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Strona główna » Instytut » Pracownicy » Tomasz Goworek

prof. dr hab. Tomasz Goworek

Zakład

Zakład Metod Jądrowych

Stanowisko:

prof. Emeritus

Kontakt:

pokój: 42B
telefon: (081) 537-62-86
e-mail: tomasz.goworek@umcs.pl

Funkcje pełnione w IF UMCS:

Profesor emerytowany

Publikacje: Ukryj abstrakty

  1. B. Zgardzińska,T. Goworek, Positronium as a probe of structure and stability of solid phases in binary n-alkane mixtures, Chemical Physics, 368(), 2010, 101 – 107

  2. B. Zgardzińska,T. Goworek, Positronium in solid phases of methanol , Chemical Physics Letters, 501(), 2010, 44-46

  3. B. Zgardzińska,T. Goworek, Positron study of the structure of nanodecane- tricosane mixed solid, Nukleonika, 55(), 2010, 47-49

  4. B. Zgardzińska,M. Paluch,T. Goworek, Positronium lifetime in supercooled 1-butanol. search for polyamorphism, Chemical Physics Letters, 491(), 2010, 160- 163

  5. T. Goworek, M. Pietrow, R. Zaleski, B. Zgardzińska, Positronium in high temperature phases of long-chain even alkanes, Chemical Physics, 355(), 2009, 123-129

  6. B. Zgardzińska, T. Goworek, J. Wawryszczuk, Positronium in alkanes at high pressure of argon and nitrogen, Material Science Forum, 607(), 2009, 45-47

  7. B. Zgardzińska, J. Wawryszczuk, T. Goworek, Positronium in the n-alkane binarymixture: Nonadecane-heneicosane, Chemical Physics Letters, 470(), 2009, 72-74

  8. Goworek, T. Zgardzińska, B., Ortho-positronium in alkanes under high pressure, Acta Physica Polonica A, 113(), 2008, 1379-1383

  9. Goworek, T., How accurate is extended tao-eldrup model ?, Czech Republic, Praha, Abstract of International workshop, Positron studies in defects, (), 2008, 59

  10. Zgardzińska, B. Goworek , T., Positronium in phase IV and rotator phase of long-chain alkanes, Acta Physica Polonica A, 113(), 2008, 1551-1555

  11. Zgardzińska, B. Goworek , T., Positronium in n-nonadecane at high pressure of nitrogen, Chemical Physics Letters, 457(), 2008, 320-322

  12. Zgardzińska, B. Goworek, T. Wawryszczuk, J., Positronium in alkanes at high pressure of argon and nitrogen, China, Positronium Chemistry PPC-9, Wuhan University, (), 2008, O5.1, 123

  13. Zgardzińska, B. Hirade, T. Goworek, T., Diffusion length of positrons and electrons, Tokyo, Abstract of 2-nd Asia-Pacific Symposium on Radiation Chemistry APSRC-2008 Waseda University Tokyo, (), 2008,

  14. Zaleski, R. Zgardzińska, B. Pietrow, M. Goworek, T., Positron irradiation effects in simple organic solids, Radiation Physics and Chemistry, 77(), 2008, 1306-1310

  15. Zgardzińska, B. Zaleski, R. Wawryszczuk, J. Pietrow, M. Goworek, T., Intermolecular free volumes and intramolecular defects in alkanes,Czech Republic, Praha, Abstract of International workshop, Positron studies in defects, (), 2008, 50

  16. Goworek, T. Wawryszczuk, J. Zaleski, R. Zgardzinska, B., Positronium in solid phases of long-chain paraffins, Radiation Physics and Chemistry, 76(2), 2007, 185-188

    Abstract:
    The study of positronium intensity rise in long-chain alkanes shows two kinds of electron traps: the first, which are discharged at low temperature (200 K); the other, which remain populated up to the transition point to the rotator phase; e.g. in C30H62 they are still observed at 328 K. In the rigid phase of even-numbered alkanes o-Ps lifetime is shorter than that of odd-numbered ones, due to the difference in the width of gap between the molecular lamellae. (c) 2006 Elsevier Ltd. All rights reserved.

