Search results

1

Positron Scattering at Thermal Energies

100%

Fedus K., Karwasz G.

Acta Physica Polonica A

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2014

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vol. 125

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issue 3

829-832

EN

Modified effective range theory in analytical form is used to extrapolate elastic cross-sections down to thermal energies for positron scattering from five atomic and molecular targets including Ar, H_2, N_2, C_6H_6 (benzene) and C_6H_{12} (c-hexane). The derived scattering parameters are then used to estimate the annihilation rates for chosen elements using a simple formula by Gribakin. We show that a combination of two simple models has a potential for accurate parameterization of coefficients describing the interaction of thermal positrons with non-polar targets of low polarizability such as Ar and N_2.

2

Porosity of Low-κ Materials Studied by Slow Positron Beam

81%

Brusa R., Macchi C., Mariazzi S., Karwasz G.

Acta Physica Polonica A

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2005

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vol. 107

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issue 4

702-711

EN

The information about porosity in low-κ materials obtainable by depth profiling with positron annihilation spectroscopy is reviewed. In particular we focus on Doppler broadening spectroscopy and 2-3γ ratio of positronium measurements on SiOCH and amorphous carbon a-C:F:H thin films produced by plasma enhanced chemical vapour deposition.

3

Low Energy Cross-Sections for Positron Interactions with Cyclic Hydrocarbons

81%

Karwasz G., Pliszka D., Brusa R., Perazzolli C.

Acta Physica Polonica A

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2005

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vol. 107

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issue 4

666-672

EN

We report total cross-section measurements for positron scattering on cyclic hydrocarbons: benzene, aniline, and cyclohexane. Measurements were done by an absolute transition method in the energy range 1.5-20 eV (0.4-20 eV for cyclohexane). High cross-sections for all measured molecules were observed at low energies. In the case of cyclohexane the cross-section in the zero energy limit tends to a constant value. In aniline a weakly accented peak slightly above the positronium formation threshold is observed. Similar but less visible bump was observed in the case of benzene. Measurement of total cross-sections in nitrogen and argon used for calibration of the apparatus are also presented.

4

Positron Annihilation Studies of Czochralski-Grown Silicon Annealed Under Pressure

81%

Karwasz G., Brusa R., Misiuk A., Zecca A.

Acta Physica Polonica A

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1999

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vol. 95

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issue 4

575-580

EN

Two positron techniques have been applied to study dynamics of oxygen precipitation in Czochralski-grown silicon, annealed under high (up to 1.4 GPa) pressure. Lifetime measurements were performed with 180 ps resolution; Doppler broadening with a variable-energy slow-positron beam. Different thermal treatings rise the mean lifetime of positrons from 222 ps in as-grown samples up to 227 ps. In samples with a high (up to 85%) amount of oxygen precipitated, an intermediate (550-800 ps) lifetime is observed.

5

Positron Scattering on Atoms and Molecules in the Limit of Low Energy

71%

Karwasz G., Brusa R., Pliszka D., Idziaszek Z., Nowakowska H.

Acta Physica Polonica A

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2006

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vol. 110

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issue 5

605-614

EN

Starting from experimental cross-sections for positron scattering in argon and nitrogen, we examine different energy ranges. In the zero-energy limit the cross-section falls with energy and can be described by modified effective range theory for polarization potential. In a few eV range the cross-sections are constant vs. energy. As far as it is possible to force the elastic scattering phase shifts in a way that both experimental differential cross-sections are reproduced and the total cross-section remains constant in energy, such a model lacks the physical justification. Only the virtual-positronium model, developed recently by Gribakin, reproduces a constant dependence of the total cross section in a few eV energy range.

6

Positronium Formation in Organic Liquids

71%

Karbowski A., Fedus K., Służewski K., Bruzdowska J., Karwasz G.

Acta Physica Polonica A

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2017

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vol. 132

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issue 5

1466-1469

EN

This paper reports on the results of positron annihilation lifetime measurements of three organic liquids: benzene (C₆H₆), cyclohexane (C₆H₁₂), and methanol (CH₃OH). The lifetime spectra are acquired at different temperatures for non-degassed, degassed, and oxygen-saturated samples, at temperatures between 5°C and 25°C. The spectra are analyzed using a standard three-exponential model. The influence of oxygen on each lifetime and intensity component is discussed.

7

Genetic Algorithms for Positron Lifetime Data

71%

Karbowski A., Fisz J., Karwasz G., Kansy J., Brusa R.

Acta Physica Polonica A

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2008

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vol. 113

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issue 5

1365-1372

EN

Recently, genetic algorithms have been applied for ultrafast optical spectrometry in systems with several convoluted lifetimes. We apply these algorithms and compare the results with POSFIT (by Kirkegaard and Eldrup) and LT programme (by Kansy). The analysis was applied to three types of samples: molybdenum monocrystals, Czochralski-grown silicon with oxygen precipitates, Si with under-surface cavities obtained by He + H ion co- implantation. In all three tests, the genetic algorithm performs very well, in particular for short lifetimes. Further developments to model the resolution function in genetic algorithms are needed.

8

Systematic Measurements of Doppler-Coincidence Spectra for Positron Annihilation in Pure Metals and Semiconductors

71%

Deng W., Pliszka D., Brusa R., Karwasz G., Zecca A.

Acta Physica Polonica A

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2002

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vol. 101

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issue 6

875-892

EN

Doppler-broadening measurements of the electron-positron annihilation line in twenty six elements are presented. The adopted coincidence technique allows to reduce the background and point out the contribution of positron annihilation with core electrons. The changes of the high momentum contribution is presented for selected examples and a semiempirical analysis of the dependence on electronic structure is performed. Measured data are in a good agreement with recent theoretical calculations and can be used to identification of impurities surrounding open volume defects.

