The influence of fragmentation models in the production of hadron jets in electron–positron annihilation

The analysis of electron–positron annihilations to hadrons at high energies shows that apart from two-jet events, there are also signs of three-jet events which are interpreted according to the QCD, as a gluon radiated by a quark. In this paper, we investigate the fragmentation of quarks and gluons to hadron jets. We show that gluon jets have a higher multiplicity compared to quark jets of the same energy. Furthermore, inclusion of different flavours in the distributions shows that quark jets are flavour-dependent, but gluon jets are not. The differences between quark and gluon jets also manifest themselves in the fragmentation functions. We observe that the fragmentation for gluon jet is softer than that for quark jet, because the radiation of soft gluons is larger for gluon jets and that gluon cannot be present as a valence parton inside a produced hadron. We provide possible explanations for these features in this paper.     

Behaviour of vacancies in dilute Fe–Re alloys: a positron annihilation study

Positron annihilation lifetime spectroscopy was used in a room temperature study of the influence of heat treatment on behaviour of vacancies in Fe_0.97Re_0.03 and Fe_0.94Re_0.06 alloys. In this experiment, the vacancies were created during the formation and further mechanical processing of the iron systems under consideration so the lifetime spectra of positrons were collected at least twice. The first samples were taken just after the melting process in an arc furnace, and the second ones were taken for the specimens annealed at 1,270 K and then cold-rolled at room temperature. After that, the spectra were measured for all studied samples after annealing at some temperatures gradually increasing from 300 to 1,270 K. It was found that vacancy-Re pairs are the dominant type of structural defects in alloys just after the melting process. In the case of alloys after a cold rolling process, the dominant type of structural defects is vacancies associated with edge dislocations. Moreover, for cold-rolled samples annealed at 473–573 K, the growth of the vacancy clusters associated with edge dislocations is observed by an increase in the mean positron lifetime. Finally, at temperatures above 573 K, vacancy clusters associated with edge dislocations as well as vacancy-Re pairs become unstable, and freely migrating vacancies sink at grain boundaries.     

Jet algorithms in electron–positron annihilation: perturbative higher order predictions

This article gives results on several jet algorithms in electron–positron annihilation: Considered are the exclusive sequential recombination algorithms Durham, Geneva, Jade-E0 and Cambridge, which are typically used in electron–positron annihilation. In addition also inclusive jet algorithms are studied. Results are provided for the inclusive sequential recombination algorithms Durham, Aachen and anti-k _t , as well as the infrared-safe cone algorithm SISCone. The results are obtained in perturbative QCD and are N³LO for the two-jet rates, NNLO for the three-jet rates, NLO for the four-jet rates and LO for the five-jet rates.     

Diffusion of Ta in α-Ti

The diffusion of Ta in the hcp (α) phase of high-purity Ti (99.99%) was studied at different temperatures from 911 K up to 1123 K. The Rutherford Backscattering Spectrometry (RBS) and Heavy Ion RBS (HIRBS) techniques were used to obtain the penetration profiles. The evolution of the diffusion coefficient, D, as a function of temperature follows prediction of the Arrhenius law. The activation energy of the diffusion process is (318±7)kJ/mol, similar to that corresponding to self-diffusion in α-Ti. On the other hand, the measured values of D are systematically lower than those corresponding to self-diffusion by a factor of approximately 5. This reduction could be explained by taking into account the mass difference between Ta and Ti. An increase of the diffusion coefficient was measured when the diffusion proceeds on a less pure Ti (99.9%) matrix. This increment is higher at lower temperatures. Received: 12 November 2001 / Accepted: 12 March 2002 / Published online: 5 July 2002 RID="*" ID="*" RID="*" ID="*" RID="**" ID="**"Corresponding author. Fax: +54-11/6772-7362, E-mail: dyment@cnea.gov.ar RID="*" ID="*"Members of the Carrera del Investigador Científico del Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina     

Microstructural characterisation of a commercial Al–Cu–Mg alloy combining transmission electron microscopy and positron annihilation spectroscopy

Transmission electron microscopy and positron annihilation spectroscopy techniques were used to characterise the microstructure of the 2024 Al–Cu–Mg commercial alloy artificially aged with and without pre-deformation. In non-deformed samples, a very dense dispersion of small, needle-shaped particles, with mean size in the order of the nanometres, was observed homogeneously distributed in the matrix, together with a coarse distribution of S-phase precipitates. In pre-deformed samples, the needle-shape particles were not seen, only a high density of S-phase precipitates nucleated on dislocations. The needle-shaped particles were identified as solute-rich Guinier–Preston–Bagaryatski (GPB) zones by combining coincidence Doppler broadening positron annihilation measurements with TEM imaging. The relationship between the microstructure and measurements of hardness and positron lifetime evolution during artificial ageing is also discussed.     

Study of radiation-induced damage to type III–V semiconductor compounds irradiated with γ quanta and protons using positron annihilation spectroscopy

The effect of proton and ?? radiation on characteristics of the spectra for the angular distribution of annihilation photons (ADAP) have been studied in the case of positron annihilation in GaAs and GaP single crystals. Relative variations in defect accumulation and annealing under irradiation and subsequent isochronous annealing of the samples have been studied using variations in the basic parameters of the ADAP spectra. In both cases (GaAs and GaP), the variations in the ADAP spectral parameters as functions of the annealing temperature have a steplike character, which is interpreted as the formation of a certain type of defects with different annealing activation energies.     

Annealing of deffects in γ-irradiated LiF studied by positron annihilation

Positron lifetime spectra and Doppler broadened spectra have been measured for LiF single crystals gamma-irradiated at room temperature. Isothermal and isochronal annealings of the samples have been performed in a temperature range of 20–600°C.     