  17. Zaleski, R. Wawryszczuk, J. Goworek, T., Pick-off models in the studies of mesoporous silica MCM-41. Comparison of various methods of the PAL spectra analysis, Radiation Physics and Chemistry, 76(2), 2007, 243-247

    Abstract:
    The results of models, extending the Tao-Eldrup model to free volume radii R > 1 nm, are discussed. The influence of free volume size and geometry on ortho-positronium lifetime was examined. Theoretical curves were compared to available experimental data. Then the models were used to obtain information about free volume sizes in MCM-41 porous silica. Numerical analysis of MCM-41 lifetime spectra is difficult due to their considerable complexity. In order to verify the results, various methods of PAL spectra analysis, e.g., deconvolution, fitting, etc., were involved. (c) 2006 Elsevier Ltd. All rights reserved.

  18. Goworek, T., Free volumes in solids under high pressure, Radiation Physics and Chemistry, 76(2), 2007, 318-324

    Abstract:
    A review of experimental work on ortho-positronium (o-Ps) lifetime in solids under the pressures up to 1 GPa is presented. Among the effects observed at high pressure one can mention: the disappearance of the energy level for Ps at the reduction of free volume size; pressure induced phase transitions; variation of Ps formation intensity with time; increase of o-Ps lifetime after intercalation of high pressure gas to the paraffin samples. (c) 2006 Elsevier Ltd. All rights reserved.

  19. Zgardzińska, B. Goworek, T. Zaleski, R. Wawryszczuk, J., Positron annihilation and phase transitions in argon intercalated n-nonadecane, Chemical Physics, 335(), 2007, 1-6

  20. Pietrow, M. Zaleski, R. Goworek, T., Positronium in 1-tridecene, Chemical Physics, 342(), 2007, 85-89

  21. Zgardzińska, B. Hirade, T. Goworek, T., Positron formation on trapped electrons in n-heptadecane, Chemical Physics Letters, 446(), 2007, 309-312

  22. Surowiec, Z. Goworek, J. Ryczkowski, J. Budzyński, M. Wiertel, M. Sarzyński, J., Mössbauer study of magnetite nanowire in MCM-41 mesoporous silica templates, Nukleonika, 52(), 2007, S33-S36

  23. Zaleski, R. Goworek, J. Kierys, A., Temperature dependence of positronium lifetime in cylindrical pores, Physica Status Solidi B, 4(), 2007, 3814-3818

  24. Zgardzinska, B. Pietrow, M. Goworek, T. Wawryszczuk, J., Ortho-positronium in some n-alkanes; Influence of temperature and pressure, Acta Physica Polonica A, 110(5), 2006, 747-753

    Abstract:
    Positron lifetime spectra were measured for a series of odd- numbered n-alkanes from C11H24 to C19H40 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.

  25. Sniegocka, M. Jasinska, B. Goworek, T. Zaleski, R., Temperature dependence of o-Ps lifetime in some porous media. Deviations from ETE model, Chemical Physics Letters, 430(4-6), 2006, 351-354

    Abstract:
    Temperature dependence of ortho-positronium lifetime in some porous materials was investigated. Above the room temperature the extended Tao-Eldrup model works rather well, however at low temperatures one observes in all samples the lifetimes evidently longer than expected. (c) 2006 Elsevier B.V. All rights reserved.

  26. Zgardzinska, B. Wawryszczuk, J. Goworek, T., Positron annihilation in argon intercalated n-alkanes at high pressure, Chemical Physics, 320(2-3), 2006, 207-213

    Abstract:
    Positron annihilation lifetime spectroscopy was used to observe the effects of argon intercalation in some solid long-chain alkanes at high pressure. The ortho-Ps lifetime rises with argon pressure, which means increase of free volumes in the alkane structure. The range of pressures in which the rotator phase exists increases, comparing. to pure alkane. In n- heptadecane, n-nonadecane, and possibly n-heneicosane, a stepwise change of ortho-Ps lifetime and intensity at 12 MPa is observed, suggesting the transition to a new kind of the rotator phase. The transition rate is low, final lifetime value is approximate to 3.3 us. Despite a large size of free volumes corresponding to such a lifetime, their compressibility is found negligible up to the pressure of 90 MPa. At low pressures the compressibility of free volumes in the rotator phase is negative. (c) 2005 Elsevier B.V. All rights reserved.

