16O Data Research Articles

Analysis of barrier distributions data for16O + 64Zn reaction at sub-barrier energies

The barrier distribution data for the 16O + 64Zn reaction at energies spanning around the no minal barrier are examined by employing symmetric-asymmetric Gaussian barrier distribution (SAGBD) approach. The cumulative role of dominant channel couplings in the SAGBD method are determined in terms of the channel coupling parameter λ and percentage decrease of fusion barrier VCBRED with reference to nominal Coulomb barrier. The non-zero and positive values of these parameters for the studied system quantitively measure the influences of dominant intrinsic channels originated from the structure of the participants. The barrier distribution data of 16O + 64Zn reaction is quantitatively as well as qualitatively explained by SAGBD outcomes.

Thermonuclear 19F(p, α)16O Reaction Rate Revised and Astrophysical Implications

Abstract We have calculated the thermonuclear 19F(p, α γ )16O reaction rate in a wide temperature region of 0.001–10 GK by re-evaluating the available experimental data. Together with recently evaluated 19(p, α 0)16O and 19(p, α π )16O data, we have derived a new total reaction rate of 19F(p, α)16O using a Monte Carlo technique. The present rate is larger than the NACRE recommended one by factors of 36.4, 2.3, and 1.7 at temperatures of 0.01, 0.05, and 0.1 GK, respectively. This is because we have considered the enhanced low-energy astrophysical S factors in the (p, α γ ) channel, owing to the interference effect between an 11 keV resonance and the well-known 323 keV resonance. It shows that the (p, α γ ) channel dominates the total rate over the entire temperature region, except for a narrow region of 0.05–0.12 GK where the (p, α 0) channel dominates, contrary to the previous conclusion. We have investigated the impact of the 19F(p, α)16O reaction rate using a simple parametric model of extra mixing in low-mass AGB stars, which would lower the fluorine abundance produced and observed in these stars. However, models considering different temperature profiles and more sophisticated approaches, such as extra mixing induced by magnetic fields, are needed to verify the results of our preliminary tests. Interestingly, our new rate has a strong impact on destruction of 19F in the CNO cycle at low temperatures of 0.02–0.03 GK, and this general behavior needs to be analyzed further.

A new approach to monitor $$^{13}\hbox {C}$$-targets degradation in situ for $$^{13}\hbox {C}(\alpha ,\hbox {n})^{16}\hbox {O}$$ cross-section measurements at LUNA

Direct measurements of reaction cross-sections at astrophysical energies often require the use of solid targets able to withstand high ion beam currents for extended periods of time. Thus, monitoring target thickness, isotopic composition, and target stoichiometry during data taking is critical to account for possible target modifications and to reduce uncertainties in the final cross-section results. A common technique used for these purposes is the Nuclear Resonant Reaction Analysis (NRRA), which however requires that a narrow resonance be available inside the dynamic range of the accelerator used. In cases when this is not possible, as for example the 13C(alpha,n)16O reaction recently studied at low energies at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy, alternative approaches must be found. Here, we present a new application of the shape analysis of primary gamma rays emitted by the 13C(p,g)14N radiative capture reaction. This approach was used to monitor 13C target degradation {\em in situ} during the 13C(alpha,n)16O data taking campaign. The results obtained are in agreement with evaluations subsequently performed at Atomki (Hungary) using the NRRA method. A preliminary application for the extraction of the 13C(alpha,n)16O reaction cross-section at one beam energy is also reported.

ENDF/B-VIIIβ4 benchmark test with iron and concrete shielding experiments using 40 and 65 MeV neutrons at QST/TIARA

