16O Cross Section Research Articles

Measurement of the total photonuclear cross section for16O in the region of the giant dipole resonance

Using a gamma -ray spectrometer consisting of a liquid deuterium target and a photoneutron time-of-flight detector, the total absorption cross section of 16O was measured in the energy range from the 3 to 38 MeV. The total photonuclear component of the cross section integrated from 10 to 30 MeV was found to be (182+or-16) MeV mb, or (0.76+or-0.07) TRK sum-rule units. This result is compared with previous measurements, and its significance to the theory of meson-exchange currents is discussed.

Attenuation of solar irradiance in the stratosphere: Spectrometer measurements between 191 and 207 nm

Measurements made by a balloon‐borne, single dispersion spectrometer have been used to investigate the attenuation of solar irradiance in the wavelength region of the O2 Schumann‐Runge bands and the Herzberg continuum. The data cover the altitude range from 28 to 39 km. The fine spectral resolution (0.012 nm) allows transmittance windows in the Schumann‐Runge system to be resolved, facilitating the comparison of their characteristic absorption to model calculations. An atmospherically derived O2 cross section developed in this analysis is significantly lower than the Herzberg continuum values most used in photochemical models.

Photonuclear reactions at intermediate energies

The interaction mechanism between an intermediate-energy photon and a nucleus is discussed with special attention given to Siegert’s theorem and the role of exchange currents. In particular, the relative contributions of (γ, p), (γ, n), (γ, pp), (γ, nn) and (γ, pn) cross-sections for16O to the total photonuclear cross-section are studied in the region of photon energies between 40 and 140 MeV, and comparison with recent measurements is made. Numerical results for (γ, p) and (γ, n) reactions in light nuclei are discussed for photon energies extending from 40 MeV up to the Δ region.

Microscopic description of 7Li and 7Be for the DWBA treatment of cluster transfer reactions

The overlap of the αt interaction times the 7Li wave function with the product of the α and t wave functions and the analogous quantity for the charge-conjugate system are calculated on microscopic grounds. This quantity contains the structure information on the 7Li nucleus in the standard finite-range DWBA description of α- and t-stripping from and pick-up to 7Li. This nucleus is described in the generator-coordinate model assuming the α + t two-cluster structure and schematic nucleon-nucleon forces. The model reproduces the measured g.s. properties reasonably. The microscopic potential overlap is obtained to be very different from those calculated with using local macroscopic αt interactions, but the difference in the 12C( 7Li, t) 16O cross section is only dramatic at backward angles. To facilitate further DWBA analyses, several versions of the potential overlap as well as the overlap without the potential are given in an easily accessible form.

An optical potential description of 16O + 28Si elastic scattering

The differential cross sections for 16O + 28Si elastic scattering at seven energies between 21 and 35 MeV in the centre of mass are described well over the whole angular range from 20° to 180° by an optical potential whose real part consists of a double-folded potential supplemented by a phenomenological model-independent correction term. This surface correction is predominantly attractive and has structure which depends only weakly on the energy. The associated imaginary potentials imply surface transparency and have detailed structure which varies rapidly with energy. However there is a systematic trend for the absorptive region to extend to larger radii as the energy increases. A simple parameterization of this trend allows the main features of the excitation function for 180° scattering to be reproduced.

Coulomb nuclear interference effects in the elastic and inelastic scattering of16O and18O ions on70Ge

Excitation functions of the elastic and inelastic scattering cross sections of16O and18O nuclei on70Ge have been measured at angles between 45° and 175° and at beam energies from 31.5 to 57 MeV. A DWBA analysis has been performed in which the composite structure of18O has been taken into account according to the approach developed by Landowne and Wolter. Starting from an optical model potential determined from the elastic scattering data in the16O+70Ge channel, a consistent description of the16O+70Ge (2+, 3−) target excitation, the18O+70Ge elastic scattering and18O(2+)+70Ge projectile excitation cross sections near the Coulomb barrier has been obtained.

