Effective Cross-section Values Research Articles

Depth-dependent heterogeneity of water flow in sandy soil under grass

AbstractThe non-equilibrium water flow was studied on an aeolian sandy soil under grass using the dye tracer infiltration experiments. Two experiments simulating infiltration of 20 mm and 70 mm of rain water were conducted. The improved digital image analysis technique for estimating the effective cross section (ECS) and degree of preferential flow (DPF) from stained soil profiles was used to analyze the depth dependency and variation of the ECS and DPF parameters. It was found that the heterogeneity of water flow in sandy soil under grass increased with depth. Extremes of heterogeneity parameters (minimum value of the ECS and maximum value of the DPF) were almost the same for the two cumulative infiltrations, and the depth, in which the extremes of heterogeneity parameters occurred, increased with an increase in cumulative infiltration. Moreover, considerable variation of infiltration patterns was indentified in consecutive vertical sections.

Production of 223Ra from 226Ra in Tehran Research Reactor for treatment of bone metastases

In this research production of 223Ra via neutron irradiation of 226Ra nuclei was studied in Tehran Research Reactor and compared with available literature data. Dowex 1X8 and Dowex 50WX8 anion and cation exchange resins were used for isolation of produced radioisotopes. Effective cross section value for the 227Ac production as a precursor of 223Ra was calculated to 13.8 ± 0.3 barns which is in good agreement with other experimental data. The approximate elution yields of each radioisotope in every separation step were determined.

Comparison of two methods to assess heterogeneity of water flow in soils

Abstract The heterogeneity of water flow and solute transport was assessed during radioactive tracer infiltration experiment in a black clay loam soil using modified methods to estimate the effective cross section (ECS) and the degree of preferential flow (DPF). The results of field and numerical experiments showed that these parameters characterized the heterogeneity of water flow in the soils unequivocally. The ECS decreases non-linearly and the DPF increases linearly with an increase of the bypassing ratio (ratio of macropore flow rate to total flow rate). The ECS decreased and the DPF increased with depth, which suggests an increase in the heterogeneity of water flow with depth. The plot of the DPF against ECS values calculated from the tracer experiment data was consistent with the relationship obtained by the numerical simulation assuming preferential flow in the neighbourhood of three probes.

Two-photon absorption properties of conjugated supramolecular porphyrins with electron donor and acceptor

Two-photon absorption (2PA) cross-section values of supramolecular porphyrin tetramer consisting of two monoacetylene-linked bisporphyrins with and without ferrocene/C 60 groups as electron donor/acceptor through the coordination of pyridyl to zinc were measured using a nanosecond open aperture Z-scan method. The maximum effective cross-section value was observed for the compound having asymmetric terminals of ferrocene and C 60 as 2.0 × 10 5 GM. The values obtained for compounds having two C 60's, two ferrocene's, no donor/acceptor, and stacked isomer were 1.7 × 10 5 GM, 1.5 × 10 5 GM, 1.1 × 10 5 GM, and 1.3 × 10 5 GM, respectively. These results indicate that the asymmetric donor–π–acceptor structure is the best for 2PA enhancement for this series of molecules. These values are extremely large compared to those of unit dimer 2D (190 GM at 780 nm) and compounds reported hitherto in literatures.

Peculiarities of the electron-impact excitation of single-charged indium ion

An ion source has been developed to produce stable ground-state In + ion beam at high operating temperatures (900–1000) °C. The energy dependence of the effective electron excitation cross-section for the resonance λ = 158.6 nm ( 5 1 P 1 0 → 5 1 S 0 ) line of In + ion has been studied for the first time within the (7–300) eV energy range. The found structural features are due to the contribution of resonance processes. The absolute effective cross-section value was determined by normalizing the experimental curve at 300 eV energy to the theoretical data obtained by the Van-Regemorter formula.

Interpretation of James' Experiments in Plasma Theory of Solar Radar Echoes

In the framework of the plasma theory of solar radar echoes results of the known James' experiments such as the values of effective cross-sections, the anisotropy of reflections, the spectrum of reflected signals, the heights of radar scattering and others are explained. In particular in this theory reflection of radar signal with high effective cross-sections (σ = (10 − 100)πR2⊙) occur in processes t + l ⇌ t + l on the Langmuir turbulance generated by Type III electrons at altitudes (1.4—1.6)-R⊙ when electron streams propagate towards the Earth. If the electrons move away from the Earth then the scattering cross-sections due to process t + l ⇌ t + l are small (σ < πR2⊙). Reflections from the heights up to 5R⊙ can occur at scattering of radar wave on the ion-sound turbulance (processes t + s ⇌ t) accompanied the Langmuir turbulance.

