Energy-averaged Cross Sections Research Articles

High accuracy, high resolution 235U(n,f) cross section from n_TOF (CERN) from 18 meV to 10 keV

The 235U(n,f) cross section was measured in a wide energy range (18 meV–170 keV) at the n_TOF facility at CERN, relative to 6Li(n,t) and 10B(n,α) standard reactions, with high resolution and accuracy, with a setup based on a stack of six samples and six silicon detectors placed in the neutron beam. In this paper we report on the results in the region between 18 meV and 10 keV neutron energy. A resonance analysis has been performed up to 200 eV, with the code SAMMY. The resulting fission kernels are compared with the ones extracted on the basis of the resonance parameters of the most recent major evaluated data libraries. A comparison of the n_TOF data with the evaluated cross sections is also performed from thermal to 10 keV neutron energy for the energy-averaged cross section in energy groups of suitably chosen width. A good agreement, within 0.5%, is found on average between the new results and the latest evaluated data files ENDF/B-VIII.0 and JEFF-3.3, as well as with respect to the broad group average fission cross section established in the framework of the standard working group of IAEA (the so-called reference file). However, some discrepancies, of up to 4%, are still present in some specific energy regions. The new dataset here presented, characterized by a unique combination of high resolution and accuracy, low background and wide energy range, can help to improve the evaluations from the Resolved Resonance Region up to 10 keV, also reducing the uncertainties that affect this region.

Statistical model calculations of the 232Th(n,2n) reaction

On the context of the Cern n_TOF collaboration the 232Th(n,2n)231Th reaction cross section has been measured relative to the 56Fe(n,p)56Mn and 27Al(n,a)24Na reaction cross sections by the activation method for neutron energies up to 11 MeV. The neutrons were produced via the 2H(d,n) reaction using a deuterium filled gas-cell, at the 5.5MV TANDEM Accelerator of NCSR "Demokritos". In addition to the experimental work, theoretical Statistical model calculations have been performed using the computer code STAPRE/F. The code STAPRE is designed to calculate energy averaged cross sections for particle induced nuclear reactions with several emitted particles and gamma rays under the assumption of sequential evaporation. For the first evaporation step preequilibrium emission is taken into account while population of states resulting from the first equilibrium evaporation step is calculated using the Hauser-Feschbach theory. Fission process competition is also taken into account in the evaporation steps. Sensitive parameters for the calculation, like level density parameters, have been adopted after fitting experimental data for the competing (n,f) reaction. The results are being compared to the experimental data.

First Measurement of the Total Neutron Cross Section on Argon between 100 and 800MeV.

We report the first measurement of the neutron cross section on argon in the energy range of 100-800MeV. The measurement was obtained with a 4.3-h exposure of the Mini-CAPTAIN detector to the WNR/LANSCE beam at LANL. The total cross section is measured from the attenuation coefficient of the neutron flux as it traverses the liquid argon volume. A set of 2631 candidate interactions is divided in bins of the neutron kinetic energy calculated from time-of-flight measurements. These interactions are reconstructed with custom-made algorithms specifically designed for the data in a time projection chamber the size of the Mini-CAPTAIN detector. The energy averaged cross section is 0.91±0.10(stat)±0.09(syst) b. A comparison of the measured cross section is made to the GEANT4 and FLUKA event generator packages, where the energy averaged cross sections in this range are 0.60 and 0.68b, respectively.

Advanced Bondarenko method for resonance self-shielding calculations in deterministic reactor physics code system CBZ

The advanced Bondarenko method for resonance self-shielding calculations is devised and proposed. This method is based on three numerical methods; the Bell factor optimization for accurate fuel escape probability representation, extension of resonance interference factors and correction factors for current-weighted total cross sections. A 107-group library for light water reactor applications based on the advanced Bondarenko method is generated for a reactor physics code system CBZ. Performance of the CBZ code with this 107-group library is examined against a suit of light water reactor cell problems. The infinite neutron multiplication factors calculated with CBZ agree with reference continuous-energy Monte Carlo solutions within 0.15%Δk/kk′ differences, and no significant biases on fuel compositions and geometrical specifications are observed. Energy-averaged cross sections are also examined. Numerical tests reveal that significant accuracy improvements in resonance self-shielding calculations are realized by adopting the advanced Bondarenko method without any significant increase of computational burden.