Fusion cross sections for 16O + 13C at low energies

Fusion cross sections for 16O + 13C have been measured by the total γ-ray yield method over the c.m. energy interval 4.6–9.1 MeV. The cross sections are observed to be enhanced with respect to the 16O + 12C system for more than the increase in the nuclear radius suggests. The enhancement is attributed to the attractive action of the loosely bound neutron of the 13C nucleus.

Shape resonance and electronic autoionization in vibrationally resolved partial photoionization cross sections of O2

Vibrationally resolved partial photoionization cross sections of O2 leading to the b4Σ−g and B2Σ−1g(3σ−1g) tonic states have been measured at high resolution in a wide photon energy range from 0.5 eV above threshold to 24.5 eV. The σu shape resonance is observed around 21.5 eV, in fairly good agreement with “one-electron” theoretical predictions. The series of resonances from 19 to 20 5 eV result from an autoionization process with a strong vibrational selectivity, explained by the similar geometries of the resonance states and the b ionic state. The large resonance widths originate from a strong Rydberg-valence orbital mixing Calculations using multichannel quantum defect theory reproduce the most important features.

O2 absorption cross sections (187–225 nm) from stratospheric solar flux measurements

The absorption cross sections of molecular oxygen are calculated in the wavelength range from 187 to 230 nm from solar flux measurements obtained within the stratosphere. Within the Herzberg continuum wavelength region the molecular oxygen cross sections are found to be about 30% smaller than the laboratory results of Shardanand and Rao (1977) from 200 to 210 nm and about 50% smaller than those of Hasson and Nicholls (1971). At wavelengths longer than 210 nm the cross sections agree with those of Shardanand and Rao. The effective absorption cross sections of O2 in the Schumann‐Runge band region from 187 to 200 nm are calculated and compared to the empirical fit given by Allen and Frederick (1982). The calculated cross sections indicate that the transmissivity of the atmosphere may be underestimated by the use of the Allen and Frederick cross sections between 195 and 200 nm. The ozone column content between 30 and 40 km and the relative ozone cross sections are determined from the same solar flux data set. The calculated ozone absorption cross sections agree with those of Inn and Tanaka (1959) and Bass and Paur (1981) to within 3%.

Solar irradiance in the stratosphere: Implications for the Herzberg continuum absorption of O2

The third Solar Absorption Balloon Experiment performed measurements of the attenuated solar irradiance between 200 and 210 nm as the payload ascended from 32 to 39 km. Comparison of these data with calculations based on ozone and pressure measurements made from the payload shows more solar radiation reaching the middle stratosphere than is predicted by O2 and O3 cross sections which are widely used in photochemical modeling. Consideration of the uncertainties in the balloon data and laboratory experiments indicates that the discrepancy can best be removed by adopting Herzberg continuum cross sections for O2 which are equal to or slightly less than the smallest values which have appeared in the literature.

Dissociative photoionization cross sections of O2 from threshold to 120 Å

The photoionization cross sections for producing the O2+ parent ion and the O+ fragment ion from neutral O2 are presented from 650 to 120 Å. A new technique was used that eliminated the serious problem of identifying the true abundance of the O+ ions. These ions are generally formed with considerable kinetic energy and because most mass spectrometers discriminate against energetic ions true O+ abundances are difficult to obtain. In the present work the relative cross sections for producing O2+ are obtained and normalized against the total cross sections in a spectral region where dissociative ionization is not possible. The fragmentation cross sections for O+ were then obtained by subtraction of the O2+ cross section from the total photoionization cross section.

