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Using a Bent Tube as an Energy Filter for a Positron Beam. Simulations on Determining the Optimum Angle of the Bend

100%
Dinescu D., Djourelov N.
Acta Physica Polonica A
|
2017
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vol. 132
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issue 5
1624-1627
EN
One way to perform lifetime measurements at a DC positron beam is to pass positrons through a pulsing device. The compressed e⁺ bunches are accelerated to a desired energy for depth profiling of the studied sample. A fraction of the e⁺ that are backscattered from the sample surface can travel back through the uniform magnetic field that is generated along the central axis of the beam transport lines and reach the accelerator. The backscattered e⁺ that reach the accelerator can be re-accelerated towards the sample and re-implant in it with a delay from the initial implanted e⁺ bunch. These e⁺ that are re-implanted into the sample with a delay cause distortions in the lifetime spectra. A setup which uses a bent tube to act as an energy filter to reduce the effect of the backscattered e⁺ is studied by simulations. The simulations are performed in order to find the optimum geometry for minimum distortions.
2
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Effect of Annealing on Free-Volume Properties of Poly(ethylene Terephthalate) Studied by Positron Annihilation

81%
Misheva M., Djourelov N., Seganov I.
Acta Physica Polonica A
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2001
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vol. 99
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issue 3-4
429-433
EN
The effect of annealing on free-volume properties of poly( ethylene terephthalate) was studied using a set of samples annealed for two hours in air at eight temperatures between 80ºC and 220ºC. The fractional free-volume in the samples was estimated from ortho-positronium lifetime and its relative intensity. The variations of fractional free-volume and of the macroscopic specific volume with annealing temperature were compared. The variation of the pore volume probability density functions with the annealing temperature was found. The interdependence between S- and W-parameters was also discussed.
3
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Study of Defects in GeTe and (GeTe)_{1-x}(AgBiTe_2)_x Solid Solutions by Positrons

81%
Misheva M., Avramova I., Plachkova S., Djourelov N.
Acta Physica Polonica A
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2001
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vol. 99
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issue 3-4
423-428
EN
Positron lifetimes and Doppler broadening of the annihilation line measurements were performed to study the vacancy type defects in polycrystalline GeTe and (GeTe)_{(1-x)}(AgBiTe_2)_x (x=0,0.03,0.05,0.1,0.15,0.2, and 1) solid solutions. The values of lifetimes obtained are explained as due to positron and positronium saturated trapping at vacancies and vacancy clusters. The interdependence between mean positron lifetime and hole concentration is discussed.
4
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Study of multiblock polyamide-6/Poly-(isoprene) copolymers by positron annihilation spectroscopy

81%
Bryaskova R., Mateva R., Djourelov N., Krasteva M.
Open Chemistry
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2008
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vol. 6
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issue 4
575-580
EN
Positron annihilation lifetime spectroscopy (PALS) has been used to determine the free volume in multiblock polyamide-6/poly-(isoprene) copolymers (PA-6/PI), synthesized via activated anionic bulk copolymerization. The diisocyanate functionalized telechelic PI, blocked with caprolactam (CL) has been used as a commoner and an activator at the same time. The elastic PI block incorporated into the main chain of PA-6 affects the amorphous and crystal phase of the copolymer leading to changing in degree of crystallinity. The positron annihilation lifetime spectroscopy (PAL) and Doppler broadening of annihilation line (DBAL) technique in a set of pure PA-6 and PA-6/PI copolymers with two different compositions have been applied and evaluation of the size of free-volume holes (pores), localized mainly in the disordered regions of the PA-6/PI copolymer by measuring the o-Ps lifetime (τ3) and o-Ps intensity (I3) has been performed. [...]
5
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Is Kapton Really That Simple?

71%
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.
6
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Application of Coincidence Doppler Broadening Spectroscopy to Hydrocarbons at Different Substance States

61%
Yu R., Suzuki T., Ito Y., Djourelov N., Kondo K., Shantarovich V.
Acta Physica Polonica A
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2005
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vol. 107
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issue 4
697-701
EN
Coincidence Doppler broadening spectroscopy was applied to study n-hexane (C_6H_{14}), cyclohexane (C_6H_{12}) and cyclohexanone (C_6H_{10}O), at solid, as well as liquid states. The experiments were performed in parallel with traditional positron annihilation lifetime spectroscopy. Significant positron trapping by oxygen was observed in cyclohexanone; the oxygen effect is stronger in liquid state than in solid one, which can possibly be attributed to positron solvation in polar solution. Coincidence Doppler broadening results for n-hexane and cyclohexane indicate an existence of higher energetic annihilation electrons in Ps-bubble forming liquids than that in solids.
7
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Some Aspects of Free Volume Studies in Molecular Substances Using Positron Annihilation Experiments

61%
Shantarovich V., Suzuki T., Djourelov N., Shimazu A., Gustov V., Kevdina I.
Acta Physica Polonica A
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2005
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vol. 107
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issue 4
629-634
EN
Positron annihilation lifetime spectroscopy is accepted now as a method for the studies of elementary free volumes (free volume holes) in solids, in polymers in particular. The aim of this paper is to discuss some problems, the difficulties on the way of this application and to illustrate them by several examples obtained by the authors.
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