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1
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Positrons and Positronium Atoms in a Condensed Phase Close to Its Melting Point

100%
Stepanov S., Zvezhinskiy D., Byakov V., Duplâtre G., Stepanov P.
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vol. 125
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issue 3
691-695
EN
Investigation of the local heating effect at the terminal part of a fast positron track sheds light on the accumulation of defects in a frozen medium close to its melting point. Estimations of the formation energies of the structural defects which may trap the Ps atom as well as a contribution of these defects to the enthalpy of fusion are obtained.
2
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Is Kapton Really That Simple?

100%
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
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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.
3
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Numerical Solution of a Two-Particle Model of Positronium Confined in Small Cavities

100%
Tanzi Marlotti G., Castelli F., Consolati G.
Acta Physica Polonica A
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2017
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vol. 132
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issue 5
1575-1578
EN
The positronium atom (Ps) is widely used as a probe to characterize nanoporous and mesoporous materials. Existing theoretical models for describing Ps annihilation rates by pick-off processes generally treat Ps as a point particle confined in a potential well. Hence these models do not justify any change in the internal structure of Ps, which is experimentally accessible by means of the contact density parameter. Recently we formulated a two-particle model in which only the electron is confined in the cavity, while the positron is moving freely and feels the medium via a positive work function. We present here a numerical treatment of the problem of calculating contact densities and pick-off annihilation rates, by using a variational method. Results are in agreement with experimental data for a large class of materials, and suggest a way to connect these data with pore sizes and positron work functions.
4
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The Evidence of Quasi-Free Positronium State in GiPS-AMOC Spectra of Glycerol

100%
Zvezhinskiy D., Butterling M., Wagner A., Krause-Rehberg R., Stepanov S.
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vol. 125
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issue 3
821-824
EN
We present the results of processing of age-momentum correlation spectra that were measured for glycerol by the gamma-induced positron spectroscopy facility. Our research has shown that the shape of experimental s(t) curve cannot be explained without introduction of the intermediate state of positronium (Ps), called quasi-free Ps. This state yields the wide Doppler line near zero lifetimes. We discuss the possible properties of this intermediate Ps state from the viewpoint of developed model. The amount of annihilation events produced by quasi-free Ps is estimated to be less than 5% of total annihilations. In the proposed model, quasi-free Ps serves as a precursor for trapped Ps of para- and ortho-states.
5
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Formation of Quasi-Free and Bubble Positronium States in Water and Aqueous Solutions

100%
Stepanov S., Duplâtre G., Byakov V., Zvezhinskiy D., Subrahmanyam V.
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vol. 125
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issue 3
770-774
EN
It is shown that in aqueous solutions a positronium atom is first formed in the quasi-free state, and, after 50-100 ps, becomes localized in a nanobubble. Analysis of the annihilation spectra of NaNO_3 aqueous solutions shows that the hydrated electron is not involved in the positronium (Ps) formation.
6
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Positron Annihilation Studies of Mesoporous Iron-Molybdenum Modified MCM-41 Silica

88%
Surowiec Z., Wiertel M., Gac W., Budzyński M.
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vol. 125
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issue 3
789-792
EN
Iron-molybdenum modified MCM-41 mesoporous materials were obtained by the application of direct hydrothermal method. An incorporation of iron and molybdenum ions in the synthesis stage led to structural changes of the MCM-41 support. With an increasing metals content, cylindrical pores of silica are destroyed. X-ray diffraction investigations indicated that both iron and molybdenum ions were first embedded in the silica walls and then formed highly dispersed species in the pores. The porous structure of the studied samples was examined by means of N_2 adsorption/desorption and PALS methods. The longest-lived o-Ps lifetime component remains constant but its intensity monotonically decreases, except for the sample with the highest metals content. These results indicate that an effect of o-Ps formation inhibition occurs.
7
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Positron Annihilation in MnFe_2O_4/MCM-41 Nanocomposite

75%
Wiertel M., Surowiec Z., Gac W., Budzyński M.
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vol. 125
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issue 3
793-797
EN
In the paper results of studies of MnFe_2O_4/MCM-41 nanocomposites have been presented. The influence of manganese ferrite loading on changes of porous properties of mesoporous MCM-41 structure was studied by means of N_2 sorption/desorption method and positron annihilation lifetime spectroscopy. Disappearance of the longest-lived ortho-positronium component (τ_5) of pure MCM-41 mesoporous material in the positron annihilation lifetime spectra of MnFe_2O_4/MCM-41 measured in vacuum is a result of either o-Ps quenching or the Ps inhibition mechanism. Filling of pores in the studied nanocomposites by air at ambient pressure causes partial reappearance of the (τ_5) component except for the sample with maximum ferrite content. Both the (τ_5) component lifetime and intensity are suppressed together with increasing MnFe_2O_4 content by chemical quenching and inhibition of Ps formation occur. Observed anti-quenching effect of air is a result of two processes: neutralization of some surface active centres acting as inhibitors and considerably weaker paramagnetic quenching by O_2 molecules.
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