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Wyświetlanie 1-10 z 286
Tytuł:
Understanding electron-positron momentum densities in solids : effect of the positron distribution
Autorzy:
Rubaszek, A.
Powiązania:
https://bibliotekanauki.pl/articles/147180.pdf
Data publikacji:
2015
Wydawca:
Instytut Chemii i Techniki Jądrowej
Tematy:
positron annihilation
electronic band structure
positron states
Opis:
The influence of the positron distribution on the electron-positron (e-p) momentum densities (MDs) is studied in terms of the l (l = s, p, d, f) character of the initial electronic state. The effect of the positron is discussed for momenta in the extended and reduced zone scheme on the example of Al (simple metal), Cu (metal with almost filled d shell), and Cr (transition metal). Present result indicates a weak sensitivity of the e-p MD inside the central Fermi surface to the positron charge density for the delocalized s and p electrons. In the case of d states, the effect of the positron on the relevant contribution to the e-p MD directly reflects the degree of localization of d electrons in the electron density of states.
Źródło:
Nukleonika; 2015, 60, No. 4, part 1; 759-763
0029-5922
1508-5791
Pojawia się w:
Nukleonika
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Some remarks on positron/positronium diffusion models
Autorzy:
Świątkowski, W.
Powiązania:
https://bibliotekanauki.pl/articles/148428.pdf
Data publikacji:
2003
Wydawca:
Instytut Chemii i Techniki Jądrowej
Tematy:
diffusion
diffusion length
positron
positronium
Opis:
Positrons thermalized near the sample boundary (as well as Ps atoms formed in the sample) can reach the boundary as a result of diffusive movement. Observations of such effects as positron reemission or positron surface trapping as well as positronium emission allow one to calculate respective diffusion lengths. From the presented analysis it follows that some models used in such calculations give quite wrong results.
Źródło:
Nukleonika; 2003, 48, 3; 141-144
0029-5922
1508-5791
Pojawia się w:
Nukleonika
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Slow positron beam at the JINR, Dubna
Autorzy:
Horodek, P.
Kobets, A. G.
Meshkov, I. N.
Sidorin, A. A.
Orlov, O. S.
Powiązania:
https://bibliotekanauki.pl/articles/147134.pdf
Data publikacji:
2015
Wydawca:
Instytut Chemii i Techniki Jądrowej
Tematy:
positron beam
positron injector
Opis:
The Low Energy Positron Toroidal Accumulator (LEPTA) at the Joint Institute for Nuclear Research (JINR) proposed for generation of positronium in flight has been adopted for positron annihilation spectroscopy (PAS). The positron injector generates continuous slow positron beam with positron energy range between 50 eV and 35 keV. The radioactive 22Na isotope is used. In distinction to popular tungsten foil, here the solid neon is used as moderator. It allows to obtain the beam intensity of about 105 e+/s width energy spectrum characterized by full width at half maximum (FWHM) of 3.4 eV and a tail to lower energies of about 30 eV. The paper covers the characteristic of variable energy positron beam at the LEPTA facility: parameters, the rule of moderation, scheme of injector, and transportation of positrons into the sample chamber. Recent status of the project and its development in the field of PAS is discussed. As an example, the measurement of the positron diffusion length in pure iron is demonstrated.
Źródło:
Nukleonika; 2015, 60, No. 4, part 1; 725-728
0029-5922
1508-5791
Pojawia się w:
Nukleonika
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Positron annihilation in liquid crystals
Autorzy:
Dryzek, E.
Juszyńska-Gałązka, E.
Powiązania:
https://bibliotekanauki.pl/articles/148118.pdf
Data publikacji:
2015
Wydawca:
Instytut Chemii i Techniki Jądrowej
Tematy:
liquid crystals
positron annihilation
phase transitions
Opis:
Positron annihilation studies of liquid crystals are reviewed with particular reference to thermotropic liquid crystals with rod-like molecules. The studies of compounds exhibiting smectic A or smectic E phases indicate that local arrangement of dipole molecules play an important role because high electron density at the end group of molecules can influence substantially formation and annihilation of positronium. The obtained ortho-positronium lifetimes in these phases can be explained by antiparallel pairing of molecules in case of the smectic A phase or a structure with a nanosegregation of alkyl chains and others parts of molecules into sublayers and liquid-like state of alkyl chains in case of the smectic E phase.
