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  1. 58 Acta Physica Polonica A

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  1. 9 Zgardzińska B.

  2. 7 Goworek T.

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1
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Current Problems and Unknowns in Positronium Dynamics

100%
Duplâtre G.
Acta Physica Polonica A
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2008
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vol. 113
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issue 5
1347-1358
EN
A survey is made on positronium dynamics in liquids. From literature data, it is shown that applying conventional kinetics to positronium reactions leads to quite satisfactory results in a large variety of solvents, solutes, reaction types and temperatures. On the opposite, applying the theoretically well established concepts of positronium bubble state and time-dependent reaction rate coefficient would imply the use of unrealistic solute radii in the related kinetic equations, with possible exception of large solutes (micelles). Some solutes displaying unexpectedly high reaction rate constants are shown to be refractory to any of the above quantitative approaches. The particular case of reversible positronium bound-state formation is further examined, the most consistent results arising, again, from the conventional analysis of the data.
2
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Some Comments on p-Ps and Density Parameter

100%
Goworek T., Wawryszczuk J.
Acta Physica Polonica A
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1995
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vol. 88
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issue 1
135-138
EN
The constraint-free analysis of several representative positron lifetime spectra was performed with the resolution time of 144 ps. The role of p-Ps parameters in understanding of the positronium structure in solids is discussed.
3
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Prospects of Positronium Spin Rotation Using for Medium Investigation

100%
Baryshevsky V. G., Metelitsa O. N.
Acta Physica Polonica A
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1995
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vol. 88
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issue 1
67-72
EN
The feasible applications of a new positron spectroscopy technique based on measuring of the time oscillation under positronium annihilation is ana­lyzed.
4
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To the Possibility of Measurements of Orthopositronium Annihilation Amplitudes for QED Testing

100%
Baryshevsky V. G., Metelitsa O. N.
Acta Physica Polonica A
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1995
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vol. 88
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issue 1
73-78
EN
In connection with previously revealed disagreement of theoretically cal­culated and measured orthopositronium decay rate in vacuum we suggest the experiment to clarify which of the positronium annihilation amplitudes R_{m},m = 0,±1 causes this disagreement. The developed for this purpose experimental procedure can also be used as the basis for the new area of positron spectroscopy, connected with the investigation of the anisotropic (spin dependent) interaction of positronium in the media.
5
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Three-Quantum Annihilation in Porous Vycor Glass

63%
Jasińska B., Wawryszczuk J., Zaleski R.
Acta Physica Polonica A
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2005
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vol. 107
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issue 5
821-825
EN
Three methods of determination of 3γ o-Ps decay intensity are compared. The estimate of 3γ fraction obtained from classical lifetime measurements can be distorted due to a higher absorption of theγ quanta from the continuous spectrum of three-quantum decay, compared to two-quantum one.
6
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Positronium Formation in Solid Long-Chain Alkanes

63%
Zaleski R., Wawryszczuk J., Goworek T.
Acta Physica Polonica A
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2005
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vol. 107
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issue 4
635-641
EN
This paper represents a fragment of systematic studies on positron annihilation in solid saturated hydrocarbons (alkanes, paraffins) from C_{17}H_{36} to C_{30}H_{62}. The Tao-Eldrup model and its extensions allow to identify the places in the crystalline structure where positronium is trapped, and estimate respective trap sizes. In the rigid phase I positronium locates mainly in two-dimensional empty spaces between the lamellae of crystal structure. Additional free volumes exist in high temperature phases due to appearance of n-alkane conformers representing a non-planar molecule distortion. The rise of ortho-positronium intensity with the sample irradiation dose, which was seen earlier in polymers, is observed also in n-alkanes. One can find similarities between the ortho-positronium annihilation in crystalline alkanes and polymers, like exponential character of that rise, and the trap bleaching by illumination. It confirms the same origin of the intensity growth, i.e. the accumulation of trapped electrons. Two kinds of electron traps were found. The time constant of electron accumulation in the traps depends on the carbon chain length and varies with temperature in different way for even- and odd-numbered alkanes.
7
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Pressure Effects in ortho-Positronium Annihilation

63%
Goworek T., Wawryszczuk J., Zaleski R.
Acta Physica Polonica A
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2005
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vol. 107
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issue 4
608-614
EN
A short review of experiments concerning the influence of high pressure on positron annihilation in molecular solids is given. To this kind of experiments belong e.g. pressure inhibition of positronium formation, pressure induced phase transitions, and free volume swelling in the media intercalated by high pressure gas.
8
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Positronium Trapping in Porous Solids: Means and Limitations for Structural Studies