  27. Goworek, T. Wawryszczuk, J. Zaleski, R., Pressure effects in ortho-positronium annihilation, Acta Physica Polonica A, 107(4), 2005, 608-614

    Abstract:
    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.

  28. Zaleski, R. Wawryszczuk, J. Goworek, T., Positronium formation in solid long-chain alkanes, Acta Physica Polonica A, 107(4), 2005, 635-641

    Abstract:
    This paper represents a fragment of systematic studies on positron annihilation in solid saturated hydrocarbons (alkanes, paraffins) from C17H36 to C30H62. 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 a 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.

  29. Dlubek, G. Sen Gupta, A. Wawryszczuk, J. Kilburn, D. Pionteck, J. Krause-Rehberg, R. Goworek, T. Alam, M. A. Kaspar, H. Lochhaas, K. H., The temperature and pressure dependence of the free volume in fluoroelastomers from PALS and PVT experiments, Acta Physica Polonica A, 107(4), 2005, 685-689

    Abstract:
    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(+) (tau(2)) and o-Ps (tau(3)) lifetime, the mean, < v(h)>, and the width (standard deviation), sigma(h1) 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(1), is estimated. The data from thermal expansion and pressure experiments show complete agreement when taking into account the compressibility of the occupied volume.

  30. Goworek, T. Wawryszczuk, J. Zaleski, R., Negative compressibility of free volumes in argon intercalated n-nonadecane, Chemical Physics Letters, 402(4-6), 2005, 367-369

    Abstract:
    Positron annihilation lifetime spectroscopy was applied to study modification of sub-nanometer voids in one of n-alkanes subjected to high pressure. Without a contact with gaseous atmosphere the sizes of voids behaved as expected - decreased with the rise of pressure. When the pressure was applied with argon penetrating the samples - the voids expanded. After the phase transition the structure damage allowed positronium, trapped in voids, to escape out of a damaged sample. (C) 2004 Elsevier B.V. All rights reserved.

  31. Dlubek, G. Wawryszczuk, J. Pionteck, J. Goworek, T. Kaspar, H. Lochhaas, K. H., High-pressure dependence of the free volume in fluoroelastomers from positron lifetime and PVT experiments, Macromolecules, 38(2), 2005, 429-437

    Abstract:
    The microstructure of the free volume and its high-pressure dependence in fluoroelastomeric copolymers of tetrafluoroethylene (TFE) and perfluoro(methyl vinyl ether) (PMVE), PFE, as well as vinylidene fluoride (VDF) and hexafluoropropylene (HFP), VDF/HFP22, was studied by pressure- volume-temperature experiments (PVT, P = 0.1-200 MPa) and positron annihilation lifetime spectroscopy (PALS, P = 0.1-448 MPa, all at 22.5 degreesC). Employing the Simha-Somcynsky equation of state the excess (hole) free volume fraction h and the specific free and occupied volumes, V-f = hV and V-occ = (1- h)V, were estimated from the specific total volume V. The pressure variation of these volumes and their compressibilities are discussed. We found that the occupied volume V-occ exhibits a remarkable compressibilitY7 K-occ approximate to 2.4 x 10(-4) MPa-1, while it shows (for T > T-g) almost no thermal expansion. The PALS spectra were analyzed using the new routine LT9.0 assuming dispersion in both the positron (tau(2)) and orthopositronium (o-Ps) lifetimes (tau(3)). We speculate that positrons (e(+)) may show Anderson localization at sites of the entire empty volume possibly assisted by an affinity to fluorine atoms. From the o-Ps lifetime parameters the (excess) free volume hole size distribution, its mean value , and dispersion sigma(h) were calculated. From a comparison of with V-f the specific hole number N-h' was estimated. N-h' was determined to be constant and the same in compression and thermal expansion experiments. When taking correctly into account the compressibility of the occupied volume V-occ the discrepancy in V vs plots between compression and thermal expansion experiments discussed in the literature disappears.