Źródło:
Nukleonika; 2015, 60, No. 4, part 1; 703-708
0029-5922
1508-5791
Pojawia się w:
Nukleonika
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Gamma Induced Positron Annihilation: History, Current, and Future Developments
Autorzy:
Selim, F.
Powiązania:
https://bibliotekanauki.pl/articles/1030020.pdf
Data publikacji:
2017-11
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Tematy:
gamma induced positron spectroscopy
positron transient measurements
thermal neutron capture
proton capture
electron accelerator
Van de Graaff accelerator
Opis:
Positron annihilation spectroscopy is often performed using radioactive sources for bulk measurements or positron beams for depth resolved measurements. Both have many advantages and great capabilities for a variety of applications. In the recent history, we have shown that positron annihilation spectroscopy can be carried out directly using high energy photons without the need for positron source or positron beam. This approach brings unique capabilities for some specific applications and promotes the use of positron annihilation spectroscopy in new areas of materials science and probably in industrial applications. Some of the important applications include developing new nondestructive highly penetrating sensitive probe for structural and engineering materials. It can also greatly advance positron applications in bulk semiconductors, electronic and photonic materials as well as in polymers, ceramics, and liquids. The recently developed γ-induced positron spectroscopy in HZDR in Dresden provides an example of an excellent facility for many of these applications. When incorporated with pulsed accelerators, γ-induced positron annihilation spectroscopy may trigger novel studies of transient states in matter and explore several solid-state processes that take place on short time scale. In this article I will review the history and development of the technique and its incorporation in a wide range of accelerators including table top electron accelerators, pulsed electron accelerators, and Van de Graaff accelerators. Then I will introduce a design for a new γ-induced positron annihilation spectroscopy facility based on using small nuclear research reactors or neutron generators. The paper presents all the possible approaches for γ-induced positron annihilation spectroscopy and discusses its potential and limitations to guide the efforts in further development of the technique and illustrate the unique aspects that the technique can bring to positron science and applications.
Źródło:
Acta Physica Polonica A; 2017, 132, 5; 1450-1455
0587-4246
1898-794X
Pojawia się w:
Acta Physica Polonica A
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Principles of positron porosimetry
Autorzy:
Zaleski, R.
Powiązania:
https://bibliotekanauki.pl/articles/146520.pdf
Data publikacji:
2015
Wydawca:
Instytut Chemii i Techniki Jądrowej
Tematy:
porous materials
positron annihilation lifetime spectroscopy (PALS)
positronium
extended Tao-Eldrup model
Opis:
The paper deals with positron porosimetry (PP), which is based on positron annihilation lifetime spectroscopy (PALS). The numerical analysis of positron lifetime spectra for PP is more demanding than in most of other applications of PALS. The resulting intensity distributions of ortho-positronium (o-Ps) lifetimes are interpreted in terms of the extended Tao-Eldrup (ETE) model, which provides the dependence between the o-Ps lifetime and pore size. Additionally, the relation between the intensity of an o-Ps component and the pore volume allows obtaining pore size distribution (PSD). The value of the empirical parameter Δ, which is dependent on material, can be estimated from the temperature dependence of an o-Ps lifetime. The most unique feature of PP among other techniques that allow determination of PSDs is its ability to perform measurements in almost any conditions. This makes this method suitable for various in situ studies. In this review article, both the capabilities and the limitations of PP are discussed. The methods to overcome some of the limitations are presented.
Źródło:
Nukleonika; 2015, 60, No. 4, part 1; 795-800
0029-5922
1508-5791
Pojawia się w:
Nukleonika
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Momentum distributions in positron annihilation with tightly bound electrons in Al
Autorzy:
Boroński, E.
Powiązania:
https://bibliotekanauki.pl/articles/148016.pdf
Data publikacji:
2013
Wydawca:
Instytut Chemii i Techniki Jądrowej
Tematy:
positron annihilation
electronic structure
Opis:
An approach based on Carbotte, Salvadori and Chiba two-body schemes for calculation of electron-positron (e-p) momentum densities ro(p) for core electrons in solids is developed. The approach allows to avoid such common approximations as the local density approximation (LDA) etc. in determination of ro(p) of deeper atomic shells and reduces the contribution to ro(p) for higher momenta. Thus, the final calculations of the Doppler spectra of annihilation radiation yield lower values for greater momenta than those obtained within the LDA scheme. The exemplary calculations for Al are presented.