The amplification and excited state absorption of Nd-doped nonstoichiometric crystals with fluorite structure in the 1.3 μm region

The spectroscopic characteristics in the 1.3 μm region of nonstoichiometric fluorite phases M 1- x R x F m (1- x)+ nx, doped by Nd 3+ ions were investigated. The direct measurement of excited state absorption and amplification in 1.3 μm region were carried out. The halfwidth of amplification spectrum in Na 0.5- x Y 0.5+ x F 2+2 x and Ca 1- x Y x F 2+ x crystals was found to be less than that the halfwidth of the luminescence spectrum due to excited state absorption (ESA). The maxima of amplification bands are located at ∼1365 nm with halfwidth of ∼40 nm for Na 0.5- x Y 0.5+ x F 2+2 x crystals and ∼60 nm for Ca 0.9Y 0.1F 2.11 crystals. The maximum values of effective cross sections σ eff=σ em-σ esa for these crystals are 2.2 ± 0.7 × 10 -21cm 2 and 4.5 ± 0.9 × 10 -21 cm 2, respectively.

Cross sections for the 93Nb(n,2n)92mNb reaction

The cross section of the 93Nb(n,2n)92mNb reaction has been studied by use of the activation method from the threshold energy of 8.8–19.8 MeV. Measurements are relative to the known cross-section values for the reactions H(n,n)H, 32S(n,p)32p, and 27Al(n,α)24Na. The cross-section value increases smoothly with energy and reaches a maximum value of 444 ± 18 mb at about 14.5 MeV then decreases to values of 293 ± 14 mb at 19.8 MeV. An effective cross-section value for a fission neutron spectrum calculated from the results is 0.321 ± 0.019 mb. The activation of Nb as a transfer standard for 14 MeV neutrons is discussed.

Photodissociation of alkali dimers Na2and K2into atoms in ground2S and resonance2P states

The paper contains a study of photodissociation of Na2 and K2 molecules in ground 2S and resonance 2P states under conditions of continuous laser excitation of alkali vapour in the visible spectral range. Absolute effective cross section values of photodissociation have been determined, as well as their dependence on excitation frequency and vapour temperature sigma ( omega ,T). Laws governing positions of frequency peaks of cross sections and the nature of spectral and temperature dependences have been ascertained. Experimental dependences for cross sections sigma ( omega ) have been compared with theoretical calculations according to quantum mechanical and classical theory. Classical formulae for cross sections have been used for reconstruction of the potential for the B 1 Pi u state of the Na2 molecule from obtained sigma ( omega ) dependences. Distribution of photofragments over finestructure components 2P3/2 and 2P1/2 of the resonance atomic state has been studied. Photodissociation of near-adiabatic nature has been found for atoms flying asunder at energies under 1 eV. This is manifest in the population of the 2P3/2 component. From an analysis of spectral dependences sigma ( omega ) and the distribution of photofragments the symmetry of molecular states has been determined, through the continuum of which the dissociation of molecules proceeds. These have been identified as B 1 Pi u and A 1 Sigma u+ states.

Practical Formalisms for Nuclear Data Representation in Evaluated Nuclear Data Files in the Unresolved Resonance Energy Region

The currently used formalism for neutron cross-section representation in the unresolved resonance energy range is based on the statistical parameters of the population of Breit--Wigner resonances. The present work introduces practical formalisms, based on parametric representation of the shielding factor curves, by which the values of effective cross sections can be obtained simply and quickly in the unresolved range, and suggests their use for neutron data representation. These formalisms were found to be compatible with such existing codes as MC/sup 2/, ETOX, HAMMER, ENDRUN, and MIGROS, and with such existing nuclear data files as ENDF/B and KEDAK. Each formalism is based on one interpolation scheme in temperature and one in sigma/sub 0/. The accuracy of four schemes in temperature and three schemes in sigma/sub 0/ was checked. Of these, three temperature schemes and one sigma/sub 0/ scheme were found to have better than 1% accuracy in the entire unresolved region; a formalism with better than 2% accuracy for representation was thus obtained. Observed spatially dependent self-shielding factors are transformed into pseudo-background cross-section-dependent (Bondarenko-type) self-shielding factors. Numerical values of the transformation for /sup 235/U and /sup 239/Pu self-shielding factors are given. It is shown that the formalisms can be used formore » the preprocessing of current nuclear data files in the unresolved range. 1 figure, 1 table.« less

Collisional dissociation of H2+ and H3+ on molecular targets leading to formation of excited hydrogen atoms

Experimental measurements have been made of the cross sections for dissociation of fast (100–700 keV) H2+ and H3+ ions to give fast H atoms in the 3s excited state; targets for this work were O2, CO, CO2, CH4, C2H6, and C3H8. The technique involved quantitative measurement of Balmer-alpha radiation emitted from fast moving fragments of the dissociated projectile. Contributions to the Balmer-alpha emission from the 3p and 3d levels were eliminated by a method that utilizes the different lifetimes of the 3s, 3p, and 3d states. It is observed that cross sections for H(3s) formation by dissociation on the hydrocarbons all exhibit essentially the same variation with impact energy and increase in magnitude with the complexity of the target; similar behavior is exhibited by the O2, CO, and CO2 group of targets. Comparison of the various cross sections suggests that one cannot assign an effective cross-section value to the constituent atoms of the target molecule.

