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1

80 Years of Positron Annihilation Radiation

100%

Goworek T.

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vol. 125

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issue 3

685-687

EN

Short history of the investigations of annihilation radiation is presented, from early experiments with ThC" to the introduction of two main methods: angular correlation of annihilation quanta and positron lifetime spectroscopy.

2

Ortho-Positronium in Alkanes under High Pressure

64%

Goworek T., Zgardzińska B.

Acta Physica Polonica A

|

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.

3

Some Comments on p-Ps and Density Parameter

64%

Goworek T., Wawryszczuk J.

Acta Physica Polonica A

|

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.

4

Positronium Formation with Trapped Electrons in n-Alkanes

64%

Zgardzińska B., Goworek T.

Acta Physica Polonica A

|

2011

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vol. 119

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issue 3

328-331

EN

Positronium (Ps) formation during self-irradiation by positrons was observed in n-alkanes of various carbon chain length. The rise of Ps intensity with time of irradiation is the result of positronium formation with trapped excess electrons. In the range T < 190 K the saturation value of positronium intensity changes with temperature mainly due to the change of positron mobility. With the increase of the irradiation dose the process of charging traps is slowing down and this effect is ruled mainly by interaction of excess electrons with ionization products during electron's migration to the traps.

5

Positronium in Phase IV and Rotator Phase of Long-Chain Alkanes

64%

Zgardzińska B., Goworek T.

Acta Physica Polonica A

|

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.

6

Positron Annihilation in p-Terphenyl Doped with Benzofluorene

64%

Goworek T., Rybka C.

Acta Physica Polonica A

|

2001

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vol. 99

|

issue 3-4

369-371

EN

In earlier measurements of momentum distribution of annihilating e^+e^- pairs the intensity of o-Ps component seemed to be much higher than that expected from the lifetime data. In this paper we show that such a deviation disappears if one assumes that the pick-off component of angular correlation of annihilation radiation is narrower by 10% than the component related to free annihilation.

7

Positronium in Small and Large Voids in Vycor Glass

51%

Ciesielski K., Dawidowicz A., Goworek T.

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vol. 95

|

issue 4

483-486

EN

The intensities of ortho-positronium components related to the pores and to small voids in the bulk were determined for a series of Vycor glasses. The ratio of "pore fraction" to the total o-Ps yield depends on the specific surface area of the porous medium. The diffusion coefficient for o-Ps was found equal to 3.7×10^{-5} cm^{2}/s.

8

Distortion of the Positron Lifetime Spectra in BaF_{2} Spectrometers

51%

Goworek T., Górniak W., Wawryszczuk J.

Acta Physica Polonica A

|

1993

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vol. 83

|

issue 3

307-311

EN

High efficiency of BaF_{2} scintillators creates the risk of distortion of the positron lifetime distributions due to summing and backscattering. In the 180° geometry it leads to the appearance of a spurious shortlived component.

9

Void Shapes and o-Ps Lifetime in Molecular Crystals

51%

Jasińska B., Kozioł A., Goworek T.

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vol. 95

|

issue 4

557-561

EN

The size and shape of the free volume in molecular crystals was estimated from crystallographic data. The o-Ps lifetime values were calculated from respective models and compared with the experimental data. The best agreement for channel-like voids has been obtained for the value of DR parameter equal 0.19 nm. The threshold value of radius allowing Ps trapping in the void was found to be (0.12-0.13) nm.

10

Positronium Formation in Solid Long-Chain Alkanes

51%

Zaleski R., Wawryszczuk J., Goworek T.

Acta Physica Polonica A

|

2005

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vol. 107

|

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.

11

Ortho-Positronium in Some n-Alkanes; Influence of Temperature and Pressure

51%

Zgardzińska B., Pietrow M., Goworek T., Wawryszczuk J.

