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1

Vacancy Cluster Distributions in He Implanted Silicon Studied by Slow Positron Annihilation Spectroscopy

100%

Brusa R.

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vol. 95

|

issue 4

474-478

EN

Doppler broadening measurements performed by a slow positron beam on p-type Si samples implanted with He at 20 keV and at a fluence of 5×10^{15} and 2×10^{16} cm^{-2} are reviewed and discussed. The evolution of the open volume defects distribution was studied as a function of isochronal and isothermal annealing of the samples. In the as implanted samples the majority of the open volume defects produced by implantation was passivated by He. The open volume defects density decreases, reaching a minimum at 250°C. In the 250-650°C temperature range there is an increase in defects due to the appearance of vacancy clusters. At the higher annealing temperatures (700-900°C) the vacancy clusters disappear only in the samples implanted at 5×10^{15} cm^{-2}.

2

Modification of Free Volume due to Rigidness of Polymeric Matrix and Effects on Gas Transport Properties

64%

Checchetto R., Brusa R.

Acta Physica Polonica A

|

2017

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vol. 132

|

issue 5

1490-1495

EN

Gas barrier properties of polymeric membranes with different rigidness of their matrix where studied by gas phase permeation measurements. Gas transport results in membranes made of epoxy resin with different cross linking densities and epoxy resin with dispersed few layer graphene fillers were discussed and compared in the framework of the free volume theory of diffusion. Transport in cellulose membranes was found to occur in the diffusion configurational regime. The physical description of the transport properties was based on positron annihilation lifetime spectroscopy measurements which allowed to evaluate experimentally the fractional free volume in epoxy resin membranes and the size of rigid elongated cavities in cellulose thin films.

3

Positron Annihilation Studies of Czochralski-Grown Silicon Annealed Under Pressure

51%

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

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vol. 95

|

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.

4

Porosity of Low-κ Materials Studied by Slow Positron Beam

51%

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

Acta Physica Polonica A

|

2005

|

vol. 107

|

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.

5

Low Energy Cross-Sections for Positron Interactions with Cyclic Hydrocarbons

51%

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

Acta Physica Polonica A

|

2005

|

vol. 107

|

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.

6

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

45%

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

Acta Physica Polonica A

|

2002

|

vol. 101

|

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.

7

Genetic Algorithms for Positron Lifetime Data

45%

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

Acta Physica Polonica A

|

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

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

45%

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

Acta Physica Polonica A

|

2006

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vol. 110

|

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.

9

Production of Cold Positronium Atoms

39%

Brusa R., Mariazzi S., Consolati G., Dupasquier A., Quasso F., Giammarchi M.

Acta Physica Polonica A

|

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.

10

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

39%

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

Acta Physica Polonica A

|

2011

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vol. 120

|

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

Structural Studies of Titanium Oxide Multilayers

39%

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

Acta Physica Polonica A

|

2005

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vol. 107

|

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.

12

Depth Profiling of Defects in He Implanted SiO_2

39%

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

|

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.

13

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

33%

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

Acta Physica Polonica A

|

2001

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vol. 99

|

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.

14

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

33%

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

Acta Physica Polonica A

|

2005

|

vol. 107

|

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.

15

Positronium for Antihydrogen Production in the AEGIS Experiment

15%

Consolati G., Aghion S., Amsler C., Bonomi G., Brusa R., Caccia M., Caravita R., Castelli F., Cerchiari G., Comparat D., Demetrio A., Di Noto L., Doser M., Evans C., Fanì M., Ferragut R., Fesel J., Fontana A., Gerber S., Giammarchi M., Gligorova A., Guatieri F., Haider S., Hinterberger A., Holmestad H., Kellerbauer A., Khalidova O., Krasnicky D., Lagomarsino V., Lansonneur P., Lebrun P., Malbrunot C., Mariazzi S., Marton J., Matveev V., Mazzotta Z., Müller S., Nebbia G., Nedelec P., Oberthaler M., Pacifico N., Pagano D., Penasa L., Petracek V., Prelz F., Prevedelli M., Ravelli L., Rienaecker B., Robert J., Røhne O., Rotondi A., Sandaker H., Santoro R., Smestad L., Sorrentino F., Testera G., Tietje I., Widmann E., Yzombard P., Zimmer C., Zmeskal J., Zurlo N.

Acta Physica Polonica A

|

2017

|

vol. 132

|

issue 5

1443-1449

EN

The primary goal of the Antihydrogen Experiment: Gravity, Interferometry, Spectroscopy (AEGIS) collaboration is to measure for the first time precisely the gravitational acceleration of antihydrogen, H̅, a fundamental issue of contemporary physics, using a beam of antiatoms. Indeed, although indirect arguments have been raised against a different acceleration of antimatter with respect to matter, nevertheless some attempts to formulate quantum theories of gravity, or to unify gravity with the other forces, consider the possibility of a non-identical gravitational interaction between matter and antimatter. We plan to generate H̅ through a charge-exchange reaction between excited Ps and antiprotons coming from the Antiproton Decelerator facility at CERN. It offers the advantage to produce sufficiently cold antihydrogen to make feasible a measurement of gravitational acceleration with reasonable uncertainty (of the order of a few percent). Since the cross-section of the above reaction increases with n⁴, n being the principal quantum number of Ps, it is essential to generate Ps in a highly excited (Rydberg) state. This will occur by means of two laser excitations of Ps emitted from a nanoporous silica target: a first UV laser (at 205 nm) will bring Ps from the ground to the n=3 state; a second laser pulse (tunable in the range 1650-1700 nm) will further excite Ps to the Rydberg state.

Refine search results

Vacancy Cluster Distributions in He Implanted Silicon Studied by Slow Positron Annihilation Spectroscopy

Modification of Free Volume due to Rigidness of Polymeric Matrix and Effects on Gas Transport Properties

Positron Annihilation Studies of Czochralski-Grown Silicon Annealed Under Pressure

Porosity of Low-κ Materials Studied by Slow Positron Beam

Low Energy Cross-Sections for Positron Interactions with Cyclic Hydrocarbons

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

Genetic Algorithms for Positron Lifetime Data

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

Production of Cold Positronium Atoms

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

Structural Studies of Titanium Oxide Multilayers

Depth Profiling of Defects in He Implanted SiO_2

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

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

Positronium for Antihydrogen Production in the AEGIS Experiment