High-resolution electron microscopy and X-ray diffraction study of intergrowth structures in α- and β-type YbAlB4 single crystals

Structural features of layered boride YbAlB₄ single crystals with YCrB₄-type (α-type) and ThMoB₄-type (β-type) phases derived from a hexagonal AlB₂-type structure were investigated by electron diffraction, high-resolution electron microscopy and X-ray diffraction. X-ray diffraction experiments indicate the existence of some structural motifs. High-resolution images clearly show that the structural motifs build the intergrown lamellar structures in the matrix. The lamellar structures can be characterized by a coherent tiling of deformed Yb hexagons, which are a common structure unit in the α- and β-type structures. The characteristic intergrown nanostructure is similar to that observed in the β-type TmAlB₄ polycrystalline sample.     

Experimental determination of U diffusion in α-Ti

α-spectrometry was used in order to measure the diffusion of U in bulk α-Ti in the temperature range 863–1123?K (540–850?°C). A straight Arrhenius plot was found, giving diffusion parameters Q?=?297?kJ/mol and D ₀?=?5?×?10^?3?m²/s, which are similar to the α-Ti self-diffusion ones, when measured in Ti samples with a similar impurity content than presently. This behaviour is compatible with the hypothesis of U diffusing via a vacancy-assisted mechanism in the α-Ti lattice and contrasts with older results in which the activation energy is almost a third the self-diffusion one, even lower than the vacancy formation energy.     

Study of strange charmed baryons in electron–positron collisions

A modern classification of charmed baryons is presented, and the quark model is briefly described for the ground states and energy levels of excited states. In addition, a survey of experimentally observed states of strange charmed baryons is given.     

Binding enthalpies of nitrogen-vacancy complexes in irradiated α-Fe

Abstract

Dilute iron-nitrogen alloys have been irradiated at low temperatures with electrons or neutrons to varying doses. In the temperature range above Stage III (T  200 K) a previously unknown family of small clusters composed of N interstitials and radiation-induced point defects, i.e. vacancies, could be resolved by means of a sensitive magnetic after-effect technique. Analysis of formation and dissociation kinetics of these clusters leads to nitrogen-vacancy binding energies between 0.08 and 0.5 eV, depending on cluster size. These relatively low values, together with further details of the precipitation kinetics, are shown to support the assumption of a two-interstitial recovery model.     

Study of ageing in Al–Mg–Si alloys by positron annihilation spectroscopy

In many common Al–Mg–Si alloys (6000 series) intermediate storage at or near ‘room temperature’ after solutionising leads to pronounced changes of the precipitation kinetics during the ensuing artificial ageing step at ≈180 °C. This is not only an annoyance in production, but also a challenge for researchers. We studied the kinetics of natural ‘room temperature’ ageing (NA) in Al–Mg–Si alloys by means of various different techniques, namely electrical resistivity and hardness measurement, thermoanalysis and positron lifetime and Doppler broadening (DB) spectroscopy to identify the stages in which the negative effect of NA on artificial ageing might appear. Positron lifetime measurements were carried out in a fast mode, allowing us to measure average lifetimes in below 1 min. DB measurements were carried out with a single detector and a ⁶⁸Ge positron source by employing high momentum analysis. The various measurements show that NA is much more complex than anticipated and at least four different stages can be distinguished. The nature of these stages cannot be given with certainty, but a possible sequence includes vacancy diffusion to individual solute atoms, nucleation of solute clusters, Mg agglomeration to clusters and coarsening or ordering of such clusters. Positron lifetime measurements after more complex ageing treatments involving storage at 0 °C, 20 °C and 180 °C have also been carried out and help to understand the mechanisms involved.     

Axially magnetized electron–positron and electron plasma competition on the self focusing of intense laser beam

The spot-size evolution of circularly polarized intense laser beam propagating through the axially magnetized electron–positron (EP) and electron plasmas is discussed, in mildly relativistic and weakly non-linear (a² ? 1) regime. The non-linear current density source terms are obtained by making used of the perturbative technique. The variational principle approach method is applied to the solution of the non-linear Schrodinger wave equation. It is shown that the laser beam spot size decreases for the left and increases for the right handed polarized beams with increasing the external magnetic field, owing to the beam passages inside the electron plasma. Furthermore, it is revealed that the self focusing property strongly enhanced in the EP plasma in comparison to the electron plasma. Moreover, self focusing of linearly polarized laser beam is investigated for EP plasma by superposition of the right and left circularly polarized beams.     

Anisotropy of optical absorption of α-MnS single crystal

The optical absorption spectra of an α-MnS single crystal and their temperature behavior in the range from 86 to 300 K are investigated for the (100) plane in the energy range from 8×10³ to 22×10³ cm^?1 for the first time. Comparison of these spectra with those for the (111) plane reveals an essential absorption anisotropy in unpolarized light. The anisotropy is manifested in a much stronger splitting of the lowest energy band for the (100) plane in comparison with that for the (111) plane. With decreasing temperature, the splitting becomes smaller. Possible mechanisms for the anisotropy revealed are proposed.     

Point defects patterning in irradiated α-zirconium: numerical study in the framework of the rate theory

We study point defects patterning in irradiated α-zirconium numerically. In our consideration, we exploit reaction-rate theory by taking into account sink density dynamics and a change in internal stress fields due to the presence of defects. Dynamics of defect populations are studied at different irradiation conditions. It is found that point defects patterning is accompanied by a formation of vacancy clusters; their morphology change is governed by irradiation temperature and damage rate. By using statistical analysis of spatially distributed vacancy clusters, it was shown that the characteristic size of these clusters is of several nanometers. Vacancy clusters' occupation densities and distributions over their sizes are studied in detail.