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Puerto Serrano, Spain

Kossert K.,Physikalisch - Technische Bundesanstalt | Grau Carles A.,Institute Fisica Fundamental Csic
Applied Radiation and Isotopes | Year: 2010

The methods to compute the counting efficiency of electron-capture nuclides in liquid scintillation counting have been improved in several previous studies. The main improvements comprise a more realistic treatment of the ejection of photoelectrons and subsequent rearrangement processes in the atomic shell as well as a more detailed atomic rearrangement model. The latter was realized in the MICELLE code by means of a new stochastic approach. This new model was also developed to account for energy deposits within micelles.The recent improvements have now been combined in an updated version of the MICELLE code, which also makes the computation of the counting efficiency of complex decay schemes possible.In this paper, we describe and discuss recent extensions and improvements of the models and further corrections. The calculated counting efficiencies of selected radionuclides are compared with the experimental data obtained by liquid scintillation counting. For the measurements, we use standard solutions, which were calibrated by other methods. © 2009 Elsevier Ltd.

De Lara-Castells M.P.,Institute Fisica Fundamental Csic | Stoll H.,University of Stuttgart | Mitrushchenkov A.O.,University Paris Est Creteil
Journal of Physical Chemistry A | Year: 2014

As a prototypical dispersion-dominated physisorption problem, we analyze here the performance of dispersionless and dispersion-accounting methodologies on the helium interaction with cluster models of the TiO2(110) surface. A special focus has been given to the dispersionless density functional dlDF and the dlDF+Das construction for the total interaction energy (K. Pernal, R. Podeswa, K. Patkowski, and K. Szalewicz, Phys. Rev. Lett. 2009, 109, 263201), where Das is an effective interatomic pairwise functional form for the dispersion. Likewise, the performance of symmetry-adapted perturbation theory (SAPT) method is evaluated, where the interacting monomers are described by density functional theory (DFT) with the dlDF, PBE, and PBE0 functionals. Our benchmarks include CCSD(T)-F12b calculations and comparative analysis on the nuclear bound states supported by the He-cluster potentials. Moreover, intra- and intermonomer correlation contributions to the physisorption interaction are analyzed through the method of increments (H. Stoll, J. Chem. Phys. 1992, 97, 8449) at the CCSD(T) level of theory. This method is further applied in conjunction with a partitioning of the Hartree-Fock interaction energy to estimate individual interaction energy components, comparing them with those obtained using the different SAPT(DFT) approaches. The cluster size evolution of dispersionless and dispersion-accounting energy components is then discussed, revealing the reduced role of the dispersionless interaction and intramonomer correlation when the extended nature of the surface is better accounted for. On the contrary, both post-Hartree-Fock and SAPT(DFT) results clearly demonstrate the high-transferability character of the effective pairwise dispersion interaction whatever the cluster model is. Our contribution also illustrates how the method of increments can be used as a valuable tool not only to achieve the accuracy of CCSD(T) calculations using large cluster models but also to evaluate the performance of SAPT(DFT) methods for the physically well-defined contributions to the total interaction energy. Overall, our work indicates the excellent performance of a dlDF+Das approach in which the parameters are optimized using the smallest cluster model of the target surface to treat van der Waals adsorbate-surface interactions. © 2014 American Chemical Society.

De Lara-Castells M.P.,Institute Fisica Fundamental Csic | Mitrushchenkov A.O.,University Paris Est Creteil
Journal of Physical Chemistry C | Year: 2011

An accurate finite cluster approach based on localized Wannier orbitals is applied here to bulk TiO2 as a prototypical extended transition metal oxide. The quasi-particle band structure, an extended property of the infinite solid, is successfully reproduced within a finite local cluster approach at the Hartree-Fock level. Post-Hartree-Fock calculations on selected clusters are then performed to account for correlation effects. The computational implementation, which is a modified version of the "cluster-in-solid" embedding technique (Birkenheuer, U., Fulde, P.; Stoll, H. Theor. Chem. Acc.2006, 116, 398) along with the frozen local hole approximation (Pahl, E.; Birkenheuer, U. J. Chem. Phys.2006, 124 214101), is described and possible further extensions to calculate local defects in bulk materials and excited states in adsorbate/metal oxide surfaces are discussed. © 2011 American Chemical Society.

Honvault P.,University of Burgundy | Honvault P.,University of Franche Comte | Jorfi M.,University of Le Havre | Gonzalez-Lezana T.,Institute Fisica Fundamental Csic | And 2 more authors.
Physical Chemistry Chemical Physics | Year: 2011

Ortho-para H 2 conversion reactions mediated by the exchange of a H + proton have been investigated at very low energy for the first time by means of a time independent quantum mechanical (TIQM) approach. State-to-state probabilities and cross sections for H + + H 2 (v = 0, j = 0,1) processes have been calculated for a collision energy, E c, ranging between 10 -6 eV and 0.1 eV. Differential cross sections (DCSs) for H + + H 2 (v = 0, j = 1) → H + + H 2 (v′ = 0, j′ = 0) for very low energies only start to develop a proper global minimum around the sideways scattering direction ( ≈ 90°) at E c = 10 -3 eV. Rate coefficients, a crucial information required for astrophysical models, are provided between 10 K and 100 K. The relaxation ortho-para process j = 1 → j′ = 0 is found to be more efficient than the j = 0 → j′ = 1 conversion at low temperatures, in line with the extremely small ratio between the ortho and para species of molecular hydrogen predicted at the temperature of interstellar cold molecular clouds. The results obtained by means of a statistical quantum mechanical (SQM) model, which has previously proved to provide an adequate description of the dynamics of the title reactions at a higher collision energy regime, have been compared with the TIQM results. A reasonable good agreement has been found with the only exception of the DCSs for the H + + H 2 (v = 0, j = 1) → H + + H 2 (v′ = 0, j′ = 0) process at very low energy. SQM cross sections are also slightly below the quantum results. Estimates for the rate coefficients, in good accord with the TIQM values, are a clear improvement with respect to pioneering statistical studies on the reaction. © the Owner Societies 2011.

Kossert K.,Physikalisch - Technische Bundesanstalt | Nahle O.J.,Physikalisch - Technische Bundesanstalt | Carles A.G.,Institute Fisica Fundamental Csic
Applied Radiation and Isotopes | Year: 2011

The Physikalisch-Technische Bundesanstalt (PTB) investigated the low-energy beta emitter 241Pu within the scope of an international key comparison on the activity concentration of the same solution. The activity concentration was measured by means of liquid scintillation counters with two and three photomultiplier tubes (PMT). The counting efficiencies were determined with two established techniques, which are based on a free parameter model. The free parameter is determined via 3H-efficiency tracing in systems with two PMTs, or it is derived from the triple-to-double coincidence ratio (TDCR) in a system with three PMTs. Both methods require an accurate computation of the beta emission spectrum of the first-forbidden (non-unique) transition. In this work, the experimental outcome of a recent measurement from Loidl et al. (2010) with cryogenic magnetic calorimeters was used to determine a shape-factor function. The computed beta spectrum is in good agreement with the measured data when the shape-factor function C(W)=1-1.9582W+0.96078W2 and an end-point energy EΒ,max=21.6keV are used.The activity concentrations determined with the two methods agree well when using the new shape-factor function, whereas a considerable discrepancy is found when assuming C(W)=1, as for an allowed beta transition. Consequently, the difference between the efficiency tracing method and the TDCR method, as observed by other researchers, could be resolved. © 2011 Elsevier Ltd.

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