51%
Kajcsos Z., Liszkay L., Duplâtre G., Varga L., Lohonyai L., Pászti F., Szilágyi E., Lázár K., Kótai E., Pál-Borbély G., Beyer H., Caullet P., Patarin J., Azenha M., Gordo P., Lopes Gil C., de Lima A., Ferreira Marques M.
Acta Physica Polonica A
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2005
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vol. 107
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issue 5
729-737
EN
Positron annihilation techniques are used for the structural investigation of solids but the interpretation of results in grainy and porous media is still unclear. A unique picture can be obtained assuming that the dominant process is Ps trapping in competing "extended free volume" sites. In samples with a large amount of free volumes near-saturation Ps trapping will rule the lifetime pattern, and very long lifetimes of over 100 ns might arise from o-Ps trapped in mesopores. It is shown that lifetime parameters must be corrected for the 3γ/2γ counting efficiency ratio. The results demonstrate the high sensitivity of Ps to mesopores in zeolites but also that Ps-trapping poses limitations on the applicability of lifetime to structural investigation in porous systems. The evolution of the lifetime spectra upon changes in the sample and measuring conditions should be considered in a complex way, observing not only changes in some selected components but in the whole lifetime pattern simultaneously.
9
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Polypropylene Films Studied with Variable Positron Beams

51%
Dębowska M., Coleman P., Gidley D.
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vol. 95
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issue 4
496-504
EN
Doppler broadening and positron lifetime spectra have been measured for polypropylene films of constant thickness (≈ 50 μm) and different morphology that crystallized from melt at temperature 0, 100, and 120°C. The S -parameter vs. energy curves do not change in a systematic way with crystallinity of samples. It is supposed that decrease in the fractional free volume for samples of greater crystallinity is not seen in S-values because of their increase resulting from annihilation in defects present both in the crystalline regions and on the lamellae surfaces. Intensities of the components in the positron lifetime spectra: I_{2} (τ_{2}=1.3 ns) and I_{3} (τ_{3}≈ 2.3÷2.8 ns) show changes with crystallinity of samples. The evident trend is observed for the longer-lived τ_{3}-component intensity to decrease with crystallinity while the reverse is true for the shorter, fixed τ_{2}-component. The decrease in the intensity of the longer-lived component can result from reduction of the fractional free volume.
10
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Discrete and Continuous Analysis of Positron Lifetime Spectra: a Comparison

51%
Consolati G., Quasso F.
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vol. 95
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issue 4
505-508
EN
Positron lifetime spectra in poly[1-(trimethylsilyl)]propine membranes were collected at different temperatures and various number of accumulated counts, in order to compare the results from analyses obtained by different decay models: discrete (that is, purely exponential) or continuous (with at least one distribution of lifetimes). Analysis into four discrete components is inadequate, unless a low number of counts (1 to 2 millions) is accumulated. Conversely, deconvolutions which allow the ortho-positronium lifetime to be distributed give satisfactory fits, even at the highest statistics (60 millions). An equivalent good fit in terms of χ^{2} test is obtained with a five discrete components analysis, from which it is possible to extract information on the first moments (centroid and standard deviation) of ortho -positronium lifetime distribution.
11
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Ortho-Positronium in Alkanes under High Pressure

51%
Goworek T., Zgardzińska B.
Acta Physica Polonica A
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2008
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vol. 113
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issue 5
1379-1383
EN
Phase transitions in n-alkanes were observed by using the positron annihilation lifetime method. In the case of alkanes, application of pressure is equivalent to temperature lowering (1 K corresponds to ≈4 MPa). When argon at high pressure penetrates the sample one observes a non-monotonous dependence of the melting point on pressure. This effect is observed also in even-numbered alkanes, in which there is no rotator phase.
12
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Is Kapton Really That Simple?

51%
Dauwe C., De Baerdemaeker J., Van Waeyenberge B., Djourelov N., Laforest N.
Acta Physica Polonica A
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2008
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vol. 113
|
issue 5
1315-1319
EN
Age momentum correlation experiments in kapton has revealed the existence of two positron states corresponding to the trapping of positrons on the partially negatively charged O and/or N sites in the polymer chain. A young age broadening of the annihilation line shape is also observed and is attributed to the annihilation in flight of the positrons.
13
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Production of Cold Positronium Atoms

51%
Brusa R., Mariazzi S., Consolati G., Dupasquier A., Quasso F., Giammarchi M.
Acta Physica Polonica A
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2008
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vol. 113
|
issue 5
1301-1306
EN
Positronium will play a primary role in the next generation of antimatter experiments through the following antihydrogen production reaction: p̅ + Ps* → p̅e⁺ + e¯. In order to study antimatter physical properties (CPT (charge, parity, time) invariance and principle of equivalence test) it is necessary to keep this system at the lowest possible (sub-kelvin) temperatures. This requires the generation of a suitable flux of cold Ps atoms in a vacuum, a non-trivial requirement at the light of the present experimental results. In this paper we discuss the state of the actual knowledge on positronium formation and consequent emission from metallic surfaces and insulators and we show the opportunity to use suitable porous materials to cool positronium through collisions with the inner walls of the pores. We get a rough indication on the geometrical parameters of the pore and we propose a simple experiment to obtain the kinetic energy - and therefore the equivalent temperature - of emitted positronium without using a positron beam.
14
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Polyamide 6/Layered Silicate Nanocomposites

