The Westcott gf-factor for 239Pu and its temperature dependence

The Westcott g_f-factor, which is important for the interpretation of measurements with Maxwellian neutron spectra and reactor spectra, was calculated using our recent accurate neutron induced fission cross-section measurements. The value we obtained is g_f = 1.053 ± 0.003.We further calculated the temperature dependence of g_f from 0°C to 1000°C.     

Electron impact excitation of the 6s S1/2 state of In atom at small scattering angles

We measured the differential cross sections (DCSs) for the electron-impact excitation of the resonance transition 5p²P_1/2-6s²S_1/2 of In atom at small scattering angles using a crossed electron-atom beam technique. The incident electron energies were E₀ = 10, 20, 40, 60, 80 and 100 eV, while the small scattering angles ranged from 1° to 10° in steps of 1°. The forward scattering function method has been used for normalizing the generalized oscillator strengths (GOS) to the known optical oscillator strength and obtaining the absolute DCS values.     

Measurement of the 239Pu(n,f) cross section from thermal up to 30 keV neutron energy

At GELINA measurements of the ²³⁹Pu fission cross-section were performed covering the neutron energy region from thermal up to 30 keV. Fission fragment as well as fission neutron detection techniques were used. Also for the neutron flux determination different methods were applied. From the σ_f-data, several fission integrals were calculated and compared with other results.     

Solution of one-speed neutron transport equation for strongly anisotropic scattering by TN approximation: Slab criticality problem

In this study, a recently proposed version of Chebyshev polynomial approximation which was used in spectrum and criticality calculations by one-speed neutron transport equation for slabs with isotropic scattering is further developed to slab criticality problems for strongly anisotropic scattering. Backward–forward-isotropic model is employed for the scattering kernel which is a combination of linearly anisotropic and strongly backward–forward kernels. Further to that, the common approaches of using the same functional form for scattering and fission kernels or embedding fission kernel into the scattering kernel even in strongly anisotropic scattering is questioned for T_N approximation via taking an isotropic fission kernel in the transport equation. As a starting point, eigenvalue spectrum of one-speed neutron transport equation for a multiplying slab with different degrees of anisotropy in scattering and for different cross-section parameters is obtained using Chebyshev method. Later on, the spectra obtained for different degree of anisotropies and cross-section parameters are made use of in criticality problem of bare homogeneous slab with strongly anisotropic scattering. Calculated critical thicknesses by Chebysev method are almost in complete agreement with literature data except for some limiting cases. More importantly, it is observed that using a different kernel (isotropic) for fission rather than assuming it equal to the scattering kernel which is a more realistic physical approach yields in deviations in critical sizes in comparison with the values presented in literature. This separate kernel approach also eliminates the slow convergency and/or non-convergent behavior of high-order approximations arising from unphysical eigenspectrum calculations.     

12C(α,α)12C resonant elastic scattering at 5.7 MeV as a tool for carbon quantification in silicon-based heterostructures

The incorporation of carbon into substitutional sites in Si or Si_1−xGe_x attracts increasing interest due to the enhanced possibilities in strain and band gap engineering of group IV heterostructures. Precise and accurate measurement of carbon concentration is, however, quite difficult to achieve. We focused our attention on the study of the alpha resonant elastic scattering in the 5.7 MeV energy region. We measured the scattering cross-section in the range 5.4–6.0 MeV at a laboratory scattering angle of 170°. The results indicate that the cross-section value is enhanced with respect to the Rutherford one of an almost constant factor (×130) in an energy interval about 100 keV wide. This allows a more accurate measurement of carbon concentration than with the normally used 4.265 MeV resonance. The experimental procedure to deal with non-Rutherford scattering of Si has been also determined. The resonant scattering at 5.72 MeV has been used, in combination with Rutherford Backscattering Spectrometry (RBS) at 3.0 MeV, to determine the carbon content of three Si_1−x−yGe_xC_y samples. This has also been used, in channelling geometry, to determine the substitutional carbon fraction of the samples.     

