Cluster Model Parameters Research Articles

Clustering effect in description of the alpha and heavier decay half-life

We examine the alpha and heavier cluster decay half-lives of the nuclei for 87 ≤ Z ≤ 96 using the analytical formula obtained by the Wentzel-Kramers-Brilliouin (WKB) method within the framework of the cluster model for the modified harmonic oscillator and spherical Coulomb potential. We correlate the clustering effect in the nuclei with the nuclear potential depth only and systematically investigate the alpha and heavier cluster decay half-lives. In addition to half-lives, the branching ratios relative to alpha decay half-lives are also calculated and results are compared with experimental data. The rms deviations of the alpha and cluster decay half-lives show a good agreement with the universal decay law (UDL), Horoi and one single line of universal (UNIV) curve models. Utilizing the optimized cluster model parameters, we also predict the half-lives of 14C, 20O, 24,25Ne, 28,30Mg and 32Si cluster emissions from various isotopes for guiding future experiments.

Dynamic Control of Mobile Multirobot Systems: The Cluster Space Formulation

The formation control technique called cluster space control promotes simplified specification and monitoring of the motion of mobile multirobot systems of limited size. Previous paper has established the conceptual foundation of this approach and has experimentally verified and validated its use for various systems implementing kinematic controllers. In this paper, we briefly review the definition of the cluster space framework and introduce a new cluster space dynamic model. This model represents the dynamics of the formation as a whole as a function of the dynamics of the member robots. Given this model, generalized cluster space forces can be applied to the formation, and a Jacobian transpose controller can be implemented to transform cluster space compensation forces into robot-level forces to be applied to the robots in the formation. Then, a nonlinear model-based partition controller is proposed. This controller cancels out the formation dynamics and effectively decouples the cluster space variables. Computer simulations and experimental results using three autonomous surface vessels and four land rovers show the effectiveness of the approach. Finally, sensitivity to errors in the estimation of cluster model parameters is analyzed.

Hierarchical kernel spectral clustering

Kernel spectral clustering fits in a constrained optimization framework where the primal problem is expressed in terms of high-dimensional feature maps and the dual problem is expressed in terms of kernel evaluations. An eigenvalue problem is solved at the training stage and projections onto the eigenvectors constitute the clustering model. The formulation allows out-of-sample extensions which are useful for model selection in a learning setting. In this work, we propose a methodology to reveal the hierarchical structure present on the data. During the model selection stage, several clustering model parameters leading to good clusterings can be found. These results are then combined to display the underlying cluster hierarchies where the optimal depth of the tree is automatically determined. Simulations with toy data and real-life problems show the benefits of the proposed approach.

Clustering Pre-equilibrium Model Analysis for Nucleon-induced Alpha-particle Spectra up to 200 MeV

The clustering exciton model of Iwamoto and Harada is applied to the analy- sis of pre-equilibrium (N; x ) energy spectra for medium-to-heavy nuclei up to 200 MeV. In this work, we calculate alpha-particle formation factors without any approximations that appear in the original model. The clustering process is also considered in both the primary and second pre-equilibrium emissions. We optimize the exciton and the cluster- ing model parameters simultaneously by looking at the experimental (N; xN) and (N; x ) energy spectra. The experimental alpha-particle spectra are well reproduced with a unique set of clustering model parameters, which is independent of incident neutrons/protons. The present analysis also implies that the clustering model parameter is not so di erent between the medium and heavy nuclei. Our calculations reproduce experimental data gen- erally well up to the incident energy of 150 MeV, but underestimations are seen above this energy. The exciton model, as it is frequently used in the nuclear data evaluation (1), predicts pre-equilibrium (N; xN) spectra generally well. However, a long-standing problem exists in the model calculation of the pre-equilibrium composite-particle spectra due to rather complicated reaction mechanism. The phenomenological models proposed by Kalbach (2,3) describe nucleon-transfer reaction process in the phase-space, in which many adjustable parameters are involved to fit experimental particle energy- spectra. Although a useful global parameterization is reported (2,3), extrapolation of the global pa- rameters beyond the experimental range needs much attention. We apply the clustering exciton model of Iwamoto and Harada (4) to calculate nucleon-induced pre-equilibrium alpha-particle spectra from medium-to-heavy nuclei. The model describes the pickup process within the exciton model framework, where the alpha-particle formation factor is calculated by the overlap integral of wave functions between the alpha-particle and four nucleons (include the nu- cleons below Fermi level). In this work, the formation factors are calculated by numerical integrations, while the original Iwamoto-Harada model employed the root-mean-square approximation (4) where no correlation exists between the coordinates in the phase-space. The clustering process is also con- sidered in both the primary and second pre-equilibrium emissions. We optimize the pre-equilibrium and the clustering model parameters simultaneously by looking at the experimental nucleon and alpha- particle energy spectra. In this paper, di erences between the exact calculation and the approximation are exhibited for the alpha-particle formation factor. Through the comparisons with experimental spectra at incident energies up to 200 MeV, we show the behavior of the clustering model parameter, the applicability and limitation of the model.