Resonance scattering and the passage to bound states in the field of near-black-hole objects

In this paper, we examine the spectrum of a massive scalar particle interacting with the strong gravitational field of a static, spherically symmetric object which is not quite massive enough to be a black hole. As was found in the case of massless particles, there exists a dense spectrum of long lived resonances (meta-stable states), which leads to an energy-averaged cross-section for particle capture which approaches the absorption cross-section for a Schwarzschild black hole. However, the generalization to nonzero mass introduces new phenomena, along with important qualitative changes to the scattering properties. In contrast to the massless case, there exists a spectrum of bound states with almost identical structure to that of the resonances, allowing for the possibility of radiative transitions and particle capture. The resonance lifetimes for elastic processes are parametrically larger than for massless particles, meaning the absorption cross-section approaches the black hole case faster than for massless scalars.

Extending the Kawai-Kerman-McVoy Statistical Theory of Nuclear Reactions to Intermediate Structure via Doorways

Kawai, Kerman, and McVoy have shown that a statistical treatment of many open channels that are coupled by direct reactions leads to modifications of the Hauser- Feshbach expression for energy-averaged cross section (Ann. of Phys. 75, 156 (1973)). The energy averaging interval for this cross section is on the order of the width of sin- gle particle resonances, 1 MeV, revealing only a gross structure in the cross section. When the energy-averaging interval is decreased down to a width of a doorway state, 0:1 MeV, a so-called intermediate structure may be observed in cross sections. We extend the Kawai-Kerman-McVoy theory into the intermediate structure by leveraging a theory of doorway states developed by Feshbach, Kerman, and Lemmer (Ann. of Phys. 41, 230 (1967)). As a by-product of the extension, an alternative derivation of the central result of the Kawai-Kerman-McVoy theory is suggested. We quantify the e ect of the ap- proximations used in derivation by performing numerical computations for a large set of compound nuclear states.

Nucleosynthesis driven by Coulomb explosion of nanodroplets

This paper presents a computational and theoretical study of the yields for the C12(p,γ)N13 and C12(d,n)N13 nucleosynthesis reactions driven by high-energy (MeV) Coulomb explosion (CE) of (CH4)n and (CD4)n nanodroplets (n=104–107, radii R0=100–500Å) in ultraintense near-infrared Gaussian laser fields (peak intensities IM=1018–1019Wcm−2, pulse length τ=25fs). Efficient nucleosynthesis with C12 nuclei requires the production of protons and deuterons in the energy range of 0.5–2.0MeV, which is accomplished by CE of nanodroplets. The energy distributions of the nuclei were obtained from scaled molecular dynamics simulations and were utilized to calculate the energy averaged cross sections and the reaction yields for nucleosynthesis. Two distinct reaction modes were established pertaining to reactions inside the plasma filament volume (IF), and to reactions outside the plasma filament volume (OF). The dominating contribution to the total yields originates from the OF mode. The time resolved γ-ray emission for C12(p,γ)N13 will be exhibited in distinct time intervals of 2–100ps for the OF mode and of 65fs–3ps for the IF mode. Estimates of the yields were provided by incorporating the effects of nanodroplet size distributions that result in a slow size dependence of the yields in the range R0>250Å, and the effects of laser intensity attenuation in an assembly of nanodroplets that result in the decrease of the OF and IF yields due to the decrease of the effective plasma filament volume. For 400Å nanodroplets at IM=4×1018–1019Wcm−2, under realistic experimental conditions, the C12(p,γ)N13 nucleosynthesis driven by CE of (CH4)n nanodroplets is characterized by a yield (per laser pulse) of ∼100, while the yield for the C12(d,n)N13 nucleosynthesis driven by CE of (CD4)n clusters is ∼9×104. Table-top nucleosynthesis driven by nanodroplet CE is amenable to experimental observation.

Generation of neutron multigroup constants in the energy range [formula omitted]–10 MeV for light and heavy water at four different temperatures

On the basis of the cross section models of neutron scattering in liquid H 2O and D 2O, which have been developed in the previous papers, we have generated eight sets of multigroup constants (energy-averaged cross sections) for each liquid at 5, 27, 52 and 77 ∘ C . The multigroup constants cover a wide energy range 0.1 μ eV –10 MeV with 140 energy groups at equal logarithmic intervals and represent an angular scattering distribution by the Legendre polynomial expansion up to order 3. Major characteristics of neutron scattering inherent to liquid water are fully included in terms of coherent and incoherent properties and temperature-dependent quasi-elastic and inelastic scattering. These characteristics are assured by comparison with relevant experimental results of scattering cross section and also by neutron slowing-down and thermalization analysis in liquid H 2O and D 2O. It is shown that the present multigroup constants serve for analysis of neutron moderation from fission/spallation to ultra-cold energies, in combination with the already-generated ones for liquid 4He, H 2, D 2 and CH 4 and solid CH 4.