Źródło:
Nukleonika; 2013, 58, 1; 211-214
0029-5922
1508-5791
Pojawia się w:
Nukleonika
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Positron annihilation studies of mesoporous iron modified MCM-41 silica
Autorzy:
Wiertel, M.
Surowiec, Z.
Budzyński, M.
Gac, W.
Powiązania:
https://bibliotekanauki.pl/articles/147950.pdf
Data publikacji:
2013
Wydawca:
Instytut Chemii i Techniki Jądrowej
Tematy:
iron
MCM-41 silica mesoporous materials
N2 sorption
positron annihilation
positron lifetime spectra
Opis:
MCM-41 silica materials modified by iron incorporation in the stage of its synthesis were investigated. The aim of the studies was determination of the nature of iron species and the influence of its content on the structural changes of materials and following the changes of their properties. For this purpose, the N2 sorption/desorption method and positron annihilation lifetime spectroscopy (PALS) were used. Disappearance of the longest-lived ortho-positronium (o-Ps) component (tau5) present in the PALS spectra of the initial MCM-41 material in the spectra of Fe-modified MCM-41 measured in vacuum is a result of a strong chemical o-Ps quenching and/or the Ps inhibition mechanism. Filling of pores by air or N2 at ambient pressure causes reappearance of the (tau5) component with lifetime shortened in comparison to that observed in vacuum for pure MCM-41 to the extent which can be explained by usual paramagnetic quenching in air. In contrary to the tendency observed for (tau5) lifetime which is practically independent of Fe content, the relevant intensity I5 monotonically decreases. This fact suggests that only inhibition of Ps formation occurs for the samples in air. Observed anti-quenching effect of air seems to be a result of competition of two processes : neutralization of surface active centres acting as inhibitors and considerably weaker paramagnetic quenching by O2 molecules.
Źródło:
Nukleonika; 2013, 58, 1; 245-250
0029-5922
1508-5791
Pojawia się w:
Nukleonika
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Effect of electronelectron correlations on positron-electron momentum density distributions
Autorzy:
Boroński, E.
Powiązania:
https://bibliotekanauki.pl/articles/146328.pdf
Data publikacji:
2010
Wydawca:
Instytut Chemii i Techniki Jądrowej
Tematy:
positron annihilation
electronic structure
many-body physics
Opis:
The calculations of the influence of the self-energy effects on the electron-positron (e-p) enhancement factors and the e-p momentum distributions are presented. The approach bases on the novel formulation of the Bethe- -Goldstone (B-G) equation for the positron in an electron gas where the scattering of the electrons into the states below the Fermi surface due to the self-energy effects is allowed. This equation has been solved and the corresponding e-p enhancement factors and momentum distributions have been found. The agreement between the absolute values of the theoretical calculations and experimental data has improved noticeably.
Źródło:
Nukleonika; 2010, 55, 1; 3-8
0029-5922
1508-5791
Pojawia się w:
Nukleonika
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Investigation of corrosion defects in titanium by positron annihilation
Autorzy:
Pietrzak, R.
Szatanik, R.
Powiązania:
https://bibliotekanauki.pl/articles/147338.pdf
Data publikacji:
2015
Wydawca:
Instytut Chemii i Techniki Jądrowej
Tematy:
positron annihilation spectroscopy
corrosion defects
Opis:
The positron annihilation method was used to study the formation of defects in titanium samples during their corrosion in the vapor of a 3% HCl solution. In particular, the distribution of defects depending on the distance from the corroding surface and the impact of an external magnetic fi eld on the concentration of vacancies forming during the corrosion of titanium layers close to the surface were determined.
Źródło:
Nukleonika; 2015, 60, No. 4, part 1; 755-758
0029-5922
1508-5791
Pojawia się w:
Nukleonika
Dostawca treści:
Biblioteka Nauki
Artykuł
Wyświetlanie 1-10 z 286

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