A simulator for adaptive parallel applications

Dynamically allocating computing nodes to parallel applications is a promising technique for improving the utilization of cluster resources. Detailed simulations can help identify allocation strategies and problem decomposition parameters that increase the efficiency of parallel applications. We describe a simulation framework supporting dynamic node allocation which, given a simple cluster model, predicts the running time of parallel applications taking CPU and network sharing into account. Simulations can be carried out without needing to modify the application code. Thanks to partial direct execution, simulation times and memory requirements are reduced. In partial direct execution simulations, the application's parallel behavior is retrieved via direct execution, and the duration of individual operations is obtained from a performance prediction model or from prior measurements. Simulations may then vary cluster model parameters, operation durations and problem decomposition parameters to analyze their impact on the application performance and identify the limiting factors. We implemented the proposed techniques by adding direct execution simulation capabilities to the Dynamic Parallel Schedules parallelization framework. We introduce the concept of dynamic efficiency to express the resource utilization efficiency as a function of time. We verify the accuracy of our simulator by comparing the effective running time, respectively the dynamic efficiency, of parallel program executions with the running time, respectively the dynamic efficiency, predicted by the simulator under different parallelization and dynamic node allocation strategies.

Estimation of time of arrival of UWB multipath clusters through a spatial correlation technique

This study estimates the time of arrival (TOA) of ultra wideband multipath components (MPCs) by analysing the spatial correlation of multipath signals captured by a 7×7 antenna array. In this study, both line-of-sight and non-line-of-sight radio propagation paths were considered. Each power-delay profile bin from all the array positions was used to calculate the spatial correlation between itself and the adjacent receiving point bins. Consequently, some MPC time delays and clusters, associated with high correlation values, were selected and compared with the TOA computed by the Sensor-CLEAN algorithm. A close agreement of the TOA estimated by both techniques was obtained. Finally, multipath cluster distributions and clustering model parameters are presented and compared with previously published results.

Breaking Cosmological Degeneracies in Galaxy Cluster Surveys with a Physical Model of Cluster Structure

Forthcoming large galaxy cluster surveys will yield tight constraints on cosmological models. It has been shown that in an idealized survey, containing > 10,000 clusters, statistical errors on dark energy and other cosmological parameters will be at the percent level. It has also been shown that through parameters describing the mass-observable relation and cosmology can be simultaneously determined, though at a loss in accuracy by about an order of magnitude. Here we examine the utility of an alternative approach of self-calibration, in which a parametrized ab-initio physical model is used to compute cluster structure and the resulting mass-observable relations. As an example, we use a modified-entropy (pre-heating) model of the intracluster medium, with the history and magnitude of entropy injection as unknown input parameters. Using a Fisher matrix approach, we evaluate the expected simultaneous statistical errors on cosmological and cluster model parameters. We study two types of surveys, in which a comparable number of clusters are identified either through their X-ray emission or through their integrated Sunyaev-Zel'dovich (SZ) effect. We find that compared to a phenomenological parametrization of the mass-observable relation, using our physical model yields significantly tighter constraints in both surveys, and offers substantially improved synergy when the two surveys are combined. These results suggest that parametrized physical models of cluster structure will be useful when extracting cosmological constraints from SZ and X-ray cluster surveys. (abridged)

Chemical bond and hybridization in nickel compounds

By means of ultraviolet reflectance and valence band photoemission one can get a scale of ionicity for nickel halides. The fraction of ionic character of the chemical bond, from 0.72 in ${\mathrm{NiI}}_{2}$ to 0.80 in ${\mathrm{NiCl}}_{2},$ evaluated with the Phillips theory has been related to the ground state d electron counts $〈{n}_{\mathrm{dn}}〉$ calculated from $\mathrm{Ni}2p$ core photoemission. While the chemical trends of cluster model parameters are in general well reproduced, a systematic discrepancy in band gap values (2--3 eV) has been found by comparing core-level results with optical and photoconductivity data.

Cluster Model Description of Polarized Cu K-Edge Spectra of Nd2-xCexCuO4-δ

Single crystals Nd 2-x Ce x CuO 4-δ with x=0.0 and 0.2 are studied by polarized X-ray absorption measurements at Cu K-edge. The positions and intensities of the 1s→ 4pσ transition features in the K-edge spectra are analyzed with a modified cluster model and functional curve fitting procedure. All of the experimental results including the energies of the spectral features, the energy separations between features and the variation of feature intensities, can be well explained by the cluster model parameters obtained from the curve fitting. Particularly, it is found that the charge transfer energy gap Δ in the Nd 2-x Ce x CuO 4-δ system decreases from about 1.4 eV at x=0 to 0.5 eV at x = 0.2. Our analysis in this paper provides a method for quantitative X-ray absorption spectroscopy (XAS) study of the charge transfer gap for electron-doped cuprate superconductors and related materials.

