Thermal conductivities of hypostoichiometric (U, Pu, Am)O2−x oxide

The thermal conductivities of (U_0.68Pu_0.30Am_0.02)O_2.00−x solid solutions (x = 0.00-0.08) were studied at temperatures from 900 to 1773 K. The thermal conductivities were obtained from the thermal diffusivities measured by the laser flash method. The thermal conductivities obtained experimentally up to about 1400 K could be expressed by a classical phonon transport model, λ = (A + BT)⁻¹, A(x) = 3.31 × x + 9.92 × 10⁻³ (mK/W) and B(x) = (−6.68 × x + 2.46) × 10⁻⁴ (m/W). The experimental A values showed a good agreement with theoretical predictions, but the experimental B values showed not so good agreement with the theoretical ones in the low O/M ratio region. From the comparison of A and B values obtained in this study with the ones of (U,Pu)O_2−x obtained by Duriez et al. [C. Duriez, J.P. Alessandri, T. Gervais, Y. Philipponneau, J. Nucl. Mater. 277 (2000) 143], the addition of Am into (U, Pu)O_2−x gave no significant effect on the O/M dependency of A and B values.     

Relativistic multireference Møller-Plesset perturbation theory calculations of the energy levels and transition probabilities in Ne-like xenon, tungsten, and uranium ions

Relativistic multireference many-body Møller-Plesset perturbation theory (MR-MP) calculations have been performed on neonlike xenon, tungsten, and uranium ions. The 2s⁻¹n? and 2p⁻¹n? (n ? 5, ? ? 4) energy levels, lifetimes and transition probabilities are reported. The second-order MR-MP calculation of energy levels included mass shifts, frequency-dependent first-order Breit correction and Lamb shifts. The calculated transition energies are compared with other theoretical and experimental data. The synthetic radiative spectra is presented for different wavelength regions.     

Revised transition probabilities for Fe XXV: Relativistic CI calculations

Revised data are provided for transition probabilities between fine-structure components of levels with n ? 6 in Fe XXV. Earlier published data for transitions between fine-structure levels in Fe XXV are found to be in error, especially for certain classes of transitions. The purpose of the present note is to provide a corrected database for transitions in Fe XXV. Wavefunctions and energies for states with n ? 6 and J = 0, 1, 2, 3 are determined using a relativistic configuration interaction (CI) expansion that includes the Breit interaction. To measure and control the numerical accuracy of the calculations, we compare our CI energies and matrix elements with values calculated using relativistic second-order many-body perturbation theory (MBPT), also including the Breit interaction. We obtain good agreement between our CI and MBPT calculations but disagree with earlier calculations for transitions with ΔL = 2 and for intercombination transitions (ΔS = 1). We provide wavelengths, line strengths, and transitions rates for fine-structure transition between levels with n ? 6 in Fe XXV.     

Allowed and forbidden transition parameters for Fe XV

A comprehensive set of fine structure energy levels, oscillator strengths (f), line strengths (S), and radiative decay rates (A) for bound-bound transitions in Fe XV is presented. The allowed electric dipole (E1) transitions were obtained from the relativistic Breit-Pauli R-matrix method which is based on the close coupling approximation. A total of 507 fine structure energy levels with n ? 10, l ? 9, and 0 ? J ? 10 are found. They agree within 1% with the available observed energies. These energy levels yield a total of 27,812 E1, same-spin multiplets and intercombination transitions. The A values are in good agreement with those compiled by NIST and other existing values for most transitions. Forbidden transitions are obtained from a set of 20 configurations with orbitals ranging from 1s to 5f using the relativistic code SUPERSTRUCTURE (SS) in the Breit-Pauli approximation. From a set of 123 fine structure levels, a total of 6962 S and A values are presented for forbidden electric quadrupole (E2), electric octupole (E3), magnetic dipole (M1), and magnetic quadrupole (M2) transitions. The energies from SS calculations agree with observed energies to within 1-3%. A values for E2, M1 transitions agree very well with the available values for most transitions while those for M2 transitions show variable agreement. The large set of transition parameters presented should be applicable for both diagnostics and spectral modeling in the X-ray, ultraviolet, and optical regions of astrophysical plasmas.     