The latest beta version of ENDF/B-VIII, ENDF/B-VIIIβ4 was released in February 2017 from the Cross Section Evaluation Working Group. We benchmarked the iron and concrete shielding experiments with 40 and 65MeV neutrons at QST/TIARA by using ENDF/B-VII.1, JENDL-4.0/HE and ENDF/B-VIIIβ4 to investigate whether the 56Fe and 16O data in ENDF/B-VII.1 were properly improved in ENDF/B-VIIIβ4 or not. The drastic overestimation in the calculated result with ENDF/B-VII.1 was resolved in that with ENDF/B-VIIIβ4 in the iron experiment with 40MeV neutrons, because of the revision of the non-elastic scattering cross section data of 56Fe in ENDF/B-VIIIβ4. However, the calculated result with ENDF/B-VIIIb4 significantly underestimated the measured one in the experiment with 65MeV neutrons. The drastic overestimation in the calculated results with ENDF/B-VII.1 still appeared in those with ENDF/B-VIIIβ4 in the concrete experiments with 40 and 65MeV neutrons, due to no improvement of 16O data particularly above 20MeV in ENDF/B-VIIIβ4. We figured out that the effect of the 16O data replacement was very large and those of 28Si and 27Al data replacements were also not so small. The official release of ENDF/B-VIII should be produced based on this study.

Analyses of iron and concrete shielding experiments at JAEA/TIARA with JENDL/HE-2007, ENDF/B-VII.1 and FENDL-3.0

IAEA released a new Fusion Evaluated Nuclear Data Library, FENDL-3.0, in 2012. FENDL-3.0 extends the neutron energy range from 20MeV to greater than 60MeV. Now there is increasing interest in nuclear data above 20MeV. Thus we have analyzed the iron and concrete shielding experiments with the 40 and 65MeV neutron sources at TIARA in Japan Atomic Energy Agency with the latest high-energy nuclear data libraries, JENDL/HE-2007, ENDF/B-VII.1 and FENDL-3.0. The Monte Carlo code MCNP-5 and ACE files of JENDL/HE-2007, ENDF/B-VII.1 and FENDL-3.0, which are supplied from JAEA, LANL and IAEA, respectively, were used for this analysis. The collimated neutron beam and test shields were modeled in the analysis. The measured source neutron data were adopted in the analysis. The followings are found out from the results; (1) Iron experiments: The calculation result with FENDL-3.0 agrees with the measured one best. That with JENDL/HE-2007 fairly agrees with the measured one. On the contrary that with ENDF/B-VII.1 drastically overestimates the measured one. It is confirmed that this overestimation is due to the smaller non-elastic scattering data of 56Fe in ENDF/B-VII.1. (2) Concrete experiments: The calculation result with ENDL/HE-2007 agrees with the measured one best, while those with FENDL-3.0 and ENDF/B-VII.1 drastically overestimate the measured one. It is confirmed that this overestimation is due to both the larger elastic and smaller non-elastic scattering data of 16O in FENDL-3.0 and ENDF/B-VII.1.

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.

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.

Isotopic effects in elastic and inelastic 12C + 16, 18O scattering

Complete angular distributions of the 12C + 18O elastic and inelastic scattering were measured at the energy E lab(18O) = 105 MeV (E c.m. = 42 MeV) . Comparison of these elastic-scattering data with those from previously measured 12C + 16O data show their large-angle cross-sections to differ by as much as a factor of 100 with the 16O data being the largest. These and the 12C + 18O scattering data taken from the literature at the energies E c.m. = 12.9-56 MeV were analysed within the optical model and coupled-reaction-channels methods. Sets of Woods-Saxon 12C + 18O optical potential parameters were obtained and their energy dependence was deduced. A similar analysis was carried out for 12C + 16O where it was shown that over a wide energy range, the primary difference in the 16O and 18O scattering potentials is in their imaginary parts. The large-angle enhancement for the 12C + 18O elastic-scattering was shown to arise from the transfer of nucleons. The inelastic-scattering data were well described over the entire angular range as arising from collective excitations of the states in the target and projectile nuclei.

Higher order correlation of the pions and their target excitation dependence in relativistic and ultrarelativistic nuclear collisions

The target excitation dependence of higher order correlation of the pions is investigated in the present work using the data of 16O–AgBr interactions at 2.1 and 60 A GeV and 32S–AgBr interactions at 200 A GeV. The data are divided into different sets of ng which correspond to different degrees of the target excitation. The target excitation dependence was evident at the lower projectile energy—at the higher projectile energy no target excitation dependence was revealed by the data.