Cold neutron production in solid and liquid CH 4 moderators

A numerical study of cold neutron production in cryogenic moderators composed of solid or liquid CH 4 at temperatures ranging from 20.4 to 111.7 K is made. The study is performed by a multigroup neutron transport analysis with a set of energy-averaged cross-sections (group constants) generated by the authors. Particular attention is paid to the total and angular properties of a space–energy cold-neutron flux, especially in terms of an equilibrium energy spectrum at each temperature, a storage property of high-density cold neutrons and an anisotropic emission of cold neutrons from the moderator surface.

Absolute cross section forSi2+(3s3p3Po→3s3p1Po)electron-impact excitation

We have measured the absolute energy-averaged cross section for electron-impact excitation of Si{sup 2+}(3s3p {sup 3}P{sup o}{yields}3s3p {sup 1}P{sup o}) from energies below threshold to the turn-on of the 3s3p {sup 3}P{sup o}{yields}3p{sup 2} {sup 3}P transition. A beams modulation technique with inclined electron and ion beams was used. Radiation at 120.65 nm from the decay of the excited ions to the 3s{sup 2} {sup 1}S ground state was detected using an absolutely calibrated optical system. The fractional population of metastable Si{sup 2+}(3s3p {sup 3}P{sup o}) in the incident ion beam was determined to be 0.256{+-}0.035(1.65{sigma}). The experimental energy spread ranged from 0.85 eV (full width at half maximum) at the lowest energies to 0.56 eV at the highest. Resonance features consistent with 12-state close-coupling R-matrix calculations are seen.

Cross-section model for cold neutron scattering in solid and liquid methane. II: group-constant generation

A set of energy-averaged cross-sections (group constants) for neutron scattering in solid and liquid CH 4 is generated: 80 groups between 0.1 μeV and 10 eV at 20.4, 50, 90.7 and 111.7 K . Use is made of the double-differential scattering cross-section model developed by the authors, which includes molecular translations and rotations as well as intramolecular vibrations. In view of the computed results on scattering and slowing-down properties, it is expected that the group constants may serve as a powerful tool for design of an advanced cold neutron source.

Author Index

Using the cross-section model for neutron scattering in liquid H2 and D2, a neutron transport analysis is performed for spherical cold-neutron sources composed of either para H2, normal H2 or normal D2. A special effort is made to generate a set of energy-averaged cross-sections (80 group constants between 0.1μeV and 10eV) for liquid H2 and D2 at melting and boiling points. A number of conclusions on the spherical cold-neutron source configurations are drawn. It is especially shown that the highest cold-neutron flux is obtainable from the normal D2 source with a radius of about 50 cm, while the normal- and para-H2 sources with radii around 3–4cm produce maximum cold-neutron fluxes at the center.

Neutronic study of spherical cold-neutron sources composed of liquid hydrogen and liquid deuterium

Abstract Using the cross-section model for neutron scattering in liquid H2 and D2, a neutron transport analysis is performed for spherical cold-neutron sources composed of either para H2, normal H2 or normal D2. A special effort is made to generate a set of energy-averaged cross-sections (80 group constants between 0.1 μeV and 10 eV ) for liquid H2 and D2 at melting and boiling points. A number of conclusions on the spherical cold-neutron source configurations are drawn. It is especially shown that the highest cold-neutron flux is obtainable from the normal D2 source with a radius of about 50 cm, while the normal- and para-H2 sources with radii around 3– 4 cm produce maximum cold-neutron fluxes at the center.

Electron recombination with multicharged ions via chaotic many-electron states

We show that a dense spectrum of chaotic multiply-excited eigenstates can play a major role in collision processes involving many-electron multicharged ions. A statistical theory based on chaotic properties of the eigenstates enables one to obtain relevant energy-averaged cross sections in terms of sums over single-electron orbitals. Our calculation of the low-energy electron recombination of Au$^{25+}$ shows that the resonant process is 200 times more intense than direct radiative recombination, which explains the recent experimental results of Hoffknecht {\em et al.} [J. Phys. B {\bf 31}, 2415 (1998)].