MgO single-crystal as an efficient thermal neutron filter

A formula is given which, for neutron energies in the range 10⁻⁴ < E < 10 eV, permits calculation of the nuclear capture, thermal diffuse and Bragg scattering cross-sections as a function of magnesium oxide (MgO) temperature and crystal parameters. Computer program has been developed to calculate the total neutron cross-section and transmission through mono-crystalline MgO. The calculated neutron transmission and effective attenuation coefficient values for MgO-single crystal at different temperatures are compared with measured ones. An overall agreement is indicated between the formula fits and experimental data. A feasibility study for the use of MgO-single crystal is discussed in terms of the optimum MgO-single crystal thickness, mosaic spread, temperature and cutting plane for efficient transmission of thermal-reactor neutrons, and also for rejection of the accompanying fast neutrons and gamma rays.     

Radiation shielding of concretes containing different lime/silica ratios

The shielding of γ-rays and fast neutrons by concrete has been studied for concretes containing different lime/silica ratios. Calculations were carried out for six different concrete samples. The total mass attenuation coefficients (μ/ρ, cm² g⁻¹) have been computed at photon energies of 1 keV to 100 GeV using the personal computer software package WinXCom. Also the macroscopic effective fast neutron removal cross-sections (Σ_R, cm⁻¹) have been calculated using MERCSF-N program and the removal cross-section database for all required elements. The obtained results showed that the lime/silica ratio of concrete has significant and insignificant effects on μ/ρ and Σ_R values, respectively.     

Neutron radiation effects on metal oxide semiconductor (MOS) devices

The main purpose of this study is to provide the knowledge and data on Deuterium-Tritium (D-T) fusion neutron induced damage in MOS devices. Silicon metal oxide semiconductor (MOS) devices are currently the cornerstone of the modern microelectronics industry. However, when a MOS device is exposed to a flux of energetic radiation or particles, the resulting effects from this radiation can cause several degradation of the device performance and of its operating life. The part of MOS structure (metal oxide semiconductor) most sensitive to neutron radiation is the oxide insulating layer (SiO₂). When ionizing radiation passes through the oxide, the energy deposited creates electron-hole pairs. These electron-hole pairs have been seriously hazardous to the performance of these electronic components. The degradation of the current gain of the dual n-channel depletion mode MOS caused by neutron displacement defects, was measured using in situ method during neutron irradiation. The average degradation of the gain of the current is about 35 mA, and the change in channel current gain increased proportionally with neutron fluence. The total fusion neutron displacement damage was found to be 4.8 × 10⁻²¹ dpa per n/cm², while the average fraction of damage in the crystal of silicon was found to be 1.24 × 10⁻¹². All the MOS devices tested were found to be controllable after neutron irradiation and no permanent damage was caused by neutron fluence irradiation below 10¹⁰n/cm². The calculation results shows that (n,α) reaction induced soft-error cross-section about 8.7 × 10⁻¹⁴ cm², and for recoil atoms about 2.9 × 10⁻¹⁵ cm², respectively.     

Low-energy electron elastic scattering cross sections for excited Au and Pt atoms

Electron elastic total cross sections (TCSs) and differential cross sections (DCSs) in both impact energy and scattering angle for the excited Au and Pt atoms are calculated in the electron impact energy range 0 ? E ? 4.0 eV. The cross sections are found to be characterized by very sharp long-lived resonances whose positions are identified with the binding energies of the excited anions formed during the collisions. The recent novel Regge-pole methodology wherein is embedded through the Mulholland formula the electron-electron correlations is used together with a Thomas-Fermi type potential incorporating the crucial core-polarization interaction for the calculations of the TCSs. The DCSs are evaluated using a partial wave expansion. The Ramsauer-Townsend minima, the shape resonances and the binding energies of the excited Au⁻ and Pt⁻ anions are extracted from the cross sections, while the critical minima are determined from the DCSs.     