Measurements of Excitation of the O<SUB>2</SUB>(a<SUP>1</SUP>Δ<SUB>g</SUB>) State for Excitation of Atomic Iodine Laser by Low Energy Electrons

The excitation coefficients of the O2 (a1Δg) state by low energy electrons in O2-Ar, O2-N2-Ar, O2-N2 mixtures and pure O2 have been measured using a drift tube technique.The time dependence of the absolute intensity of the 1.27μm band emission was measured for 0.05% O2 in Ar, for (1-5)% O2 in N2-Ar, for 20% O2 in N2 at total gas density of (2.3-16)×1018cm-3 and for pure O2 at total gas density of (0.97-6.44)×1017cm-3. The electric field to gas density ratio E/N was varied from (0.5-5)×10-17 V-cm2 for O2-Ar mixtures, from (0.2-3)×10-16 V-cm2 for O2-N2-Ar, O2-N2 mixtures and from (0.8-7)×10-16 V-cm2 for pure O2. The measured decay constants yield a normalized diffusion coefficient for O2 (a1Δg) in Ar of DN≈1.65×1018 cm-1 sec-1 and rate coefficients for quenching by O2 and Ar of 1.51×10-18 cm3 sec-1 and 1.86×10-20cm3 sec-1 respectively. Our experimental electron excitation coefficients agree satisfactorily with values calculated using derived sets of electron collision cross sections for O2, N2 and Ar.

Excitation of the O2(a 1Δg) state by low energy electrons

Coefficients for the excitation of the a 1Δg metastable state of O2 by low energy electrons in O2–Ar mixtures have been measured using a drift tube technique. The time dependence of the absolute intensity of the 1.27 μm band emission was measured for 1% and 5% O2 in Ar at total gas densities of (1–20)×1024 m−3. The electric field to gas density ratio E/N was varied from (0.3–10)×10−21 V m2, corresponding to calculated mean electron energies between about 0.8 and 4 eV. The measured decay constants yield a normalized diffusion coefficient for O2(a 1Δg) in Ar of (2.8±0.2)×1020 m−1 s−1 and a rate coefficient for quenching by O2 of (1.57±0.1)×10−24 m3 s−1. Our experimental electron excitation coefficients agree satisfactorily with values calculated using previously derived sets of electron collision cross sections for O2 and Ar.

Photodestruction of ions containing sulfur dioxide

Photodestruction cross sections have been measured for six ions containing SO2 over the photon energy range 1.55–3.5 eV using a drift tube mass spectrometer and rare gas ion and dye lasers. The cross section for SO2− increases smoothly with photon energy from (0.9 to 2.4) ×10−18 cm2. The photodestruction spectra of SO2 (SO2)− and O2 (SO2)+ consist of broad, structureless bands. These bands are attributed to direct dissociation through repulsive excited states. Cross section for O2 (SO2)−, NO2 (SO2)−, and NO3 (SO2)− are measurably different from zero only at the upper end of the photon energy range.

Nuclear reaction cross sections of 16O + 16O obtained by γ-ray detection

The partial production cross sections for reaction residues produced by the fusion of 16O with 16O have been measured at E c. m = 9–30 MeV by detecting the characteristic γ-rays with a Ge(Li) detector. The dominant products are 24Mg and 27A1 corresponding to 2α and αp emission from the compound nucleus, respectively. The total γ-producing cross sections σ R were also derived by summing the partial cross sections after correction for the observed (average) γ-ray angular distributions. The trend in the total cross sections is very similar to the trends derived from an optical model or a statistical-evaporation model calculation. The partial production cross sections were compared with other experimental results at 11.9 MeV and 30 MeV and with the results of the statistical-model calculation. It is concluded that the treatment of angular momentum in the calculation is inadequate for describing the partial cross sections. Structure in the partial and total cross section excitation functions is observed with minima occurring at E c.m. = 27, 24, 20, 17.5, and possibly 15 MeV. Some of this structure is well established by the statistical accuracy of the data and most, but perhaps not all of it, is correlated in the various channels. This structure is compared with that observed in another experiment and some of its implications are discussed.