  32. Zgardzinska, B. Zaleski, R. Wawryszczuk, J. Goworek, T., Positronium in solid phases of n-pentacosane, Physics Letters A, 333(3-4), 2004, 341-346

    Abstract:
    Positron annihilation lifetime spectra in solid n-pentacosane were measured as a function of the temperature and positron irradiation time. Phase IV was found to be well visible in the cooling runs due to its easy supercooling; a small increase of ortho-Ps lifetime in phase IV compared to low-temperature phase indicates that predominant intramolecular defects are end- gauche ones. The effect of excess electron accumulation, typical mainly of polymers, was also observed; two kinds of electron traps were found. The main trap component had activation enthalpy 0.6 eV and disappeared at about 250 K; an additional fast accumulating component existed up to the temperatures near the transition to the rotator phase. (C) 2004 Elsevier B.V. All rights reserved.

  33. Goworek, T. Zaleski, R. Wawryszczuk, J., Observation of intramolecular defects in n-alkanes C25H52- C29H60 by the positron annihilation method, Chemical Physics Letters, 394(1-3), 2004, 90-92

    Abstract:
    The lifetime of ortho-positronium (o-Ps) in solid phases of long-chain n-alkanes was measured. In even-numbered alkanes, only one phase transition is observed; a very long lifetime is related to large free volumes near kink-shaped conformers. In odd-numbered alkanes (n less than or equal to 25), another solid phase (phase IV) is characterized by slightly increased o-Ps lifetime, which means the domination of end-gauche and double-gauche intramolecular defects. In C29H60 the structure of defects changes continuously in the whole range of phase IV. (C) 2004 Elsevier B.V. All rights reserved.

  34. Goworek, T. Wawryszczuk, J. Zaleski, R., Positronium inhibition in naphthalene at high pressures, Chemical Physics Letters, 387(4-6), 2004, 433-435

    Abstract:
    The lifetime Of ortho-positronium in solid naphthalene shortens with the rise of pressure as a result of diminishing the free spaces in which it is trapped. At the pressure approximate to100 MPa the size of these spaces is too small to accommodate the Ps atom (no energy level in the well), and the intensity of ortho-Ps component. initially weakly dependent on pressure, abruptly drops to zero. The narrow range in which ortho-Ps disappears indicates uniformity of Ps-accommodating voids. The nonmonotonous dependence of free positron lifetime is discussed in the framework of the blob model. (C) 2004 Elsevier B.V. All rights reserved.

  35. Djourelov, N. Goworek, T. Kondo, K. Suzuki, T. Zaleski, R., Enhancement of positronium formation by trapped electrons in solid n-nonadecane. Light bleaching effect, Physics Letters A, 323(3-4), 2004, 165-168

    Abstract:
    The increase of positronium, formation probability with the time of exposure to positron radiation, observed until now mainly in polymers, is shown for the case of polycrystalline n- nonadecane. Sample illumination by visible light restores the initial low value of positronium intensity owing to elimination of trapped electrons. Slowing down the growth of positronium intensity with prolonged positron irradiation is ascribed to inhibiting action of radicals. (C) 2004 Elsevier B.V. All rights reserved.

  36. Goworek, T. Zaleski, R. Wawryszczuk, J., Odd-even differences in o-Ps properties in some solid long- chain alkanes, Positron Annihilation, Icpa-13, Proceedings, (), 2004, 298-300

    Abstract:
    Positron lifetime spectra in solid long-chain alkanes were measured as a function of the storage time with positron source between the samples, and as a function of temperature and pressure, Large differences of the time constant of I-3 increase were observed for odd-numbered and even-numbered molecules. High pressures also induced the I-3 instability, but in this case the I-3 intensity decreased. In the rigid solid phase the o-Ps lifetime in even-numbered alkanes was shorter than in odd ones, which was due to different thickness of the empty layer between the lamellas in the crystal structure.