Allowed and forbidden transition parameters for Fe XXII

Radiative transitions for photo-excitations and de-excitations in Fe XXII are studied in the relativistic Breit-Pauli approximation. A comprehensive set of fine structure energy levels, oscillator strengths (f), line strengths (S), and radiative decay rates (A) for electric dipole (E1), same spin multiplicity and intercombination, fine structure transitions is presented. These are obtained from the first calculations in the close coupling approximation using the Breit-Pauli R-matrix method for this ion, all existing theoretical results having been obtained from various other atomic structure calculations. The present work obtains a set of 771 fine structure energy levels with n ? 10, l ? 9, and 1/2 ? J ? 17/2, only 52 of which have been observed. The f, S, and A values are reported for 70,372 allowed E1 transitions, exceeding by far those published previously. The calculated fine structure levels have been identified spectroscopically using a procedure based on quantum defect analysis. The energies agree with the available observed energies to within less than one to a few percent. The A values for E1 transitions are in good agreement with other existing values for most transitions. Using the atomic structure code SUPERSTRUCTURE (SS), S and A values are also presented for 38,215 forbidden transitions of the types electric quadrupole (E2), electric octupole (E3), magnetic dipole (M1), and magnetic quadrupole (M2) among 274 fine structure levels formed from 25 configurations with orbitals ranging from 1s to 4f. Some of these levels lie above the ionization limit and hence can form autoionizing lines. Such lines for 1s-2p K_α transitions have been observed in experiments. The energies from the SS calculations agree with observed energies within a few percent. The A values for E2 and M1 transitions agree very well with the available values. The atomic parameters for both allowed and forbidden transitions should be applicable for diagnostics as well as complete spectral modeling in the X-ray, ultraviolet, and optical regimes of astrophysical and laboratory plasmas.     

Non-dipole second order parameters of the photoelectron angular distribution for elements Z = 1-100 in the photoelectron energy range 1-10 keV

Presented here are the photoelectron angular distribution non-dipole parameters associated with the terms of the second order O [(kr)²] (k is the photon energy and r is the radius of the ionized atomic shell) for both unpolarized and linearly polarized radiation. The parameters are given for atomic shells with binding energies lower than 2 keV of all elements 1 ? Z ? 100 for four values of photoelectron energy in the range 1−10 keV. In this range, the second-order terms are shown to make a significant contribution (up to ∼30%) to the angular differential cross section. The inclusion of these terms becomes all the more important in calculations of the differential cross section ratio for the fixed geometry of angles which is measured experimentally in the case of linearly polarized radiation. The Dirac-Fock-Slater potential is used in the calculations. The hole left by the emitted electron is taken into account in the frozen orbital approximation.     

Atomic data and spectral line intensities for S XIII

Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for S XIII. The configurations used are 2s², 2s2p, 2p², 2l3l′, 2l4l′ and 2s5l′, with l = s, p and l′ = s, p, d, giving rise to 92 fine-structure levels in intermediate coupling. Collision strengths are calculated at seven incident energies (10, 20, 45, 90, 135, 180, and 225 Ry) for the transitions within the three lowest configurations, and five incident energies (45, 90, 135, 180, and 225 Ry) for transitions between the lowest five levels and the n = 3, 4, 5 configurations. Calculations have been carried out using the distorted wave approximation. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates of the present work, and R-matrix results for the 2s², 2s2p, 2p² configurations available in the literature, statistical equilibrium equations for level populations are solved at electron densities covering the range of 10⁸-10¹⁴ cm⁻³ at an electron temperature of log T_e(K) = 6.4, corresponding to the maximum abundance of S XIII. Spectral line intensities are calculated, and their diagnostic relevance is discussed. Observed line ratios indicate electron temperatures of the emitting plasma close to log T_e(K) = 6.4. This dataset will be made available in the next version of the CHIANTI database.     

Thermal and structural properties of low-fluence irradiated graphite

The release of Wigner energy from graphite irradiated by fast neutrons at a TRIGA Mark II research reactor has been studied by differential scanning calorimetry and simultaneous differential scanning calorimetry / synchrotron powder X-ray diffraction between 25 and 725 °C at a heating rate of 10 °C min⁻¹. The graphite, having been subject to a fast-neutron fluence from 5.67 × 10²⁰ to 1.13 × 10²² n m⁻² at a fast-neutron flux (E > 0.1 MeV) of 7.88 × 10¹⁶ n m⁻² s⁻¹ and at temperatures not exceeding 100 °C, exhibits Wigner energies ranging from 1.2 to 21.8 J g⁻¹ and a Wigner energy accumulation rate of 1.9 × 10⁻²¹ J g⁻¹ n⁻¹ m². The differential-scanning-calorimeter curves exhibit, in addition to the well known peak at ∼200 °C, a pronounced fine structure consisting of additional peaks at ∼150, ∼230, and ∼280 °C. These peaks correspond to activation energies of 1.31, 1.47, 1.57, and 1.72 eV, respectively. Crystal structure of the samples is intact. The dependence of the c lattice parameter on temperature between 25 and 725 °C as determined by Rietveld refinement leads to the expected microscopic thermal expansion coefficient along the c axis of ∼26 × 10⁻⁶ °C⁻¹. At 200 °C, coinciding with the maximum in the differential-scanning-calorimeter curves, no measurable changes in the rate of thermal expansion have been detected - unlike its decrease previously seen in more highly irradiated graphite.     