Elastic and inelastic scattering of 7Li + 18O versus 7Li + 16O

Angular distributions of the 7Li + 18O elastic and inelastic scattering were measured at the energy E lab ( 18O) = 114 MeV (32 MeV c.m.) in inverse kinematics. This technique allowed both small and large angle data to be collected simultaneously. The data were analyzed within the optical model and coupled-reaction-channels method to determine the potential parameters of 7Li + 18O scattering and reaction channels dominating the scattering. 18O inelastic scattering deformation parameters were obtained. The present data show that the 7Li + 18O system has a much stronger absorption when compared to previously measured 7Li + 16O data. The derived optical model parameters clearly show the scattering difference between two systems.

18O/ 16O mapping and hydrogeology of a short-lived (≈10 years) fumarolic (>500°C) meteoric–hydrothermal event in the upper part of the 0.76 Ma Bishop Tuff outflow sheet, California

18O/ 16O data from the 200-m-thick, 0.76 Ma Bishop Tuff outflow sheet provide evidence for a vigorous, short-lived (≈10 years), high-temperature, fumarolic meteoric–hydrothermal event. This is proved by: (1) the juxtaposition in the upper, partially welded Bishop Tuff of low- 18O groundmass/glass ( δ 18O=−5 to +3) with coexisting quartz and feldspar phenocrysts having magmatic δ 18O values (+8.7±0.3; +7.5±0.3); and (2) the fact that these kinds of 18O/ 16O signatures correlate very well with morphological features and mapped zones of fumarolic activity. Profiles of δ 18O with depth in the Bishop Tuff within the fumarole area define a 40- to 50-m-thick, low- 18O, stratigraphic zone that is sandwiched between the essentially unwelded near-surface portion of the tuff and an underlying, densely welded black tuff that displays magmatic 18O/ 16O values. Shallow-dipping columnar joints and other fumarolic features (i.e., subhorizontal tubular conduits and steep fissures) correlate very well with these pervasively devitrified, low- 18O zones. The base of the low- 18O zone is extremely sharp (∼3‰ per meter) and is located directly above the transition from partially welded tuff to densely welded black tuff. The observed average whole-rock 18O-depletions within this low- 18O zone are about 6–7‰, requiring meteoric water/rock ratios in excess of 0.24 in mass units. Rainfall on the surface of the tuff would not have been high enough to supply this much H 2O in the short lifetime of fumarolic activity, suggesting that some recharge must have been from groundwater flow through the upper part of the tuff, above the sloping (1°–5°) top of the impermeable lower zone. This is compatible with the observation that the fumarolic areas roughly correlate with the preeruptive regional drainage pattern. Some of this recharge may in part have been from the lake that filled Long Valley caldera, which was dammed by the Bishop Tuff up to the level of this boundary between the partially and densely welded zones (≈7000 ft, the elevation of the highest Long Valley Lake shorelines). Gazis et al. had previously shown that the 2.8-Ma intracaldera Chegem Tuff from the Caucasus Mountains exhibits exactly the same kind of 18O-signature that we have correlated with fossil fumaroles in the Bishop Tuff outflow sheet. Although not recognized as such by McConnell et al.; 18O/ 16O data from drill-hole samples from the intracaldera Bishop Tuff in Long Valley also display this characteristic 18O signature (i.e., analogous δ 18O-depth profiles, as well as low- 18O groundmass coexisting with high- 18O feldspar phenocrysts). This fumarolic 18O/ 16O signature is observed to much greater depths (≈650–750 m) in the intracaldera tuffs (≈1500 m thick) than it is in the ≈200-m-thick Bishop Tuff outflow sheet (≈80 m depth).

