structure were examined. DC magnetron and rf diode sputtering were used to deposit MoSi2 and SiC, respectively. The total number of sublayers of each kind was 90. The nominal thickness of the MoSi2 sublayers was 10 and that of the SiC 3 nm. The microstructure of the as-deposited samples was amorphous whereas annealing at 500 °C for I h resulted in the transformation of MoSi2 into C40 type hexagonal structure as determined using cross-section transmission electron microscopy (TEM). However, silicon carbide still remained amorphous. Hardness and Young's modulus were determined by using a nanoindentation technique. Hardness of 11.6 and 20.8 GPa were obtained for the asdeposited and annealed structure, respectively. Corresponding Young's moduli were 220 and 290 GPa, which yield values of 232 and 332 GPa for Young's moduli of amorphous and crystalline MoSi2. Oxidation tests were carried out in wet oxidation conditions at 400 and 500 °C. The degree of oxidation was determined by measuring oxygen concentration on the surface using a nuclear reaction 16O(d,p) 17O at a deuterium energy of 950 keV. The MoSi2/SiC coating gave good protection against oxidation for unalloyed steel. As compared to a single MoSi2 coating a slightly higher degree of oxidation and a significantly different mechanism of oxidation was observed. This was also supported by secondary ion mass spectroscopy (SIMS) measurements. Cracking of the coating caused by a thermal mismatch and transformation stresses was worse in the case of the single MoSi2 layer than in the case of the composite coating. Thus cracking of the coating can be significantly reduced by using a nanolayered structure.

Noble gases in six GSJ igneous rock samples.

Six igneous rocks of the GSJ reference samples were analyzed mass spectrometrically to determine the elemental abundances and isotopic compositions of noble gases. The concentrations of noble gases except for radiogenic 4He and 40Ar are low and in the range reported for various types of igneous rocks. In JR-1 obsidian a significant enrichment in Ne was found, which might be caused by diffusion of atmospheric Ne into the sample. K-Ar ages of JR-1, JGb-1 and JA-2 calculated using the 40Ar contents measured in this study and K in literature are 0.94 ± 0.09, 96 ± 10 and 13 ± 1 Ma, respectively. They agree with the ages previously reported. JG-1 has excesses of 21Ne and heavy Xe isotopes, which can be attributed to the nuclear reaction 18O(α, n)21Ne and 238U spontaneous fission, respectively. Since the 3He/4He ratio of JP-1 peridotite is similar to the ratio of MORB, which is higher than the ratio associated with arc volcanism, this sample may provide information about the noble gas state in the wedge mantle beneath the Japanese Islands. The low 40Ar/36Ar ratio of 384 for this sample implies that 99.8% of total 36Ar came from atmosphere. If the contaminative Ar was transported from sea water to the subducting oceanic plate, 0.2 mg of water for each gram of peridotite is sufficient to account for the contamination. The 3He/4He ratio, however, is not changed by this contamination.

Modification of surface mechanical properties of high chromium tool steel by carbon-implanted codeposited Fe-Ti films

An iron-titanium film 300 nm thick was deposited on a tool steel (1.55% C, 0.3% Si, 0.3% Mn, 12% Cr, 0.8% Mo, and 0.8% V) by co-evaporation of iron and titanium. Subsequently this surface film was implanted with carbon at energies of 55, 120, and 200 keV to a total fluence of 1.24×10 18C + cm -2. This treatment produced a surface hardness of 15 GPa. The effect of this coating on unlubricated wear and friction was tested in air at a relative humidity of 10% in a pin-on-disc tester using a 440C pin as a counterface. The sliding mechanism of the untreated substrate was found to be based on the transfer of pin material and subsequent growth of uneven oxide hillocks on the wear track. Oxide scales were observed also on a wear scar of the pin, presumably as a result of back-deposition. In contrast, the sliding mechanism on the coated sample was drastically different. A more uniform transfer film originating in the coating was found on the pin, resulting in sliding between identical materials. No wearthrough of the coating occured during the test of 5000 cycles at a hertizian pressure of 835 MPa, and the surface of the wear track showed extreme smoothness to the very end of the test. The friction coefficient was decreased from 0.7 for the uncoated to 0.2 for the coated surface. The wear track on the coated surface was also found to be slightly oxidized, as determined by a nuclear reaction ( 16O(d,p) 17O). The reduction in friction was mainly attributed to the increased hardness of the counterfaces and an adequate but controlled oxidation.