Radiation damage in ZnO ion implanted at 15 K

Commercial O-face (0 0 0 1) ZnO single crystals were implanted with 200 keV Ar ions. The ion fluences applied cover a wide range from 5 × 10¹¹ to 7 × 10¹⁶ cm⁻². The implantation and the subsequent damage analysis by Rutherford backscattering spectrometry (RBS) in channelling geometry were performed in a special target chamber at 15 K without changing the target temperature of the sample. To analyse the measured channelling spectra the computer code DICADA was used to calculate the relative concentration of displaced lattice atoms.Four stages of the damage evolution can be identified. At low ion fluences up to about 2 × 10¹³ cm⁻² the defect concentration increases nearly linearly with rising fluence (stage I). There are strong indications that only point defects are produced, the absolute concentration of which is reasonably given by SRIM calculations using displacement energies of E_d(Zn) = 65 eV and E_d(O) = 50 eV. In a second stage the defect concentration remains almost constant at a value of about 0.02, which can be interpreted by a balance between production and recombination of point defects. For ion fluences around 5 × 10¹⁵ cm⁻² a second significant increase of the defect concentration is observed (stage III). Within stage IV at fluences above 10¹⁶ cm⁻² the defect concentration tends again to saturate at a level of about 0.5 which is well below amorphisation. Within stages III and IV the damage formation is strongly governed by the implanted ions and it is appropriate to conclude that the damage consists of a mixture of point defects and dislocation loops.     

Dependence of cluster ranges on target cohesive energy: Molecular-dynamics study of energetic Au402 cluster impacts

It has long been known that the stopping and ranges of atoms and clusters depends on the projectile-target atom mass ratio. Recently, Carroll et al. [S.J. Carroll, P.D. Nellist, R.E. Palmer, S. Hobday, R. Smith, Phys. Rev. Lett. 84 (2000) 2654] proposed that the stopping of clusters also depends on the cohesive energy of the target. We investigate this dependence using a series of molecular-dynamics simulations, in which we systematically change the target cohesive energy, while keeping all other parameters fixed. We focus on the specific case of Au₄₀₂ cluster impact on van-der-Waals bonded targets. As target, we employ Lennard-Jones materials based on the parameters of Ar, but for which we vary the cohesive energy artificially up to a factor of 20. We show that for small impact energies, E₀ ? 100 eV/atom, the range D depends on the target cohesive energy U, D ∝ U^−β. The exponent β increases with decreasing projectile energy and assumes values up to β = 0.25 for E₀ = 10 eV/atom. For higher impact energies, the cluster range becomes independent of the target cohesive energy. These results have their origin in the so-called ‘clearing-the way’ effect of the heavy Au₄₀₂ cluster; this effect is strongly reduced for E₀ ? 100 eV/atom when projectile fragmentation sets in, and the fragments are stopped independently of each other. These results are relevant for studies of cluster stopping and ranges in soft matter.     

First-principles approach to the properties of point defects and small helium-vacancy clusters in palladium

First-principles calculations based on density functional theory (DFT) have been performed to study the properties of interstitial helium atoms, the vacancy, substitutional, and small helium-vacancy clusters He_mV_n (m, n = 0-4) in palladium. The result indicates that the vacancy has the strongest ability of capturing helium atoms and the octahedral interstitial configuration is more stable than the tetrahedral one, while the energy difference between them is very small. In the palladium crystal, helium atom will migrate from one octahedral interstitial site to another one through the O-T-O path. The formation energies and binding energies of an interstitial helium atom and an isolated vacancy to the helium-vacancy clusters are also determined in palladium. It is found that the formation energies increase with the increasing of helium atoms and the binding energies mainly depend on the helium to vacancy ratio of the clusters rather than the cluster size.     

Determination of the effective atomic numbers and electron densities for YBaCuO superconductor in the range 59.5-136 keV

The effective atomic numbers and electron densities of YBa₂Cu₃O_7−δ superconductor at 59.5, 65.2, 77.1, 94.6, 122 and 136 keV were calculated by using the measured mass attenuation coefficients. Measurements were made by performing transmission experiments in a well-collimated narrow beam geometry set-up by employing Si(Li) detector with a resolution of 0.16 keV at 5.9 keV. These values are found to be in good agreement with theoretical values calculated based on XCOM data. The observed crystal structure of YBa₂Cu₃O_7−δ superconductor is close to the theoretical structure. Z_eff and N_el experimental values showed good agreement with the theoretical values for calcined and sintered YBa₂Cu₃O_7−δ.     