  37. Goworek, T. Zaleski, R. Wawryszczuk, J., Temperature, pressure and source-irradiation effects in positronium formation in some solid long-chain alkanes, Chemical Physics, 295(3), 2003, 243-253

    Abstract:
    Positron lifetime spectra in solid n-alkanes C19H40, C20H42 and C21H44 were measured. The effect of irradiation by positron source (increase of ortho-positronium intensity I-3), known from polymer studies, was found to appear also in alkanes; at about 100 K it can lead to doubling the intensity. The time constant of I-3 rise in n-eicosane increased with temperature, reaching the value of 60 h, while it decreased (down to 1 h) in its odd-numbered neighbours. The difference in ortho-Ps lifetimes between odd- and even-numbered alkanes can be explained by different thickness of interlamellar layer in the crystal structure. The variation of I-3 after application of high pressure was also observed, but in this case the intensity decreased with time. (C) 2003 Elsevier B.V. All rights reserved.

  38. Goworek, T., Mesopore characterization by PALS, Radiation Physics and Chemistry, 68(3-4), 2003, 331-337

    Abstract:
    The usefulness of positron annihilation techniques to the study of porous med a seems to be well proven. The models of pick-off annihilation for cylindrical and rectangular geometries give identical tau vs. pore size dependence, except the range of transverse size below 3 nm. Approximations contained in the model and the influence of Ps interaction with the medium (other than pick-off) on the observed o-Ps lifetime are discussed. (C) 2003 Elsevier Ltd. All rights reserved.

  39. Goworek, T. Wawryszczuk, J. Zaleski, R., Ortho-Ps annihilation in resorcinol at high pressure, Radiation Physics and Chemistry, 68(3-4), 2003, 577-579

    Abstract:
    The effect of high pressure on positron lifetime spectra in resorcinol was investigated. The o-Ps lifetime decreases with pressure continuously reaching the value which needs redefinition of DeltaR value used in the Tao-Eldrup model. The intensity of long-lived component unexpectedly rises with pressure increase, but it 450 MPa a drop of that intensity can indicate the approach of the minimal size of free volume able to trap Ps. preliminary data for hydroquinone are also presented. (C) 2003 Elsevier Ltd. All rights reserved.

  40. Zaleski, R. Wawryszczuk, J. Borowka, A. Goworek, J. Goworek, T., Temperature changes of the template structure in MCM-41 type materials; positron annihilation studies, Microporous and Mesoporous Materials, 62(1-2), 2003, 47-60

    Abstract:
    The lifetime spectra of positrons annihilating in MCM-41 ordered silica were measured as a function of temperature. It was found that in alkyltrimethylammonium templates degradation in vacuum begins at 380-400 K, that consists in breaking the long micelle cylinders, rise of gaps between the fragments and finally pore evacuation. The latter process is not completed and particularly in C12TAB the spectrum component belonging to fragmented template preserves its high intensity up to 520 K (the highest temperature in our experiment). The pore surface area over 1000 m(2)/g is achieved already at temperatures about 490 K. The lifetime of the longest-lived component indicates that practically all ortho-positronium, atoms leave the pores before annihilation. (C) 2003 Elsevier Inc. All rights reserved.

  41. Jasinska, B. Dawidowicz, A. L. Goworek, T. Wawryszczuk, J., Pore size determination by positron annihilation lifetime spectroscopy, Optica Applicata, 33(1), 2003, 7-12

    Abstract:
    Two Vycor glasses were investigated using PALS method. The model of pick-off annihilation in cylindrical free volume was applied to evaluate the pore radii. The o-Ps mean lifetime value in the pores changed from 6.5 to 40 ns in one glass, while from 13 to 51 ns in the other (softer) one. The determined pore radii, using a cylindrical shape model, were from 0.5 to 1.4 nm and from 0.7 to 1.6 nm, respectively.

  42. Zaleski, R. Wawryszczuk, J. Goworek, J. Borowka, A. Goworek, T., Vacuum removal of the template in MCM-41 silica studied by the positron annihilation method, Journal of Colloid and Interface Science, 262(2), 2003, 466-473

    Abstract:
    Positron annihilation lifetime spectra were measured for ordered silica before template (octadecyltrimethyl ammonium) removal. An intermediate lifetime component, well pronounced in the spectra, indicates the presence of structural defects in the template being the size of a surfactant molecule. At the temperature near 420 K a radical change of template structure occurs, consisting in appearance of disorder and the degradation of template; evaporation of the fragments begins. It seems that moderate heating (up to about 500 K only) in vacuum for several hours is an effective and simple method of template removal, leaving no pure carbon residue in the pore structure. The results are compared with the data from the liquid nitrogen adsorption/desorption method and elemental analysis, confirming the positron annihilation results. (C) 2003 Elsevier Science (USA). All rights reserved.