Methodology and application of the nuclear resonance reaction 16O(α, α) 16O for the profiling of titanium oxide

A method is presented for the application of the 16O(α,α) 16O resonance at 3.045 MeV to the depth profiling of oxide films. The resonance yield is translated into oxygen to metal stoichiometry as a function of the depth probed. The method is applied to anodic titanium oxide films grown in the absence and presence of tungsten anions. The oxygen depth profile is then utilized to obtain the profile of the dopants (W) incorporated in the oxide. A depth resolution of 6 μg/cm is achieved. The incorporation of tungsten anions of the order of 2 at.%, evaluated from the RBS yields, precludes the establishment of a constant oxygen to metal stoichiometry in the doped film. The tungsten atomic fractions have an estimated error of 5%.

Oxygen and silicon self-diffusion in natural olivine at T = 1300° C

Oxygen and silicon self-diffusion in natural olivine single crystals (Mg 0.89 Fe 0.11) 2 SiO 4 from San Carlos, Arizona, have been measured for the first time at T = 1300°C in controlled oxygen partial pressure pO 2 ranging from 10 −4 to 10 Pa. Polished and chemically-etched specimens were heated for several hours in a gas mixture H 2/CO 2 or H 2/H 2 18O. The latter gas mixture provides both the controlled pO 2 and the isotopic reservoir for the 18O diffusion across the gas-solid interface. For 30Si diffusion, a thin layer of isotopic forsterite Mg 2 30SiO 4 (few hundreds of angstroms) was deposited onto the olivine surface. Diffusion profiles for oxygen and silicon were determined using the nuclear reaction 18O(p, α) 15N and Rutherford back-scattering of 2 MeV α particles, respectively. The diffusion coefficients are D ox = 10 −18 ± 0.6m 2s −1 for oxygen and D Si = 10 −20m 2s −1 for silicon. We conclude from these measurements on San Carlos olivine at T = 1300°C that: (1) Si is the slowest diffusing species in natural olivine, as it is in forsterite, and (2) 30Si and 18O diffuse 30 to 200 times faster in olivine than in forsterite.

Determination of 18O by prompt nuclear reaction analysis: application for measurement of microsamples.

A method is described for the routine determination of 18O concentrations in microsamples of biological fluids. The method utilizes the prompt nuclear reaction 18O(p, alpha o)15N, and 846-keV protons from a 3-MeV Van de Graaff Accelerator are focused on approximately 2,000-A-thick Ta2O5 targets prepared by anodic oxidation from 50-microliter samples of water distilled from blood or other biological fluids. The broad cross section of the resonance peak for this nuclear reaction (47 keV) ensures high yields, especially at small reaction angles, and the high-energy alpha particles produced by the reaction (4 MeV) are readily separated from scattered protons by the use of an aluminized Mylar foil of suitable thickness. Background levels of 18O (0.204 atom%) can be detected with run times of approximately 5-8 min, and the sensitivity of the method is of the order of 0.05 atom %. Experimental error due to sample preparation was found to be 1.7%, and counting errors were close to theoretical limits so that total error was of the order of 2.5%. Duplicate samples were analyzed by use of the 18O(p, alpha o)15N reaction at Lucas Heights, Australia, and the 18O(p,n)18F reaction by the method of Wood et al. (Anal. Chem. 47: 646-650, 1975) at the University of California, Los Angeles, and the agreement was excellent (y = 1.0123x - 0.0123, r = 0.991, P less than 0.001). The theoretical limitations and the general applicability of the method in biological studies designed to estimate the rate of metabolism of free-ranging animals are discussed.