Neutron capture cross-section of 135Xe

The neutron capture cross-section of 135Xe has been measured by the activity depletion method using mass separated-ion implanted samples and high resolution gamma-ray spectrometry. An effective cross-section value was measured at a neutron temperature of 318 ± 5°K as 3·09 ± 0·07 Mbarns, relative to a 59(Co( n, γ) cross section of 37·09 barns. A calculated value for the 135Xe 2200 m/sec capture cross-section is discussed.

EXCITATION CURVES FOR THE (n, 2n), (n, p), AND (n, nα) REACTIONS OF 65Cu

Cross sections for the reactions 65Cu (n, 2n)64Cu, 65Cu(n, p)65Ni, and 65Cu(n, nα)61Co have been measured by the activation method from threshold energies up to 20.2 MeV. The measurements are relative to the known cross section for the reaction 32S(n, p)32P. The (n, 2n) cross-section curve increases smoothly with energy and reaches a maximum value of 1 085 ± 60 mb at about 18 MeV. The (n, p) reaction reaches a maximum value of 21.7 ± 1.2 mb at 13.9 MeV. The (n, nα) reaction has a minimum detectable value of 0.3 ± 0.1 mb near 14 MeV and increases to a value of 18.9 ± 0.9 mb at 19.8 MeV. Effective cross-section values for a fission-neutron spectrum calculated from these results are 0.251 ± 0.018 mb for the (n, 2n) reaction and 0.523 ± 0.030 mb for the (n, p) reaction.

EXCITATION CURVES FOR THE REACTIONS Fe56(n, p)Mn56 AND Co59(n, α)Mn56

Excitation curves for the reactions Fe56(n, p)Mn56 and Co59(n, α)Mn56 have been measured by the activation method from near threshold energies to 20.3 Mev. The measurements are relative to the known cross section for the S32(n, p)P32 reaction. Cross sections for both reactions increase smoothly with neutron energy and reach maximum values of 119 ± 4 mb for the Fe56(n, p) reaction at 13.6 Mev and 30.0 ± 0.9 mb for the Co59(n, α) reaction at 14.5 Mev. Above 15 Mev both cross sections decrease with neutron energy. From the excitation curves effective cross-section values for a fission-neutron spectrum have been calculated as 1.04 ± 0.05 mb for the (n, p) reaction and 0.140 ± 0.007 mb for the (n, α) reaction.

EXCITATION CURVES FOR THE REACTIONS Al27(n, α)Na24 AND Mg24(n, p)Na24

Excitation curves for the reactions Al27(n, α)Na24 and Mg24(n, p)Na24 have been measured by the activation method from near threshold energies to 20.3 Mev. The measurements are relative to the known cross section for the reaction S32(n, p)P32. Monoenergetic neutrons were obtained from the D(d, n)He3, T(d, n)He4, and the T(p, n)He3 reactions employing a Tandem Van de Graaff accelerator or 125-kev accelerator. Cross sections for both reactions rise above the minimum detectable value of 0.02 mb near 5 Mev and reach maximum values of 126 mb for the Al(n, α)Na24 reaction and 205 mb for the Mg24(n, p)Na24 reaction at 13.5 Mev. Above this energy both cross sections decrease. From the excitation curves effective cross-section values for a fission-neutron spectrum have been calculated as 0.61 ± 0.03 mb for the (n, α) reaction and 1.34 ± 0.07 mb for the (n, p) reaction.

Cross Sections of Threshold Reactions for Fission Neutrons: Nickel as a Fast Flux Monitor

Measured values of effective cross sections for fission neutrons obtained for n,p reactions on Ni58, Fe54, Zn64, Mg24, Fe56, Zn67, and Cu65 are 92, 54, 28, 1.2, 0.82, 0.57, and 0.36 mb, respectivel...

Effective Reactor Cross Sections in MTR Fuel Assemblies

Analyses of uranium, neptunium, and plutonium isotopes have been carried out on dissolved MTR fuel elements. Effective reactor cross sections computed from these measurements are in good agreement with differential cross-section measurements. In particular, effective cross-section values were found for U/sup 236/ and U/sup 238/ of 34 barns and 23 barns, respectively. (auth)