  43. Zaleski, R. Borowka, A. Wawryszczuk, J. Goworek, J. Goworek, T., Positron probing of the micellar template interior in MCM-41, Chemical Physics Letters, 372(5-6), 2003, 794-799

    Abstract:
    The properties of positronium annihilating in the micellar template in MCM-41 materials were investigated. For alkyltrimethylammonium bromide surfactants (C-n = 14,16,18) the template interior is liquid-like; the bubble model of positronium, elaborated for bulk liquids, can be applied. The equivalent surface tension is linearly dependent on temperature, like in classic liquids. A very long-lived component in o-Ps lifetime spectrum (approximate to 20 ns) can be ascribed to o-Ps in cracks in the elongated template structure. (C) 2003 Elsevier Science B.V. All rights reserved.

  44. Zaleski, R. Wawryszczuk, J. Goworek, T., Electron density at positron's site in MCM-41 ordered silica, Chemical Physics Letters, 372(5-6), 2003, 800-804

    Abstract:
    The lifetime of para-positronium in raw materials of MCM-41 class was measured as a function of temperature. It was found that the lifetime shortens when the template filling is removed and empty pores are created. Rough data are given also for phase transition in n-nonadecane. The electron contact density eta on the positron bound into positronium atom in the condensed media, determined in experiments, seems to be related more to the size of positronium trap than to the electronic properties of the medium. The role of contact density eta in the estimation of ortho-positronium lifetime in the bulk is discussed. (C) 2003 Elsevier Science B.V. All rights reserved.

  45. Wawryszczuk, J. Goworek, J. Zaleski, R. Goworek, T., Positron lifetime in mesoporous silica of MCM-41 type, Langmuir, 19(7), 2003, 2599-2605

    Abstract:
    Positron lifetime spectra in ordered mesoporous silica were measured. In the raw material (still containing the template) the voids in micellae reaching 1 nm in size were observed. Ortho-positronium (o-Ps) lifetime was found to be very sensitive to the carbon deposits left in pores after pyrolysis. In highly dispersed materials the relation between the pore diameter and positronium lifetime is shadowed by o-Ps escape from the pores to extragranular spaces. A very slow thermalization of o-Ps and increased lifetime of free positrons in low-density bulk were also observed.

  46. Goworek, T., Comments on the relation: positronium lifetime - free volume size parameters of the Tao-Eldrup model, Chemical Physics Letters, 366(1-2), 2002, 184-187

    Abstract:
    The author is discussing the parameters appearing in the Tao- Eldrup (TE) model describing the ortho-positronium (o-Ps) lifetime dependence on the size of free volume in which Ps is trapped. Parameter values are not universal, applicable to all media. The Ps penetration to the bulk should depend on Ps work function; the o-Ps decay rate is strongly influenced by the contact density factor. (C) 2002 Elsevier Science B.V. All rights reserved.

  47. Wawryszczuk, J. Zaleski, R. Goworek, T., Positron studies of vapour absorption and desorption by melamine-formaldehyde resin, Materials Chemistry and Physics, 76(3), 2002, 285-289

    Abstract:
    The lifetime spectra of positrons annihilating in vapour- absorbing polymers were measured in order to gain information about the change of dimensions and number of free volume holes during absorption and desorption. Absorption of water, methanol and ethanol vapours in melamine-formaldehyde resin leads to the shortening of ortho-positronium lifetime that can be explained by predominant filling larger holes by absorbed molecules, leaving smaller ones empty and accessible for ortho- positronium. Removal of molecules by outpumping the vapour increases the lifetime. It was found that the intensity of positronium formation does not follow the rule of proportionality to the number of free volume sites. (C) 2002 Elsevier Science B.V. All rights reserved.