Internal oxidation of Sb-implanted silver single crystals

Internal oxidation of Sb-implanted silver single crystals has been studied by Mössbauer spectroscopy in combination with ion-beam analysis. The concentration and depth distribution of oxygen were determined by using the nuclear reaction 18O(p, α) 15N. Strong evidence was found for the existence of an inward moving oxidation front. In crystals oxidized for less than about 5 min in −100 mbar 18O 2 at 550 K, Sb was found to be associated with two oxygen atoms, while for oxidation times longer than 30 min this ratio was 4. Channeling experiments using the same reaction show that the oxygen atoms trapped by the Sb impurities occupy octahedral interstitial sites.

Determination of oxygen in the production of nitrides by means of neutron activation

A procedure is described for the determination of oxygen impurities in ultradisperse and dense samples of titanium, aluminium, niobium, boron and silicon nitrides using the nuclear reaction 16O(n, p) 16N. The samples are placed in a sealed titanium dish and irradiated with 14 MeV neutrons produced in a neutron generator. The activity of the 16N produced is measured by 4–8 MeV γ emission in a scintillation γ spectrometer. The oxygen content to be determined was from n × 10 −1 mass % to n × 10 mass % with a relative standard deviation of 10% and 2% respectively. The results of the nitride analysis are presented.

The reactivity of ion-implanted SiC

Implantation of chromium into single-crystal or polycrystalline α-SiC produces a surface amorphous layer for displacement damage greater than about 0.2 displacements per atom at room temperature. The enhanced chemical reactivity of such specimens was studied by two methods: chemical etching rate and oxidation rate. The chemical etching rates in a saturated solution of 50%K 3Fe(CN) 6 plus 50% KOH were measured. The etching rate for the amorphous layer was 2.4−3.7 times that of the polycrystalline samples and 3.0–4.1 times that of the single-crystal samples. Polycrystalline specimens were exposed to flowing oxygen for 1 h at 1300 °C. Rutherford backscattering and the nuclear reaction 16O(d,p) 17O ∗ were used to determine the amount of oxygen on the surface. The amount of oxygen (and the thickness of oxide) over the amorphous region was 1.67 times that over the crystalline region. The relative thicknesses of the oxide on the amorphous and crystalline regions were confirmed by measuring the sputtering time required to remove the oxygen signal in an Auger spectrometer.

Application of Autoradiography in Studying the Matter Transport in Seales Formed on Metals in Oxygen and Sulphur Atmospheres

The transport of oxidant in the scale formed during hightemperature oxidation of metals has been studied with the use of a two-stage oxidation method. This method enables the observation of the distribution of labelled oxidant in the initially formed non-labelled part of the scale. Stable isotope of oxygen, 18O, and radioactive isotope of sulphur, 35S, have been used in the study. Oxygen-18 has been detected with the aid of a nuclear reaction 18O(p,n)18F. Transversal cross-sections of the oxidized samples have been bombarded with protons in a cyclotrone and then examined autoradiographically. Concentration changes of the isotope have been determined photometrically. It has been stated that in addition to the outward diffusion of metal, evidenced by other methods, the in ward transport of oxidant participates as a rule, in the process of scale growth. The character of oxidant transport has been suggested.

Comparison of the nuclear reactions 18O(p, α) 15N and 16O(d, α) 14N to study the oxygen effects in Pt silicide formation

2 MeV backscattering spectrometry (BS) and nuclear reactions (NRs) 18O(p, α) 15N induced by 1.5 MeV H 2 + and 16O(d, α) 14N induced by 900 keV D + have been used to study the effect of implanted oxygen on Pt silicide formation. The location of oxygen before and after silicide formation was deduced from NR spectra. Advantages and disadvantages of these two NRs in this application are discussed.