Cold neutron production in liquid para- and normal-H 2 moderators

A neutron transport analysis is performed for liquid H 2 moderators with 100% para and normal (ortho:para=0.75:0.25) fractions. Four sets of energy-averaged cross-sections (group constants) for liquid ortho- and para-H 2 at melting and boiling points are generated and neutron energy range between 0.1 μeV and 10 eV is broken into 80 groups. Basic moderating characteristics are studied of a model cold-neutron source in a one-dimensional bare-slab geometry. It is shown that liquid para-H 2 is superior in cold neutron production to liquid normal H 2 on account of a para-to-ortho transition (molecular rotational excitation) and a good transmission property with a mean free path of about 10 cm . In the case of neutron extraction from the inside of the source, high intensity of cold neutrons is possible with liquid normal H 2 at higher temperatures up to the boiling point.

Statistical treatment of detection cross-section uncertainties in the analysis of solar neutrino data

We propose a modification to the standard statistical treatment of the detection cross-section uncertainties in the analysis of solar neutrino data. We argue that the uncertainties of the energy-averaged cross sections of the different neutrino fluxes in the same experiment should be treated as correlated. We show that the resulting allowed regions for the neutrino oscillation parameters are significantly larger than the ones obtained with uncorrelated uncertainties.

Absolute cross section for Si2+(3s3p 3p0→3s3p 1p0) electron-impact excitation

We have measured the absolute energy-averaged cross section for electron-impact excitation of Si 2 + (3s3p 3 P 0 →3s3p 1 P 0 ) from energies below threshold to the turn-on of the 3s3p 3 P 0 →3p 2 3 P transition. A beams modulation technique with inclined electron and ion beams was used. Radiation at 120.65 nm from the decay of the excited ions to the 3s 2 1 S ground state was detected using an absolutely calibrated optical system. The fractional population of metastable Si 2 + (3s3p 3 P 0 ) in the incident ion beam was determined to be 0.256′0.035(1.65σ). The experimental energy spread ranged from 0.85 eV (full width at half maximum) at the lowest energies to 0.56 eV at the highest. Resonance features consistent with 12-state close-coupling R-matrix calculations are seen.

Semi-classical and quantal calculation of state-to-state generalized cross-sections for N+2-He mixtures

A semi-classical calculation of state-to-state generalized cross-sections has been performed for the first time and compared with quantal close-coupling calculations for the system N+ 2-He. At the total collision energy of 838.6cm-1, the semi-classical results for the cross-sections diagonal in the tensorial ranks agree with the close-coupling results within 20%, while those for the non-diagonal cross-sections give at least semi-quantitative agreement. It is expected that the agreement will improve when energy-averaged cross-sections are computed. At high collision energies, where large rotational quantum numbers are involved, quantal calculations cease to be feasible, and the semi-classical method provides a practical and reasonably accurate procedure to compute state-to-state cross-sections.

Semi-classical and quantal calculation of state-to-state generalized cross-sections for N+2-He mixtures

A semi-classical calculation of state-to-state generalized cross-sections has been performed for the first time and compared with quantal close-coupling calculations for the system N+ 2-He. At the total collision energy of 838.6cm-1, the semi-classical results for the cross-sections diagonal in the tensorial ranks agree with the close-coupling results within 20%, while those for the non-diagonal cross-sections give at least semi-quantitative agreement. It is expected that the agreement will improve when energy-averaged cross-sections are computed. At high collision energies, where large rotational quantum numbers are involved, quantal calculations cease to be feasible, and the semi-classical method provides a practical and reasonably accurate procedure to compute state-to-state cross-sections.

Dynamic behavior of highly excited molecular states in the strong monochromatic laser fields

The dynamic behavior of highly excited molecular states in an external monochromatic field has been investigated in order to establish the general trends in the Rydberg state manifestations in collisional and radiative processes. The effects of interference between direct (background) and resonant interactions and coupling between the continua on the fine structure of collision cross sections and near-threshold photoabsorption spectra are discussed. Analytical expressions for the widths and intensities of the Rydberg lines induced by mixing the field with other quasistationary states are derived and their dependence on the external field strength and frequency are analyzed. It was found that the appreciable stabilization of isolated Rydberg levels observed previously in superstrong fields is also possible in fields much weaker than atomic fields. The possibility of laser control for the energy averaged cross sections and reaction rate constants are considered. All effects are illustrated for thee− + H2+ system.