Anomaly in the 15N, 16O, 19F + 54,56Fe fusion cross sections around the Coulomb barrier energy

Fusion cross sections have been determined by detecting evaporation residues from six systems formed by projectiles of 15N, 16O and 19F and target nuclei of 54Fe and 56Fe at incident energies around the Coulomb barrier. The comparison of the experimental results with the prediction from the one-dimensional barrier-penetration model revealed that the 16O reactions were not much enhanced, whereas the 19F fusion reactions at sub-barrier energies are more than ten times higher than the model calculation. The fusion cross sections for 15N projectiles showed a medium enhancement. The one-dimensional model has reproduced to some extent the experimental data of the 15N + 54Fe and 16O + 54,56Fe systems by making corrections for the quadrupole deformation of the reacting nuclei. However, the 19F data cannot be described by this model. In order to explain uniformly the cross sections of the six systems we have performed coupled-channel calculations. The 15N and 16O data have been reproduced within the experimental error by opening channels for inelastic excitation in the reacting nuclei and those for one-nucleon transfer. For better explanation of the 19F data, however, an additional channel for the triton transfer was required.

Neutron-antineutron oscillations in 16O

We have made a careful analysis of the effect of statistical uncertainty in the form of the antineutron-nucleus potential on the value of the neutron-antineutron mixing time. Using a bias-free optical potential derived from fits to the ( p , 12 C) scattering data at 47 MeV, together with the IMB and Kamiokande results for the lifetime of 16O, we set lower limits on the neutron-antineutron mixing time τ n n of > 1.1 × 10 8 s and 1.5 × 10 8 s , respectively, both at the 90% confidence level. We adopt the theoretical approach of Dover, Gal and Richard to extract these limits from the 16O data.

Phase-shift analysis of 16O + 16O elastic scattering data

A phase-shift analysis of precisely measured elastic 16O + 16O data has been carried out in order to establish the existence of molecular resonances between E c.m = 15.5 and 18 MeV and to investigate the possible structure of these resonances.

Synthetic coral-reef terraces and variations of Quaternary sea level

Coral-reef terraces on tectonically elevated coasts provide important data on variations of sea level. In this paper a systematic approach is presented for the construction of synthetic reef terraces. Idealized reef terraces are generated assuming a constant rate of tectonic uplift ν and a harmonic variation of sea level with amplitude h wo and period τ. It is further assumed that coral terraces grow only during periods of relative sea-level rise. The behavior is governed by the parameter u = τυ/2π h wo. If u > 1 no reef terraces form. If 0.217 < u < 1 a discontinuous series of terraces form on the uplifting basement. If 0 < u < 0.217 the basement is covered with coral and a series of coral terraces forms on this coral. Uplifted coral-reef terraces on the Huon Peninsula, Papua New Guinea, compare reasonably well with terraces generated by our model with τ = 20,000 years and h wo = 21.4 m. We also compare the observed terraces with model terraces using the paleo sea level inferred from 18O/ 16O data. The observed terraces are qualitatively similar to those predicted using the oxygen isotope data, however, the model terraces require larger relative sea-level variations with a near 20,000 year period than those inferred directly from the 18O/ 16O data.

Compound nucleus fission and quasi-fission in reactions of 238U with 16O and 27Al

Fission fragment yields have been measured as a function of fragment mass, angle and energy for the 238U + 16O, 27Al fusion reactions at 6 MeV/u. The observed anisotropies for the two systems indicate that a large fraction of the 238U + 27Al cross section is associated with the quasi-fission (or fast fission) reaction, whereas the 238U + 16O data are consistent with compound nucleus formation, followed by fission.

Correlation of magnetitic susceptibility with 18O/ 16O data in late orogenic granites of the southern Appalachian Piedmont

Initial magnetic susceptibility (generally indicative of magnetite content) has been determined for 445 samples from 17 granites located in the southern Appalachian Piedmont of Georgia and South Carolina. These values have been correlated with whole rock δ 18O data from the same plutons, yielding a pronounced inverse relationship. It has previously been shown for the southern Piedmont that low oxygen isotopic ( 18O-enriched) values usually occur in S-type granites (Wenner [1], this issue). It follows, then, that I-type granites are characterized by high susceptibilities (χ > 1 × 10 −4 G/Oe), and S-type granites by low susceptibilities (χ < 1 × 10 −4 G/Oe). An interesting result of this work has been the observation that some S-type granites exhibit good within-site clusters of remanent magnetic directions while I-type granites generally do not.

