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We report on our work program for the calculation of atomic data for astrophysical and magnetic fusion diagnostic plasma modelling. We restrict ourselves to electron-impact excitation. We look at recent work with the R-matrix method, including development of the Dirac R-matrix with pseudo-states method, as well as a new implementation of the distorted wave method within the AUTOSTRUCTURE code. [ABSTRACT FROM AUTHOR]

The E2 component of the 12C(α, γ)16O cross section is investigated by a microscopic cluster model, and by R-matrix fits. The first approach provides SE2(300 keV)≈50 keV-b for ground-state transitions. In the R-matrix theory, we show that the background term plays a crucial role, and cannot be determined without ambiguity. Only an upper limit on the extrapolated S factor can be obtained [SE2(300 keV)<190 keV-b]. To constrain the R-matrix analysis, we use the GCM Asymptotic Normalization Constant (ANC) of the 21+ level. This procedure strongly reduces the uncertainties on the R-matrix fit, and we end up with a recommended value of SE2(300 keV) =42±2 keV-b. As ANC values derived from indirect methods are not consistent with the 12C(α, γ)16O cascade transitions to the 21+ state, we suggest a remeasurement of this cross section. [ABSTRACT FROM AUTHOR]

We review some aspects of R-matrix theory and its application to the semi-empirical analysis of nuclear reactions. Important applications for nuclear astrophysics and recent results for the 12C(α,γ)16O reaction are emphasized. [ABSTRACT FROM AUTHOR]

The article presents information on R-matrix, which encodes the integrable structure. The R-matrix is the foundation for much of the integrable machinery, such as the algebraic Bethe ansatz. According to researchers, this particular R-matrix is exceptional because unlike most other known R-matrices it cannot be written as a function of the difference of two spectral parameters.

An overview will be given of recent atomic and molecular collision methods developed to take advantage of modern massively parallel computers. The focus will be on direct solutions of the time-dependent Schrödinger equation for simple systems using large numerical lattices, as found in the time-dependent close-coupling method, and for configuration interaction solutions of the time-independent Schrödinger equation for more complex systems using large numbers of basis functions, as found in the R-matrix with pseudo-states method. Results from these large scale calculations are extremely useful in benchmarking less accurate theoretical methods and experimental data. To take full advantage of future petascale and exascale computing resources, it appears that even finer grain parallelism will be needed. [ABSTRACT FROM AUTHOR]

As computer power continues to increase, so does our ability to carry out extensive, non-perturbative calculations of atomic processes. Much work has been done on increasing the target representation of complex systems in step with advances in computational power. However there is also a need for coverage in terms of atomic data used in astrophysical or fusion plasma modeling. In this talk, we discuss recent R-matrix calculations of electron-impact excitation and photoionization over iso-electronic sequences. While automation of R-matrix codes and data analysis are reviewed, the talk is focused on iso-electronic trends observed in the data. Specifically, we show the effect that resonant enhancement and level mixing can have on expected iso-electronic trends. [ABSTRACT FROM AUTHOR]

In this work we present a fast and stable method for solving the one-channel Lippmann-Schwinger equation in the case of potential electron-molecular scattering. Our approach benefits from the accurate construction of the Green’s function based on the R-matrix framework. We focus on long range spherically symmetric local interactions and show how to adapt our method for accurate computation of scattering length and effective range in this case. As a byproduct of our calculations also the number of bound states supported by the potential can be easily determined. Numerical tests are performed on a class of potentials (driven by the van der Waals like terms ∼r-6 for large r) describing the ground state interaction of CS2 and 4He2 dimers. Comparison with alternative techniques is also given. Accurate results are of particular importance especially in view of the interest in Bose-Einstein condensation of ultracold gases. [ABSTRACT FROM AUTHOR]

We present here a study of the three-body, nα6He particle break-up of 11Be(10.6) following 11Li β-decay. The emitted charged particles were detected in coincidence using a cubic set-up of highly segmented silicon detectors, allowing us to measure simultaneously energy and trajectory. The three body break-up of 11Be(10.5) through the intermediate state 10Be(9.6) was modeled using the multiple-level single-channel R-Matrix formalism. [ABSTRACT FROM AUTHOR]