Moment-based probability tables for angular anisotropic scattering

This paper presents a strategy for taking into account anisotropy scattering into a Monte Carlo algorithm relying on the subgroup method and developed in the DRAGON lattice code. For the sake of consistency, we limited our Monte Carlo code to the same cross-section libraries available for deterministic methods. However Legendre moments for the transfer cross-sections cannot be directly used during the Monte Carlo random walk, due to the presence of non-positive parts into the distributions. The discrete angle method is proposed to deal with this limitation, following an approach initially introduced in the MORET multigroup Monte Carlo code. We selected a moment approach, originally employed to compute probability tables for resonant cross-sections, to derive consistent sums of Dirac distributions conserving Legendre moments of the angular distributions. A detailed analysis of the applicability of the moment approach is here mandatory. When the moment technique fails due to incoherent Legendre moments, the discrete angle technique is substituted by legacy semi-analytical methods. We illustrate the proposed method using critical benchmarks coming from the ICSBEP handbook by comparison toward S_N and other Monte Carlo results. The impact of the anisotropy scattering is also discussed on a PWR MOX assembly case.     

Application of the TN method to critical slab problem for one-speed neutrons with forward and backward scattering

The critical slab problem which includes isotropic forward and backward scattering has been studied in one-speed neutron transport equation using first kind of Chebyshev polynomials. The critical half-thicknesses are computed for different degrees of c and forward and backward scattering with Mark and Marshak boundary conditions in the uniform finite slab. It is shown that T_N method gives accurate results in one-dimensional geometry and the results are agreement P_N approximation.     

The 232Th(n, f) cross-section for thermal and 2.44 MeV neutrons

The thermal neutron induced fission cross-section of ²³²Th was measured at an intense and very pure thermal neutron beam of the Grenoble High Flux Reactor, yielding a value of (3 ± 1) μbarn. For control purposes, the same targets were used at the VandeGraaff accelerator of the CBNM (Geel), where a ²³²Th (n, f) cross-section value of (0.12 ± 0.02) barn was obtained with 2.44 MeV neutrons, in agreement with other published data.     

The integral-transform method for solving neutron transport problems with general anisotropic scattering in a cylinder of finite height

In this paper, we present a mathematical technique for solving the integral transport equation for the criticality of a homogeneous cylinder of finite height. The purpose of the present paper is two-fold : firstly, to show that our earlier formalism can be generalized to any order of anisotropy, and secondly to generate the numerical results, which could serve as benchmarks when scattering is linearly anisotropic. We expand the scattering function in spherical harmonics to retain the Lth order of anisotropy. Thereafter, we write the integral transport equations for the Fourier-transformed spherical harmonic moments of the angular flux. In conformity with the integral-transform method for multidimensional geometry, the kernels of these integral equations are represented in their respective factorized form, which consists of a series of products of suitable spherical Bessel functions. The Fourier-transformed spherical harmonic moments are also represented in their separable form by expanding them in a series of products of spherical Bessel functions, commensurate with the symmetry of finite cylindrical geometry. The criticality problem for the cylinder of finite height is then posed as a matrix eigen value problem whose eigen vector is composed of the expansion coefficients mentioned above. The general matrix element is expressed as a product of certain integrals of Bessel functions, which can be evaluated by recursion relations derived in this paper. Finally, a comparison between the present benchmark results and S_N results (twotran) in (r–z) goemetry is presented when scattering is linearly anisotropic.     

Numerical Study of Z-pinch Dynamics at Different Working Regimes

Mass sweeping and current efficiency factors (f _m and f _i ) are two important parameters in Z-pinch devices, because one can compute those to better analyze Z-pinch dynamics. In this paper, the advanced shock model was employed to calculate and compare the aforementioned parameters for ACOL Z-pinch in three different working regimes. It was found that f _m is in the range of 0.12–0.13 for hydrogen gas at p ₀ = 400 and 800 Pa pressures and f _m = 0.13 for helium gas at p ₀ = 260 Pa. Similar works have resulted f _m in the range 0.10–0.16. Therefore our computations of f _m values (range 0.1–0.13) agree with the f _m range, and it confirms our model. Therefore, the factors can be precisely being calculated using advanced shock model for various Z-pinch systems at different working regimes.     