  48. Goworek, T. Jasinska, B. Wawryszczuk, J. Zaleski, R. Suzuki, T., On possible deviations of experimental PALS data from positronium pick-off model estimates, Chemical Physics, 280(3), 2002, 295-307

    Abstract:
    The sources of possible inconsistency between the models of trapped positronium annihilation and experimental data are discussed. There are two such sources: excessive simplifications in the models of pick-off process and appearance of other processes not accounted in the models. In the case of the smallest traps, the penetration depth of ortho- positronium (o-Ps) wavefunction into the bulk can be larger than usually assumed. In traps of sizes exceeding I run the population of excited levels in the well has to be taken into account. For such large voids the sources of divergence of the experiment and model predictions can be: escape of o-Ps from open pores to outside, chemical interaction of o-Ps with pore surfaces (adsorbate layers), the presence of impurities inside free volumes. (C) 2002 Elsevier Science B.V. All rights reserved.

  49. Goworek, T., Positron annihilation in organic solids, Positron Annihilation - Icpa-12, (), 2001, 227-232

    Abstract:
    Recent results of positron annihilation studies of organic solids are reviewed. The studies concentrated around two problems: the models of pick-off process and influence of external factors on positronium lifetime (or intensity) in solids. Several variants of the pick-off model appeared and the geometries other than the spherical geometry were discussed. The positron annihilation method has proved its usefulness in investigations of structural changes in the medium. However, despite of 20 years which have passed since the first attempts to obtain positronium trap properties from annihilation experiments, the situation is not fully clear; new effects are observed at extreme temperatures and under pressure.

  50. Goworek, T. Jasinska, B. Suzuki, T., Ortho-Ps in bi-dispersional porous glass, Positron Annihilation - Icpa-12, (), 2001, 316-318

    Abstract:
    PAL spectra in porous Vycor glass with average pore radius approximate to 0.9 nm were measured as a function of temperature. Two long-lived components reflect the existence of two maxima in the pore radii distribution Relative intensities of these components drastically change with temperature.

  51. Goworek, T. Rybka, C., Positron annihilation in p-terphenyl doped with benzofluorene, Acta Physica Polonica A, 99(3-4), 2001, 369-371

    Abstract:
    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.

  52. Goworek, T. Rybka, C. Wawryszczuk, J., Positrons in doped pyrene, Physical Chemistry Chemical Physics, 3(9), 2001, 1769-1772

    Abstract:
    It was shown that the annihilation of free positrons produces a momentum distribution of annihilating pairs which is narrower for doped pyrene than for pure pyrene. We propose to explain this effect by enhanced annihilation on guest molecules; the enhancement factor for phenanthrene as admixture is of the order of 25. The increased annihilation probability on admixtures is not correlated with the respective effective numbers of electrons participating in annihilation per molecule (Z(eff)) observed in collisions of slow positrons with free molecules in vacuum.

  53. Goworek, T. Jasinska, B. Wawryszczuk, J. Ciesielski, K. Goworek, J., Mesopore characterization by positron annihilation, Characterization of Porous Solids V, (), 2000, 557-564

    Abstract:
    The model of a particle in a rectangular potential well, commonly used in positron studies of free volume distribution in polymers, was extended to larger voids, like pores. by taking into account the effects of positronium annihilation from the excited levels. The average lifetime calculated in this way (for room temperature) agrees quite well with the experimental data for various porous materials (silica gels, Vycor glass, porous polymers). The parameter DeltaR describing the penetration of positronium wavefunction into the bulk needs to be slightly enlarged in relation to that in small spherical voids.

  54. Goworek, T. Ciesielski, K. Jasinska, B. Wawryszczuk, J., Temperature variations of average o-Ps lifetime in porous media, Radiation Physics and Chemistry, 58(5-6), 2000, 719-722

    Abstract:
    Modification of the Tao-Eldrup model is proposed in order to extend its usefulness to the case of porous media. The modification consists in the transition from spherical to capillary geometry and in inclusion of pick-off annihilation from the excited states of a particle in the well. Approximated equations for pick-off constant in these states are given. The model was tested by observing the temperature dependences of o- Ps lifetime in various media. In the case of silica gels and Vycor glass with narrow pores, the model seems to work well, while for larger pores in Vycor unexpectedly long lifetimes appear in the range of lowest temperatures. (C) 2000 Elsevier Science Ltd. All rights reserved.