Oxygen isotope geochemistry of the potassic igneous rocks from the Roccamonfina volcano, Roman comagmatic region, Italy

18O/ 16O measurements were made on 80 samples of lavas, pyroclastics, and xenolithic nodules from the 0.05- to 1.54-m.y.-old Roccamonfina volcano. The δ 18O values of these magmas varied from +5.6 to +10.1 in the High-K Series (HKS) and +6.6 to +8.8 in the Low-K Series (LKS). The xenolithic nodules, many of which are HKS cumulates, crystallized from magmas with δ 18O= +7.8to+10.3. These data indicate that assimilation of high- 18O country rocks (pelitic schists, flysch, and some carbonates) was an important petrologic process during fractional crystallization of both the early, HKS magmas (leucite-bearing tephrites, leucitites, and phonolites) and the later, LKS magmas (trachybasalts, trachyandesites, and trachytes). Combined with 87Sr/ 86Sr, Pb isotope, and 143Nd/ 144Nd data obtained by other workers from the same outcrop localities, the 18O/ 16O data prove that: (1)| The assimilated country rocks had relatively high 87Sr/ 86Sr values (0.710–0.720), low 143Nd/ 144Nd values (ε Nd ≈ −10to−15), and 206Pb/ 204Pb values of about 18.70. (2)| The HKS and LKS represent completely separate trends of magmatic differentiation and different parent magmas. (3)| The HKS parent magma (leucite basanite?) probably had δ 18O≈ +6.0and 87Sr/ 86Sr≈ 0.708, while the LKS parent magma (trachybasalt?) had δ 18O≈ 6.5and 87Sr/ 86Sr≈ 0.706. (4)| The HKS and LKS parent magmas are only indirectly related genetically, perhpps in their source regions in the upper mantle, or during an earlier episode of wall rock-magma interaction in the lower crust. (5)| Although wall rock assimilation has played an important role in determining the isotopic and trace element characteristics of these magmas, the liquid line-of-descent and the changes in major element chemistry were dominated by fractional crystallization. (6)| The energy required to heat up and dissolve the assimilated rocks was provided by the latent heat associated with concurrent fractional crystallization of the cumulates (as represented by some of the plutonic nodules). (7)| The oxygen and strontium isotopic changes at Roccamonfina, although very significant, are much less than in the undersaturated magmas of the Roman Province further to the north. This is apparently because during and prior to the Quaternary potassic vulcanism, the latter terrane was invaded and heated by anatectic magmas associated with the Tuscan rhyolitic event, thus creating conditions favorable for much greater crustal assimilation.

Resonance structure in 15N(p, n 0) 15O in the region Ep = 8.5–19.0 MeV

Excitation functions at seven angles, covering the energy range E p = 8.5–19.0 MeV, have been measured for 15N(p, n o) 15O using time-of-flight methods. Angular distributions were also measured at five energies. Activation methods were used to determine the total cross section below the excited-state threshold. Ten resonances were observed, four of which have also been found in 15N(p, γ) 16O data. The energies, widths and nucleon widths have been extracted and are used, together with proton capture data, to derive radiative widths for certain levels.

Coulomb-nuclear interference with pions

We propose that Coulomb-nuclear interference experiments be analyzed directly in terms of f N( θ), where f N( θ) is defined as the difference between the complete elastic scattering amplitude and the Coulomb amplitude. The advantages are : (i) this analysis works even for heavy nuclei and (ii) the results do not depend on models of the strong interaction. Once f N is known it is possible to extract the purely strong amplitude f S( θ) using an analysis with the Bethe phase φ B. We point out some mistakes that have been commonly made in applying the West-Yennie theory for φ B. For N = Z nuclei we show how f S and φ B may be determined directly from the data without a detailed theory of φ B; applying these ideas to the 16O data of Mutchler et al., we find a substantially different result : namely that Re f S(0) = 0 at 178 ± 4 MeV, a shift of 16 MeV from the previously published number. The φ B that we extract from the data is in reasonable agreement with the West-Yennie prediction. We also make calculations to assess the validity of the West-Yennie formula for φ B within the framework of the optical model ; Re φ B is in substantial agreement for 40Ca for incident pion energies T > 100 MeV, but Im φ has less accuracy than one would like.