Interstellar clouds are mainly composed of hydrogen molecules whose molecular spectra are difficult to record. Major other components are polar molecules, whose rotational and ro-vibrational spectra are readily observed. Inelastic collisions of molecules with H2 and electrons are important processes to understand the formation and intensities of molecular spectral lines. We have calculated the interaction energy and various inelastic cross-sections and rates, for two important constituents of the interstellar media, H2O and HC3N. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Physical processes underlying X-ray spectral formation may be influenced considerably by particle energy distribution in the source. It is found that different electron distributions — Maxwellian or Gaussian — in the plasma yield significantly different X-ray line ratios. Owing to energetically uneven occurrence of dense resonances in electron impact excitation cross sections for transitions in Fe XVII, line ratios with Maxwellian and Gaussian averaged rates may differ by up to a factor of two, thereby explaining outstanding discrepancies between two independent laboratory measurements using Electron-Beam-Ion-Traps, as well as with astrophysical observations. The Close Coupling R-matrix theory which includes resonances via an ab initio wavefunction expansion, vs. several variants of the Distorted Wave theory which does not, is discussed. The R-matrix method also enables a unified and self-consistent theoretical treatment of photoionization and (e + ion) recombination (inverse) processes, that includes both the radiative and dielectronic recombination and yields total and level-specific rate coefficients valid at all temperatures. Finally, atomic databases from the Opacity Project and Iron Project at the Center de Donnees Astronomiques de Strasbourg and the Ohio Supercomputer Center are described, including a novel on-line database to compute stellar opacities for an arbitrary mixture of elements. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The analysis code SAMMY is widely used for parameterization and understanding of neutron- and charged-particle-induced cross-section data. This report describes advances made in the development of the code: An overview of the main features of SAMMY is presented, with emphasis on options recently added to the code. Among the new options are an energy-dependent direct-capture component for capture cross sections, a crystal-lattice model for Doppler broadening, improvements in the fitting procedure to incorporate more of the experimental uncertainties and covariances, streamlined input, and enhanced ENDF-related capabilities. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Better measurements and evaluations are needed for many elements where the existing evaluations or the underlying nuclear cross-section data are not sufficiently accurate for reliable calculation of criticality safety margins. Deficiencies in the existing ENDF/B-VI data evaluation for Cl led to our resonance parameter evaluation of Cl neutron cross sections in the resolved resonance region with the multilevel Reich-Moore R-matrix formalism. Our evaluation takes advantage of recent high-resolution capture and transmission measurements at the Oak Ridge Electron Linear Accelerator (ORELA) as well as older total cross-section measurements at Karlsruhe (KFK) to extend the resolved resonance energy range to 1.2 MeV with much more accurate representation of the data than previous evaluations. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The independent analysis of new experimental data for 232Th cross sections in the unresolved region performed on the basis of the existing scheme and method of evaluation confirm the previously obtained average resonance parameters. The method of statistical modeling of the resonant cross-section structure in the unresolved resonance region, proposed and developed earlier by introducing the characteristic function of R-matrix elements distribution and the presentation of this by a ladder of fixed “resonances,” is used for calculation of the self-shielding factors of 232Th. The results are given in comparison with those of the code NJOY and experimental data. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present a method for computing direct-semidirect neutron radiative capture and its applications to R-Matrix evaluations of 19F, 27Al, 28,29,30Si, 35,37Cl, and 39,41K performed by the Nuclear Science and Technology Division’s Nuclear Data group at ORNL. The method includes both direct and semidirect capture; the latter is a core-polarization term in which the giant dipole resonance is formed. We compare our method with that employed in the TEDCA code, which has been commonly used in computations related to astrophysics, as well as in nuclear data evaluations. For nuclear data evaluations in the resolved resonance region, where R-Matrix evaluations are commonly performed using the Sammy code, direct capture is significant between resonant capture peaks, as well as at very low neutron incident energy below the lowest resonance (e.g., thermal capture). The experimental data for which the R-Matrix analysis are being performed is subject to background effects that at energies below 10 keV could be sufficiently large to obscure the direct capture cross section. Consequently, an accurate evaluation of TOF data requires a theoretical model of the direct capture cross section, to which end we apply our method. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Covariance data are required to correctly assess uncertainties in design parameters in nuclear applications. The error estimation of calculated quantities relies on the nuclear data uncertainty information available in the basic nuclear data libraries, such as the U.S. Evaluated Nuclear Data File, ENDF/B. The uncertainty files in the ENDF/B library are obtained from the analysis of experimental data and are stored as variance and covariance data. The computer code SAMMY is used in the analysis of the experimental data in the resolved and unresolved resonance energy regions. The data fitting of cross sections is based on generalized least-squares formalism (Bayes’ theory) together with the resonance formalism described by R-matrix theory. Two approaches are used in SAMMY for the generation of resonance-parameter covariance data. In the evaluation process SAMMY generates a set of resonance parameters that fit the data, and, in addition, it also provides the resonance-parameter covariances. For existing resonance-parameter evaluations where no resonance-parameter covariance data are available, the alternative is to use an approach called the “retroactive” resonance-parameter covariance generation. In the high-energy region the methodology for generating covariance data consists of least-squares fitting and model parameter adjustment. The least-squares fitting method calculates covariances directly from experimental data. The parameter adjustment method employs a nuclear model calculation such as the optical model and the Hauser-Feshbach model, and estimates a covariance for the nuclear model parameters. In this paper we describe the application of the retroactive method and the parameter adjustment method to generate covariance data for the gadolinium isotopes. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