9

Structural Studies of Titanium Oxide Multilayers

62%

Karwasz G., Miotello A., Zomer E., Brusa R., Kościelska B., Armellini C., Kuzmin A.

Acta Physica Polonica A

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2005

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vol. 107

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issue 6

977-982

EN

Multilayers of titanium oxide on conductive glasses (silica, covered with indium/tin and tin oxides) were obtained by different methods (from suspension, by sol-gel, by vacuum sputtering). X-ray diffraction and positron annihilation depth-resolved characterization of these samples are presented. The data allow us to determine optimal deposition parameters, in order to obtain the anatase phase, important in practical applications in photoelectrochemical cells.

10

Positron-Annihilation, Structural and Optical Studies on Properties of Nanostructured ZrO_2, ZnO, Bi_2O_3 and ZnO-Bi_2O_3

62%

Fidelus J., Karbowski A., Grabis J., Jusza A., Piramidowicz R., Brusa R., Karwasz G.

Acta Physica Polonica A

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2011

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vol. 120

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issue 6A

A-66-A-68

EN

Nanostructured oxides, like ZrO_2, on ZnO and Bi_2O_3 show interesting electronic and photovoltaic properties. Pressed and annealed samples were obtained from nanopowders grown in hydrothermal or plasma processes. Positron annihilation (the Doppler broadening depth-resolved, positron lifetime) techniques were used to trace structural changes in samples after annealing. Photoluminescence spectra of all investigated samples show broad-band emission in the visible, with intensity depending on annealing temperature. The change in nanoporosity and positron lifetimes correlate well with changes in photoluminescence properties. The nature of broad photoluminescence bands is to be understood; further positron studies via the Doppler coincidence method would help in identifying the nature of defects in these samples.

11

Depth Profiling of Defects in He Implanted SiO_2

62%

Mariazzi S., Toniutti L., Brusa R., Duarte Naia M., Karbowski A., Karwasz G.

Acta Physica Polonica A

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2008

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vol. 113

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issue 5

1447-1453

EN

Thin layer of SiO_2 thermally grown on p-type Si was implanted with He^+ ions at 30 keV with a dose of 5×10^{15} ions/cm^2. SiO_2/Si samples were depth profiled by Doppler broadening positron annihilation spectroscopy to identify induced defects in the silicon oxide, at the interface and in the Si substrate. In one sample the silicon dioxide layer was removed by etching after implantation. It is shown that removing the silicon dioxide layer some more information about defects into the substrate can be found.

12

Amorphous Carbon Thin Films Deposited on Si and PET: Study of Interface States

52%

Mariazzi S., Macchi C., Karwasz G., Brusa R., Laidani N., Bartali R., Gottardi G., Anderle M.

Acta Physica Polonica A

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2005

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vol. 107

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issue 5

842-847

EN

Thin carbon films with various thicknesses, deposited on different substrates (Si and poly-ethylene-terephthalate) at the same operating conditions in a radio frequency plasma enhanced chemical vapour deposition system were characterized by Doppler broadening spectroscopy. The films and the substrates were depth profiled by a slow positron beam. The aim of these measurements was to study the open volume structure and the interface of the films. It was found that, independently from the substrate, the films were homogeneous and exhibited the same open volume distribution. On the contrary, the effective positron diffusion length in the Si substrate was found to change with the thickness of the carbon films. This behaviour was interpreted as a change in the electric field at the carbon/silicon interface.

13

Surfaces of Electron-Emitting Glasses Studied by a Slow Positron Beam

52%

Pliszka D., Gazda M., Kusz B., Trzebiatowski K., Karwasz G., Deng W., Brusa R., Zecca A.

Acta Physica Polonica A

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2001

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vol. 99

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issue 3-4

465-472

EN

Semi-conducting glasses used for electron multipliers and microchannel plate devices are obtained by surface modification of Pb or Bi-reach silicon-based glasses. The reduced layer extends down to 200-500 nm, much more than the effective depth of the electron-emitting layer. By the use of slow-positron beam we monitor the structural changes undergoing in near-to-surface layers after isothermal annealing. The measurements suggest a possible correlation between secondary-electron emission coefficient and the Doppler-broadening S-parameter. On these samples there were also performed atomic force microscopy, secondary electron emission, differential scanning calorimetry, and electric conductivity measurements.

Refine search results

Positron Scattering at Thermal Energies

Porosity of Low-κ Materials Studied by Slow Positron Beam

Low Energy Cross-Sections for Positron Interactions with Cyclic Hydrocarbons

Positron Annihilation Studies of Czochralski-Grown Silicon Annealed Under Pressure

Positron Scattering on Atoms and Molecules in the Limit of Low Energy

Positronium Formation in Organic Liquids

Genetic Algorithms for Positron Lifetime Data

Systematic Measurements of Doppler-Coincidence Spectra for Positron Annihilation in Pure Metals and Semiconductors

Structural Studies of Titanium Oxide Multilayers

Positron-Annihilation, Structural and Optical Studies on Properties of Nanostructured ZrO_2, ZnO, Bi_2O_3 and ZnO-Bi_2O_3

Depth Profiling of Defects in He Implanted SiO_2

Amorphous Carbon Thin Films Deposited on Si and PET: Study of Interface States

Surfaces of Electron-Emitting Glasses Studied by a Slow Positron Beam