Mechanical properties of unidirectional and crossply SiCf/SiC composites using SiC fibers with carbon interphase formed by electrophoretic deposition process

Unidirectional (UD) and crossply (CP) SiC_f/SiC composites with different fiber volume fraction were fabricated based on EPD and sheet stacking process, and their mechanical properties were evaluated at room temperature. The UD-SiC_f/SiC composites showed a pseudo-ductile fracture behavior in bending test. Bending strength and fracture energy of the UD-SiC_f/SiC composites with high V_f (66%) was higher than those of the UD-SiC_f/SiC composites with low V_f (47%). Although the CP-SiC_f/SiC composites also fractured in a non-brittle manner, their bending strength and fracture energy were much lower than the UD-SiC_f/SiC composites because the SiC fibers aligned in 90° direction did not work for toughening and strengthening the SiC_f/SiC composites. In the UD-SiC_f/SiC composites, fiber pullout was clearly observed, and the number of fiber pullout in the UD-SiC_f/SiC composites with high V_f was large rather than the UD-SiC_f/SiC composites with low V_f. On the other hand, fiber pullout did not appear in the CP-SiC_f/SiC composites with low V_f whereas large number of fiber pullout was observed in the CP-SiC_f/SiC composites with high V_f. This can be explained by the thickness of the SiC matrix-layers between SiC fiber-layers.     

Optical properties of UO2 and PuO2

We perform first-principles calculations of electronic structure and optical properties for UO₂ and PuO₂ based on the density functional theory using the generalized gradient approximation (GGA) + U scheme. The main features in orbital-resolved partial density of states for occupied f and p orbitals, unoccupied d orbitals, and related gaps are well reproduced compared to experimental observations. Based on the satisfactory ground-state electronic structure calculations, the dynamical dielectric function and related optical spectra, i.e., the reflectivity, adsorption coefficient, energy-loss, and refractive index spectrum, are obtained. These results are consistent with the available experiments.     

A new spectral nodal method based on Larsen’s extended diamond approach

In this article, we describe a new spectral nodal method for solving discrete ordinates (S_N) neutron transport problems with anisotropic scattering for arbitrary order N of angular quadrature. The key to our new spectral nodal method is a consistent derivation of nonstandard auxiliary equations that relate angular neutron fluxes only in the upwind directions. These nonstandard equations are angularly coupled extensions of very basic auxiliary equations proposed by Edward W. Larsen in his extended diamond scheme of solving S₂ problems in the presence of scattering and free from spatial truncation error. The resulting method here is also free from spatial truncation error and, in contrast to previously developed spectral nodal methods, it is compatible with an efficient use of iteration on the scattering source and is free from the storage of cell-edge angular fluxes.     

A measurement of the Li(n,α) cross-section

The relative shape of the ⁶Li(n,α)/²³⁵U(n,f) cross-section ratio has been determined in the range of incident neutron energy from 2 to 800 keV. The measurements were made by the time-of-flight method using the Harwell 45 MeV linac to provide the pulsed source of neutrons. A thin ⁶Li-glass scintillator was used to register the (n,α) events, and the (n,f) events (in a metallic sample of ²³⁵U) were registered with fission neutron detectors. The shape of the ⁶Li(n,α) cross-section was obtained by combining the measured ⁶Li(n,α)/²³⁵U(tn,f) cross-section ratio with an evaluation of the ²³⁵U(n,f) cross-section. The cross-section so derived was placed on an absolute scale by normalization in the neutron energy interval 2–10 keV, where the ⁶Li(n,α) cross-section is accurately known. The cross-section at the peak of the prominent p-wave resonance near 240 keV is found to be 3.29 ± 0.12 b. The results are compared with other measurements and also with a recent theoretical calculation of the cross-section.     