In the evaluation of low-energy cross sections, the broadening of neutron resonances due to the motion of the target nucleus was thoroughly studied. The widely used model for Doppler broadening is the Free Gas Model (FGM) that neglects atomic interactions between target atoms. When the target atoms are bound to a molecule or a crystal, the incident neutron energy can change the atomic vibrational or rotational quantum state, and a more realistic treatment is needed. In the early days of cross-section theories, Lamb derived a theoretical model, known as the Crystal Lattice Model (CLM), to account for the emission or absorption of one or more phonons (quanta of vibrational energy). This model was recently implemented in the Bayesian R-matrix fitting computer code SAMMY. In the present work, the CLM of SAMMY is tested against transmission experiments performed at GELINA (Geel ELectron LINear Acelerator) on 238U with U and UO2 samples at 23.7 K and at room temperature. The CLM of SAMMY is then used to improve the evaluation of 238U resonance parameters in progress at the Oak Ridge National Laboratory. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

An R-matrix analysis of experimental nuclear data on the reactions 4He(α,α), 4He(α,p), 4He(α,d), 7Li(p,α), 7Li(p,p), 7Li(p,n), 7Be(n,p), 6Li(d,α), 6Li(d,p), 6Li(d,n), and 6Li(d,d), leading to the 8Be intermediate state, has been completed in the last two years. About 4700 data points from 69 experimental references are included. The excitation energy above the 8Be ground state is 25 – 26 MeV for all reactions except 4He(α,α) and 7Be(n,p). The data for the reactions 4He(α,α) and 6Li(d,d) do not fit well, but the other reactions fit with a χ2/(point) of less than the overall value of 7.9. Most of the 19 resonances found in the R-matrix analysis correspond to resonances formerly known from experiment. Evaluated integrated 4He(α,p), 4He(α,d), 7Li(p,α), 7Li(p,n), 7Be(n,p), 6Li(d,α), 6Li(d,p) and 6Li(d,n) reaction cross sections are presented. Evaluated cross-section and angular dependence files in ENDF format were prepared for the twelve reactions p7Li, n7Be, d6Li → α4He, p7Li, n7Be, d6Li. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper reports the results of a collaborative effort between CEA of France and the DOE of the United States (in particular between le Laboratoire d’Etudes de Physique de Cadarache and the Nuclear Data Group at Oak Ridge National Laboratory): In preparing input for analyses of differential nuclear data using multilevel multi-channel R-matrix theory, a sometimes daunting and often error-prone task is the generation of quantum-number information for all channels for each compound nuclear state (i.e., for each “spin group,” defined by quantum numbers Jπ). For many years, the code SAMQUA has been available to users of the R-matrix code SAMMY to assist in preparation of that input; the original SAMQUA code, however, was limited to single-channel spin group information. In this paper, an improved version of the SAMQUA code is described. The new SAMQUA permits inclusion of all open reaction channels in the low-energy interaction between one particle (neutron or charged particle) and a nuclear target, and considerably simplifies the determination of the quantum numbers needed for the definition of the reaction channels. SAMQUA, in addition to its primary function of preparing quantum numbers for the SAMMY input file, also provides the possibility to visualize immediately all open reaction channels. This paper gives two examples of the use of SAMQUA, with emphasis on the notions of reaction channels and penetrability. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