Acta Physica Polonica A

|

2006

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vol. 110

|

issue 5

747-753

EN

Positron lifetime spectra were measured for a series of odd-numbered n-alkanes from C_{11}H_{24} to C_{19}H_{40} as a function of temperature and pressure. The ortho-Ps lifetimes in the rotator phase and the increase in intensity at the transition to that phase can be explained by location of Ps in the vicinity of kink-type conformers. The relation between o-Ps lifetime and molecule length can be described in the framework of extended Tao-Eldrup model. One can eliminate the effect of intensity rise in time by sample illumination or by application of high pressure. A decrease in temperature by 1 K is equivalent to an increase in pressure by about 4 MPa.

12

Pressure Effects in ortho-Positronium Annihilation

51%

Goworek T., Wawryszczuk J., Zaleski R.

Acta Physica Polonica A

|

2005

|

vol. 107

|

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.

13

Gas Pressure Induced Porosity of CYTOP Polymer. A~Positron Study

45%

Zgardzińska B., Goworek T., Śniegocka M., Zaleski R., Dlubek G.

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vol. 125

|

issue 3

782-784

EN

The pore formation in CYTOP polymer under high pressure of argon or nitrogen gases was investigated by positron annihilation spectroscopy. In the range 0-70 MPa, formation of large open pores was observed. After reduction of pressure to normal one, the free volumes inside the polymer structure remained enlarged. The structure of CYTOP can be restored by applying the temperature over 470 K at normal pressure.

14

An Attempt to Produce the Implanted ^{22}Na Source for Positron Spectrometry

39%

Latuszyński A., Yushkevich Y., Bystrov V., Misak R., Wawryszczuk J., Goworek T.

Acta Physica Polonica A

|

1993

|

vol. 83

|

issue 3

345-348

EN

Preliminary results of ^{22}Na implantation into the metal foils in order to produce the positron source for annihilation experiments are presented.

15

The Temperature and Pressure Dependence of the Free Volume in Fluoroelastomers from PALS and PVT Experiments

33%

Dlubek G., Sen Gupta A., Wawryszczuk J., Kilburn D., Pionteck J., Krause-Rehberg R., Goworek T., Alam M., Kaspar H., Lochhaas K.

Acta Physica Polonica A

|

2005

|

vol. 107

|

issue 4

685-689

EN

Positron lifetime and pressure-volume-temperature experiments were performed as function of T (100-485 K) and P (10^{-5} Pa-500 MPa) on a fluoroelastomer. From positron annihilation lifetime spectroscopy, analyzed with LT9.0 assuming a dispersion in the e^+ (τ_2) and o-Ps (τ_3) lifetime, the mean, 〈v_h〉, and the width (standard deviation), σ_h, of the hole size distribution were calculated. The pressure-volume-temperature data were analyzed using the Simha-Somcynsky hole-lattice theory to estimate the specific hole free volume V_f. From a comparison of V_f with 〈v_h〉 the specific hole number, N'_h, is estimated. The data from thermal expansion and pressure experiments show complete agreement when taking into account the compressibility of the occupied volume.

Refine search results

80 Years of Positron Annihilation Radiation

Ortho-Positronium in Alkanes under High Pressure

Some Comments on p-Ps and Density Parameter

Positronium Formation with Trapped Electrons in n-Alkanes

Positronium in Phase IV and Rotator Phase of Long-Chain Alkanes

Positron Annihilation in p-Terphenyl Doped with Benzofluorene

Positronium in Small and Large Voids in Vycor Glass

Distortion of the Positron Lifetime Spectra in BaF_{2} Spectrometers

Void Shapes and o-Ps Lifetime in Molecular Crystals

Positronium Formation in Solid Long-Chain Alkanes

Ortho-Positronium in Some n-Alkanes; Influence of Temperature and Pressure

Pressure Effects in ortho-Positronium Annihilation

Gas Pressure Induced Porosity of CYTOP Polymer. A~Positron Study

An Attempt to Produce the Implanted ^{22}Na Source for Positron Spectrometry

The Temperature and Pressure Dependence of the Free Volume in Fluoroelastomers from PALS and PVT Experiments