Atomic data and spectral line intensities for Mg IX

Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for Mg IX. The configurations used are 2s², 2s2p, 2p², 2l3l′, 2l4l′ and 2s5l′, with l = s, p and l′ = s, p, d giving rise to 92 fine-structure levels in intermediate coupling. Collision strengths are calculated at seven incident energies (6, 12, 25, 50, 75, 100, and 125 Ry) for the transitions within the three lowest configurations, and five incident energies (25, 50, 75, 100, and 125 Ry) for transitions between the ground configuration and the n = 3, 4, 5 configurations. Calculations have been carried out using the distorted wave approximation. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates of the present work, and R-Matrix results for the 2s², 2s2p, 2p² configurations available in the literature, statistical equilibrium equations for level populations are solved at electron densities covering the range of 10⁸-10¹⁴ cm⁻³ at an electron temperature of log T_e (K) = 6.0, corresponding to the maximum abundance of Mg IX. Spectral line intensities are calculated, and their diagnostic relevance is discussed. Observed line ratios indicate electron temperatures of the emitting plasma which agree with log T_e (K) = 6.0. This dataset will be made available in the next version of the CHIANTI database.     

Emission statistics of X-ray induced photoelectrons and its comparison with electron- and ion-induced electron emissions

The emission statistics of secondary electrons from a gold metal surface induced by monochromatic X-rays is studied by Monte Carlo simulations. The number distributions of emitted electrons n and their mean values γ are calculated systematically for incident photon energies from 1 to 100 keV. The results are compared with recent experimental results measured at the SPring-8 X-ray beam facility (BL15XU). We found that both theoretical and experimental results of the statistical number distributions of secondary electrons can be reproduced fairly well by Polya-type functions, showing small probabilities for one-electron emission (n = 1) and broad distributions for high-n emission. In contrast, these features can never be reproduced by Poisson statistics. Furthermore, calculated emission yields γ are found to depend rather weakly on the incident X-ray energy. These results indicate clearly that fast photoelectrons produced by high-energy X-rays are responsible for high-n emission although the photoionization cross sections are considerably smaller at higher X-ray energies. Simulations are also extended to electron and ion bombardments, and a comprehensive comparison between X-rays and charged particle impacts is given for the emission statistics of electrons from a metal surface.     

Electron impact collision strengths and oscillator strengths for Ge-, Ga-, Zn-, Cu-, Ni-, and Co-like Au ions

Energy levels, oscillator strengths, and electron impact collision strengths have been calculated for Ge-, Ga-, Zn-, Cu-, Ni-, and Co-like Au ions. For Ni-like Au, these atomic data are obtained among the levels belonging to the configurations of ([Ne])3s²3p⁶3d¹⁰, 3s²3p⁶3d⁹nl, 3s²3p⁵3d¹⁰nl, and 3s 3p⁶3d¹⁰nl (n = 4, 5; l = 0, 1, … , n − 1). For other Au ions, more levels have been obtained with special attention to atomic data up to transitions of 5f → 3d for emission or 3d → 5f for absorption. Configuration interactions are taken into account for all levels included. Collision strengths have been obtained at 20 scattered electron energies (5-40,000 eV) and they are listed at six representative energies of 100, 500, 1000, 5000, 10,000, and 20,000 eV in this work. Effective collision strengths have been obtained by assuming a Maxwellian electron velocity distribution at 10 representative temperatures ranging from 500 to 5000 eV. The present dataset should be adequate for most applications. The energy levels are expected to be accurate to within 0.5%, while oscillator strengths and collision strengths for strong transitions are probably accurate to better than 20%. The complete dataset is available electronically from http://www.astronomy.csdb.cn/EIE/.     

Low-energy electron elastic collision cross sections for ground and excited Tm, Lu and Hf atoms

Low-energy 0 ? E ? 1.0 eV electron elastic scattering from ground and excited Tm, Lu and Hf atoms has been investigated. Both total and differential cross sections have been calculated; the latter at the scattering angles θ = 0°, 90° and 180°. The recent Regge-pole methodology has been used for the calculations. In the method the crucial electron-electron correlation effects are accounted for through the Mulholland formula. We find that the total cross sections are characterized generally by shape resonances, Ramsauer-Townsend minima and dramatically sharp long-lived resonances from which we extract the binding energies of the negative ions. Our extracted binding energy of the Hf⁻ negative ion from the total cross section is compared with that of Pan and Beck [14].