Low energy ion scattering study of oxygen adsorption on a Cu{100} single crystal surface: Part II: Kinetics

The kinetics of the O₂/Cu{100} system at room temperature have been studied with Low Energy Ion Scattering. Two measuring methods have been applied: 1) A method in which the coverage is measured as a function of oxygen exposure. In this method the influence of the ion beam is kept as low as possible. 2) A method in which the oxygen coverage is measured as a function of oxygen pressure, after an equilibrium is reached between adsorption and ion-induced desorption. In both methods the O^? recoil signal and the Ne⁺|O reflection signal were evaluated. It is known from other scattering studies that at least two essentially different oxygen positions are involved at the Cu{100} surface. We tried to describe all experimental results with a model in which the two types of sites i become covered independently, with a sticking probability S(θ_i) = S_0i(1?θ_i). With this model it was possible to reproduce all experimental results using physically plausible parameter values, except for the Ne⁺|O reflection signal when using method 2.     

Electron capture into Rydberg states in collisions between multiply charged ions and hydrogen

We investigate the substate distribution σ _nlm in Rydberg manifolds (n ～ 10) following an asymmetric charge transfer reaction, C⁶⁺ + H(1s) → C⁵⁺(nlm) + H⁺, at velocities v ? 1 a.u. These calculations serve as testing ground for two novel features included in our theoretical treatment, namely, (1) the use of a multiple scattering approach, the continuum distorted wave (CDW) approximation, to describe the capture process, and (2) the explicit inclusion of the post-collision interaction (PCI) with the residual target ion to supplement the primary capture amplitude. A measure of the multiple scattering contributions is obtained by comparison with the predictions of the first Born (OBK, single scattering) approximation whereas the PCI effects are gauged by inspection of the (l, m)?distributions obtained from the resulting CDW(OBK)-PCI model with those calculated with the standard CDW (OBK) approximation.     

The finite-element method for multigroup neutron transport: Anisotropic scattering in 1-D slab geometry

Proof-tests on 1-D multigroup neutron transport problems are reported for strong anisotropic scattering. These tests have been undertaken as part of the validation of the 3-D multigroup finite-element transport code fel tran for ansisotropic scattering media. To illustrate the treatment of within-group and intergroup anisotropic scattering in the finite-element method the relevant theory is outlined. Ingroup scattering is checked using the backward-forward-isotropic (BFI) scattering law for source and eigenvalue problems. With this law anisotropic scattering problems can be transformed into equivalent isotropic scattering problems. In this way the well-validated isotropic scattering version of fel tran is used to validate the anisotropic version. Intergroup scattering effects are checked by solving few-group source problems for P₁ and P₃ scattering and the BFI scattering law. For P₁ and P₃ scattering checks are made with the discrete-ordinate finite-difference code anisn and the spherical harmonics finite-difference code marc/pn. For the BFI scattering law comparison is made with two-group exact solutions of Williams (1985) for 1-D systems.     

Inverse photoemission of uranium oxides

Understanding the itinerant-localised bonding role of the 5f electrons in the light actinides will afford an insight into their unusual physical and chemical properties. In recent years, the combination of core and valance band electron spectroscopies with theoretic modelling have already made significant progress in this area. However, information of the unoccupied density of states is still scarce. When compared to the forward photoemission techniques, measurements of the unoccupied states suffer from significantly less sensitivity and lower resolution. In this paper, we report on our experimental apparatus, which is designed to measure the inverse photoemission spectra of the light actinides. Inverse photoemission spectra of UO₂ and UO_2.2 along with the corresponding core and valance electron spectra are presented in this paper. UO₂ has been reported previously, although through its inclusion here it allows us to compare and contrast results from our experimental apparatus to the previous Bremsstrahlung Isochromat Spectroscopy and Inverse Photoemission Spectroscopy investigations.     

The CN method applied to the stationary problem of particle reflection by a purely scattering or weakly absorbing half-space; extension to the asymptotic time-dependent case

The application of the classical C_N method fails for a pure scattering or weakly absorbing medium. The asymptotic C_N method, in stationary mode, is the way to solve the transport equation in this limiting case, when c is equal or very close to 1. The asymptotic method allows us to obtain the asymptotic time-dependent emergent angular distribution for a given impinging angular intensity at t = 0 whatever c may be. The numerical results for the classical and asymptotic methods are consistent in the overlapping range.