Study of 18O diffusion in magnesium orthosilicate by nuclear microanalysis

The self‐diffusion of 18O‐enriched oxygen in forsterite was determined by measuring the bulk concentration using the nuclear reaction 18O(p,α)15N. Important technical improvements led to a more precise determination of the 18O diffusion profiles. These results indicate that the self‐diffusion coefficient D* is independent of the annealing duration and the oxygen partial pressure. In addition, the D* values are found to be lower by nearly 1 order of magnitude than those previously published (preexponential factor D0 ∼ 2.3×10−6cm2/s, activation energy, ∼70 kcal/mole).

An 18O Study of Cooperative Diffusion and Chemical Reaction during Thermal Treatments of Silica Films in Water Vapor

The mechanism of isotopic exchange of oxygen between silicon oxide films and water vapor was studied using18O as the tracer. layers of natural isotopic composition, obtained by anodic or thermal oxidation, were treated in18O enriched water vapor at 13 Torr and at temperatures ranging from 400° to 930°C. 18O profiling was carried out using the 629 keV narrow resonance in the nuclear reaction 18O(p, α) 15N. The results have been interpreted by a mechanism involving the diffusion of a mobile carrier (probably a free water molecule) reacting with the network. Two simple limit cases have been observed: one corresponds to a strong divergence between the 18O isotopic labeling of the network and that of the carrier, and the other to an isotopic quasi‐equilibrium. We have been able to deduce two parameters characteristic of the network reactivity and of the diffusivity . The values of these parameters depend on the type of oxide (thermal or anodic) and on the kind of annealing. The major effect is on the reactivity (it can be about 20 times smaller for a technological preannealed thermal oxide than for the other ones). The diffusion coefficient of the carrier, showed an activation energy of about 16–18 kcal/mol, which is close to that found for diffusion of OH in bulk silica. For the reactivity, no determination of activation energy, was yet possible; however it appears that it should be very much higher than that for .

Prompt emission of nucleon jets in time-dependent Hartree-Fock collisions

The prompt emission of fact nucleon jets is calculated in the time-dependent Hartree-Fock approximation for 16O+93Nb collisions at Elab=204 MeV. The emission time is independent of the entrance channel angular momentum and is approximately equal to the time needed for projectile nucleons to traverse the target without interacting. The mean laboratory velocity, scattering angle, and particle number of the jet are calculated and interpreted in terms of a zero temperature Fermi gas model. The neutron multiplicity in the jet in coincidence with deep inelastic scattering is found to be consistent with recent experimental measurements. NUCLEAR REACTIONS 16O+93Nb at ELab=204 MeV in the time dependent Hartree-Fock approximation. Single nucleon emission with fusion and deep inelastic scattering.

Ion beam profiling of gettered oxygen in amorphous silicon films

Thin amorphous silicon films on crystalline silicon substrates were prepared by magnetron reactive sputtering. The elastic resonant nuclear reaction 16O (α, α) 16O was used to profile the films for oxygen impurity content. Strong gettering effects were found for samples prepared at base vacuum pressures ≈10 −6 Torr. Analysis of oxygen content and profiles in the film indicated that the severe oxygen inclusion by the growing films could be eliminated by a combination of pumping, trapping and presputter gettering procedures. Films made under these precautionary conditions contained less than 0.1% oxygen.

Location of adsorbed atoms by surface channeling

The positions of oxygen and carbon adsorbed on a (101) Ni surface are determined in a surface channeling experiment with 1.2 MeV deuterons at grazing incidence. For identification the nuclear reactions 16O(d, p) 17O and 12C(d, p) 13C are used. Yield maxima with beam direction along [101̄] and minima along [010] are found for both atom species. This is consistent with a “long bridge” position at the (101) Ni surface.