Dielectric response and a.c. conductivity of synthetic dopa-melanin polymer

The dielectric spectra of dopa-melanin have been examined over the wide frequency range 100 Hz–1000 MHz at room temperature. Synthetic dopa-melanin was obtained by oxidative polymerization of L-3,4-dihydroxyphenylalanine. The purpose of the present study was to characterize polarization effects, the electrical conduction mechanism and structural arrangement of the units taking part in these processes as a basis for future biological applications. Real and imaginary components of the dielectric permittivity show an anomalous low-frequency dispersion. Values of the power-law exponents were interpreted in terms of the cluster model of potentially mobile charges and the structural arrangement of molecular units in melanin. The fractal circuit model (self-similar branched porous electrode system) was found to be equivalent to the cluster model in the description of the dielectric response in melanin. It has been shown that relations between cluster model parameters p and n, and fractal dimensionalities of melanin, are fulfilled very well. The presented results support the earlier findings based on d.c. conductivity measurements, that charge hopping is the main conduction mechanism, which contributes to the dielectric polarization in the low-frequency region. Dielectric spectra at frequencies above 10MHz show small β-relaxation features due to molecular polar segments and water molecules bound to melanin. © 1998 Kluwer Academic Publishers

The effect of clustering on the equivalent width distribution of QSO Lyman-alpha clouds

A significant discrepancy between the observed equivalent width distribution p(W) of Lyman α absorption features in intermediate-resolution QSO spectra and that expected from high-resolution spectra is reported. It is found that, if clouds are randomly distributed, line blending cannot account for this effect. The difference is interpreted as due to enhanced line blending caused by cloud-cloud clustering. Using a clustering model for the Lyman α clouds, it is shown how the apparent p(W) depends on clustering properties and an estimate of the clustering model parameters is derived by comparing the simulated p(W) to that of the real data

The electronic structure of the [CuO5]8? unit: A photoemission study of Y2BaCuO5

A photoemission study of the ‘green phase’ Y2BaCuO5, including core level spectra of the elements, CuL3VV Auger, and XPS and UPS valence band spectra has been performed. The crystal structure of Y2BaCuO5 contains discrete [CuO5]8− units with divalent copper. The surface status of the Y2BaCuO5 samples is discussed with respect to the problems encountered in photoemission studies of the YBaCuO superconductors. The photoemission data are analysed within a simple cluster configuration interaction model by considering in addition previously reported optical spectra. Y2BaCuO5 is a charge transfer insulator with the charge transfer energy Δ≈0.5 eV being much smaller than thed−d Coulomb interaction energyU≈6eV. Cluster model parameters and valence band shapes of Y2BaCuO5 are compared with those of Nd2CuO4 the structure of which contains divalent copper within an extended copper-oxygen network. More detailed cluster and impurity models for cuprates described in literature are discussed in view of the present results.

The fractal nature of the cluster model dielectric response functions

Calculable fractal circuit models are used to show that the cluster model response functions result from the combination of two types of self-similarity. The analysis is extended to the molecular scale where the cluster model is seen to be based on sequential relaxation processes. An outline is given of the physical origin for such behavior, and the self-similar processes are identified with the basic concepts of (i) an efficient (compact) exploration of a fractal lattice and (ii) self-similarity in the contacts between internally connected regions (clusters). The relationship of the cluster model parameters n and m to system dimensionalities are derived for a number of cases.

Vibronic and impurity-induced Raman scattering from a Jahn-Teller—distorted impurity

Raman scattering transitions between the hindered-rotational levels of the vibronically coupled $^{2}E_{g}$ ground state of ${\mathrm{Cu}}^{2+}$ in CaO have been studied as a function of temperature and applied uniaxial strain. The results are compared with a cluster model for the vibronic interaction, and the energies of the strain-split states fit the model well. The intensities of the transitions are calculated from the cluster-model vibronic wave functions. Only two parameters are available to fit the intensities of all observed levels as a function of applied strain. These intensity measurements are therefore a rigorous test of the accuracy of the vibronic wave functions. Good agreement is obtained for transitions observed in one of the two pertinent polarization geometries, but poor agreement is observed for the second geometry. These results are not fully understood, but they may be due simply to the inaccuracy of the cluster-model eigenfunctions. The impurity-induced Raman scattering from the vibrations of the host lattice is compared with calculations based on the shell-model lattice-dynamics eigenvectors for CaO. It is found that substantial coupling to the second-neighbor ${\mathrm{Ca}}^{2+}$ ions and the oxygen polarization are required in addition to the nearest-neighbor (oxygen) core coupling to explain the observed spectra. This apparent complexity of the electron-phonon interaction is the most likely explanation for the observation that the measured strain coupling coefficient for the ${\mathrm{Cu}}^{2+}$ ground state is a factor of 2.5 too small to be consistent with the other cluster-model parameters deduced from the fits to the vibronic levels.

Application of cluster model wave functions to the Coulomb break-up of accelerated 6Li ions

The total cross section for the Coulomb disintegration of accelerated 6Li ions into alpha particles and deuterons in the Coulomb field of a heavy target nucleus is studied as a function of the cluster model parameters and of the bombarding energy. It is found that the magnitude of the cross section decisively depends on the degree of cluster overlap in the wave functions. Despite the approximations and uncertainties involved, it is concluded that such investigations might be a useful tool for studying cluster effects in light nuclei. The need for further measurements is stressed.