Predissociation of Nitric Oxide in the Mesosphere and Stratosphere

Absorption of solar photons by nitric oxide in the wavelength ranges 181.3-183.5 and 189.4-191.6 nm leads to predissociation of the molecule in the mesosphere and upper stratosphere. Molecular oxygen controls the penetration of the required solar irradiance via absorption in the Schumann-Runge bands, while attenuation due to ozone becomes significant in the upper stratosphere. The calculation of the nitric oxide dissociation rate is complicated by the need to include all rotational fine structure in both the NO and O2 cross sections. The dissociation rate computed here for the upper mesosphere is a factor of 3.6 less than that reported in past work when currently accepted values of the oscillator strengths and solar irradiance are used. In addition, improved molecular parameters describing the O2 cross section predict less attenuation of the dissociation rate with decreasing altitude than results previously available.

Photodissociation and photodetachment of molecular negative ions. VI. Ions in O2/CH4/H2O mixtures from 3500 to 8600 Å

The photodestruction cross sections for O−, O2−, O2−⋅H2O, O3−, O3−⋅H2O, O4−, OH−, and OD− have been measured in the 3500–5400 Å and 6300–8600 Å wavelength regions. The ions were produced in a drift tube mass spectrometer and interacted with a dye laser or ion laser inside the laser cavity. The photodetachment cross sections for O− and OH−(OD−) have sharp onsets at wavelengths near 8480 and 6795 Å, respectively, and at shorter wavelengths their values are nearly constant. The photodestruction cross sections for O2−, O4−, and O2−⋅H2O increase monotonically with increasing photon energy. In contrast, in the photodestruction cross section for O3−, structure is observed over this wavelength region. The processes for creation and photodestruction of the various negative ions are discussed. Comparison is made with other measurements.

Photoelectric cross-sections for fixed-orientation atomic orbitals: Relationship to the plane-wave final state approximation and angle-resolved photoemission

Differential photoelectric cross sections for fixed-orientation p- and d-orbitals have been derived within the one-electron, central-potential model and the dipole approximation for a completely general experimental geometry. Also, for the special geometry of a polarized source with polarization parallel to the electron emission direction, it is shown that such cross sections are exactly proportional to the probability distribution of the initial orbital, a result equivalent to that derived by using a plane-wave final-state approximation. Detailed numerical calculations of cross sections for Cu3 and O2 p orbitals in various geometries and at various energies exhibit significant differences in comparison to plane-wave cross sections. By contrast, certain prior angular-resolved X-ray photoemission studies of single-crystal valence bands are found to have been carried out in an experimental geometry that fortuitously yields cross-sections close to the plane-wave predictions.

Excitation of the b 1Σ+g state of O2 by low energy electrons

Rate coefficients for excitation of the b 1Σ+g state of O2 by low energy electrons have been measured using a drift tube technique. The time dependence of the absolute intensity of the 762 nm band emission was measured for O2 densities between 1016 and 2×1018 molecules/cm3. When corrected for electron attachment, ionization, and metastable diffusion, the number of b 1Σ+g molecules produced per centimeter of electron drift and per O2 molecule calculated from the 762 nm emission varied from 1.3×10−18 cm2 at E/N=5×10−17V cm2 to 2.1×10−16 cm2 at E/N=2×10−15V cm2. These values of electric field to oxygen density ratio E/N correspond to mean electron energies of 0.75 and 6 eV, respectively. Measured decay constants for the 762 nm radiation yield a value for the product of the diffusion coefficient and the O2 density of (5.0±0.3) ×108 cm−1 sec−1 and a quenching coefficient for the b 1Σ+g state of (3.9±0.2) ×10−17 cm3 sec−1. Comparison of measured excitation coefficients with values calculated using a recommended set of electron collision cross sections for O2 show that the cross sections for direct excitation of the b 1Σ+g state are accurate near threshold and suggest that essentially all of the O2 molecules excited to levels at and above 1.63 eV result in the formation of molecules in the b 1Σ+g state.

Fragmentation cross-section of relativistic oxygen ions and determination of overlap parameter

Fragmentation cross-section of16O relativistic ions, at two different energies, was measured in CsI crystal. An experiment designed basically to study primary cosmic ray nuclei, was exposed to monoenergetic beam of16O ions of bevatron. Fragmentation of the ions was observed in a two layer CsI target. Each CsI crystal was 1 inch thick.