R-matrix formulas are used to calculate di-proton decay widths of 18Ne, 45Fe and 48Ni. This formulation allows one to relate the decay amplitude to the correlated many-body wave functions for these nuclei. The role of pairing correlations is discussed. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]

The polarization of Balmer-α radiation excited in collisions of electrons with atomic hydrogen is presented for an electron energy range from threshold to 1000 eV. Measurements are in good agreement with calculations carried out using either convergent-close-coupling (CCC) or R-Matrix with pseudo-states (RMPS) approaches. Cascade is demonstrated to have a significant effect. Balmer-α excitation function data are discussed and H(n=31) data are presented. A previous measurement of the polarization of Balmer-α following dissociative excitation of H[sub 2] by electrons is confirmed and extended. [ABSTRACT FROM AUTHOR]

Two approaches based on the R-matrix method to study electron impact ionization at intermediate incident electron energies are discussed. These are the Rmatrix with pseudo-states approach (RMPS) and the intermediate energy R-matrix method (IERM). Recent results to illustrate the key features of these methods are presented. [ABSTRACT FROM AUTHOR]

We report the recent results of R-matrix analyses of light systems directed at obtaining capture cross sections for reactions of interest in nuclear astrophysics. The two examples discussed are [sup 1]H(n,γ)[sup 2]H and [sup 12]C(α,γ)[sup 16]O. The first reaction is important in Big-Bang nucleosynthesis at energies well up into hundreds of keV. The second reaction is crucial for determining the relative abundances of carbon and oxygen at the end of the helium-burning phase, but at energies below the range of current measurements. In both cases, the direct measurements are rather sparse and sometimes difficult to interpret because of level-structure uncertainties. However, the analyses we will describe make use of auxiliary data to determine the cross sections over wide ranges of energies, including those of astrophysical interest. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Multiphoton ionization rates have been calculated for He at the KrF laser wavelength of 248 nm and for intensities up to 6×10[sup 14] W cm[sup -2] using the R-matrix Floquet method. For intensities above that at which channel closing takes place it is found that intermediate five-photon resonances with bound states strongly influence the total rate. In particular the resonance between the ground state and the 1s2p [sup 1]P[sup 0] state is found to occur at an intensity of 5×10[sup 14] W cm[sup -2]. This resonance is investigated at several neighboring wavelengths and its prominent nature, which is discussed in terms of a proposed nearby laser-induced degenerate state (LIDS), should enable it to be observed experimentally. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A unified R-matrix Floquet theory of multiphoton processes is introduced which can be used to analyze both multiphoton ionization of diatomic molecules and laser-assisted electron diatomic molecules scattering. The laser field is assumed to be monochromatic, monomode, spacially homogeneous and linearly polarized, where the molecular axis can be oriented in an arbitrary direction relative to the polarization direction. Multi-center electron-molecule scattering programs have been modified in a straightforward way to obtain the solution in the internal region and single-center atomic multiphoton propagator programs have been extended to solve the problem in the external region. The theory is illustrated by obtaining multiphoton ionization rates for a model H[sub 2] problem which enables the theory and computer programs to be tested by comparing the results against a completely independent atomic R-matrix Floquet program. The program is then used to obtain one- and two-photon ionization rates for H[sub 2] using an accurate wave function for H[sub 2]. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The absolute photoionization cross section for the astrophysically important C[sup +] ion has been measured for the first time thus allowing a test of the predicted cross section from the Opacity Project. The measurements are performed with a new ion-photon merged beam setup at the ASTRID storage ring utilizing an undulator beam line. In addition to the predicted 2s2p([sup 3]P)np [sup 2]D and [sup 2]S autoionizing resonances, the [sup 2]P states which are not included in the theoretical predictions also contribute significantly to the ionization yield below the 2s2p([sup 3]P) limit. The cross section is determined with a precision of 10% and lies 5–25% below the theoretical prediction at 24–37 eV. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A quantitative analysis of part of the experimental data reported by Wang, Chen and Elliott [1,3] who studied in barium coherent control through two-color resonant interfering paths is reported. Dynamics of the two-color photoionization process, described as an adiabatic process in the rotating wave approximation, is governed by the coherent excitation of the 6s6p and 6s7p [sup 1]P[sub 1] intermediate states. Interference effects are found to play a minor role. The required atomic parameters are obtained from a theoretical approach based on a combination of jj-coupled eigenchannel R-matrix and Multichannel Quantum Defect Theory. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]