51%
Dębowska M., Rudzińska-Girulska J., Pigłowski J., Dołęga J.
Acta Physica Polonica A
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2008
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vol. 113
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issue 5
1321-1329
EN
Polyamide 6 (PA6) and its two exfoliated nanocomposites (PA6/Nf919 and PA6/BZ-COCO), with bentonite (2.5 wt.%) organophilically treated with different cations, were studied. Improved mechanical properties, changes in crystallinity and morphology as well as higher glass transition temperature values were observed for the nanocomposites in comparison to the neat PA6. For the nanocomposite PA6/BZ-COCO, of better surface modification of platelets and better interaction between the polymeric matrix and the organobentonite, higher values of Young's modulus and yielding point together with higher contribution of larger free volume holes to free volume distributions occurred.
15
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Oscillations in Orthopositronium Annihilation in Magnetic Field in SiO_{2} Powders

51%
Berestov A. V., Talat A. S., Baranov V. V., Rudak E. A., Martsynkevich B. A.
Acta Physica Polonica A
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1995
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vol. 88
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issue 1
79-82
EN
The observation method of γ-quantum angular distribution oscillations in positronium annihilation in magnetic field is essentially improved. Oscillation parameters in the experiment are in agreement with theoretical estimations.
16
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Quadrupole Interactions and Anisotropic Magnetic Quenching of Positronium in Oriented Crystals

51%
Bondarev I. V., Kuten S. A.
Acta Physica Polonica A
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1995
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vol. 88
|
issue 1
83-90
EN
The quadrupole interaction of positronium in noncubic defectless crystals has been investigated theoretically. It is shown to lead to the weak anisotropy of positronium magnetic quenching. The temperature dependence of the quadrupole coupling constant and the estimations of the magnitude of the anisotropy are obtained for delocalized positronium in crystalline quartz. The significant magnitude of the anisotropy is predicted for "anomalous" positronium in oriented crystalline polymeric structures.
17
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Anisotropic Magnetic Quenching of Positronium Formed by Polarized Positrons in Oriented Crystals

51%
Bondarev I.
|
|
vol. 95
|
issue 4
455-460
EN
A theoretical analysis is performed of the phenomenon of anisotropic magnetic quenching of a positronium atom formed by polarized positrons in noncubic crystals oriented with respect to an external magnetic field. An initial polarization of positrons is shown to enhance an anisotropy of Ps magnetic quenching and to decrease the strength of magnetic field in which an anisotropy is maximal. A maximal anisotropic effect has been estimated for quasi-Ps in crystalline quartz and Ps complex in naphthalene single crystal.
18
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Positronium in Phase IV and Rotator Phase of Long-Chain Alkanes

51%
Zgardzińska B., Goworek T.
Acta Physica Polonica A
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2008
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vol. 113
|
issue 5
1551-1555
EN
Ortho-positronium lifetime and intensity was measured as a function of temperature and pressure for some n-alkanes in which so-called "phase IV" appears. This phase is characterized by slightly increased ortho-positronium lifetime compared to low-temperature phases. It was found that the effect of quenching positronium by light does not appear in all phases from phase IV upwards. The lifetime in the rotator phases becomes equal to that in liquids, and only the change of positronium formation intensity allows to detect the solid-liquid transition in positron lifetime measurements.
19
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Content available

Primary Chemical Reactions Induced by Transformation of Radioactive Nuclei in Solids at Low Temperatures. Investigation by Means of the Emission Mössbauer and Positron Spectroscopies

51%
Byakov V., Kulikov L., Perfil'ev Y., Stepanov S.
Acta Physica Polonica A
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2005
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vol. 107
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issue 5
792-798
EN
A set of early chemical reactions in grapes of ionization (blobs) arising in tracks of fast electrons and positrons in liquids as well as formed by the Auger electrons in frozen aqueous media around decaying Mössbauer ^{57}Co or ^{119m}Sn nuclei is suggested. The mechanism predicts a correlated variation of the formation probabilities of intrablob final products, namely Fe^{2+}-Fe^{3+} or Sn^{2+}-Sn^{4+} ions, positronium atom and molecular hydrogen with variation of temperature, degree of crystallinity, concentration of electron scavengers. These correlations indicate on similarity of chemical processes as in blobs created by ^{57}Co and ^{119m}Sn after their decay as well as in blobs produced in tracks of fast positrons and electrons.
20
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Theoretical Study of Positronic Atoms with Adiabatic Separation of Positronic Motion

51%
Strasburger K., Wołcyrz M.
Acta Physica Polonica A
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2008
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vol. 113
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issue 5
1533-1542
EN
A modified adiabatic approximation, with the positron treated as a light nucleus and its charge included only partially in the electronic Hamiltonian, was applied to compute the energies and properties of bound states of lithium positride, positronic beryllium and positronic magnesium. The electronic Schrödinger equation was solved with the configuration interaction method. No bound state was obtained for lithium positride, while the dissociation energies of positronic beryllium and positronic magnesium were underestimated by 2.6 and 7 millihartrees, respectively. Consequences for the applicability of the adiabatic approximation to describe positronic systems are discussed.
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