  55. Jasinska, B. Dawidowicz, A. L. Goworek, T., Positron lifetimes at the initial stage of pore formation in Vycor glass, Radiation Physics and Chemistry, 58(5-6), 2000, 723-726

    Abstract:
    The formation of narrow pores during leaching of Vycor glass by sulphuric acid was investigated using the positron lifetime technique. During the leaching process the pore diameter remained roughly constant (except for the case of cold leaching). The time of processing changed the total length of capillaries, but not their number; at the temperature 50 degrees C during 20 min of leaching the average leaching depth was 24 mu m. (C) 2000 Elsevier Science Ltd. All rights reserved.

  56. Jasinska, B. Dawidowicz, A. L. Goworek, T. Radkiewicz, S., Processes during liquation of Vycor glass studied by positron annihilation lifetime spectroscopy, Physical Chemistry Chemical Physics, 2(14), 2000, 3269-3274

    Abstract:
    This study concerns Vycor glass subjected to liquation in the temperature range 725-950 K and leached in hot acid in order to produce pores. Three stages of pore creation were observed with increasing liquation temperature: The initial structure of nanopores with a radius of about 0.5 nm (like in the nonliquated glass) disappears, giving way to the next group, dominant at 875 K, whose average radius is 1.15 nm, independently of the liquation time. The spread of these radii is narrow, 0.15 nm. At the highest temperatures, the intensity of this component decreases rapidly (down to 1%), and large pores begin to grow; the specific surface area diminishes. About 900 K, the total positronium formation probability decreases significantly, which can be ascribed to the partial crystallization of silica.

  57. Goworek, T. Suzuki, T. Hamada, E. Kondo, K. Ito, Y., Pressure quenching of positronium in solid biphenyl, Chemical Physics, 255(2-3), 2000, 347-351

    Abstract:
    In solid crystalline biphenyl, ortho-positronium (o-Ps), abundant at normal pressure, can be eliminated by application of a pressure of about 100 MPa. The lifetime of o-Ps shortens with the rise of pressure, as a result of diminishing of the free spaces in which it is trapped. However, the lifetime of para-Ps seems to increase, and at 80 MPa, it cannot be distinguished from that of free positrons. Another new effect is the reappearance of the long-lived component in the positron lifetime spectrum at high pressures. The same effect of positronium reappearance, although not so well pronounced, was found in biphenyl at low temperatures. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

  58. Goworek, T. Rybka, C., Positronium formation in chrysene, Journal of Radioanalytical and Nuclear Chemistry, 242(1), 1999, 225-226

    Abstract:
    The lifetime spectra of positrons in solid chrysene were measured at temperatures of 160-300 K. The temperature dependence of positronium yield suggests Ps trapping on thermally produced free volumes. Those volumes cannot be vacancies, but rather the intermolecular free spaces; thermal motions of molecules can increase these spaces to the extent making Ps creation energetically possible.

  59. Ciesielski, K. Dawidowicz, A. L. Goworek, T., Positronium in small and large voids in Vycor glass, Acta Physica Polonica A, 95(4), 1999, 483-486

    Abstract:
    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 x 10(-5) cm(2)/s.

  60. Jasinska, B. Koziol, A. E. Goworek, T., Void shapes and o-Ps lifetime in molecular crystals, Acta Physica Polonica A, 95(4), 1999, 557-561

    Abstract:
    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.

  61. Ciesielski, K. Dawidowicz, A. L. Goworek, T. Jasinska, B. Wawryszczuk, J., Positronium lifetimes is porous Vycor glass, Chemical Physics Letters, 289(1-2), 1998, 41-45

    Abstract:
    The ortho-positronium lifetimes in porous Vycor glass were measured in a wide range of pore radii. A good correlation of the most probable lifetime with the model accounting for the population of upper levels of a particle in the potential well was observed. For channel-like voids (capillaries in porous media) a new value of the empirical parameter, Delta R, entering in the semi-empirical Eldrup equation, is proposed: Delta R = (0.191 +/- 0.007) nm; it explains some of the discrepancies previously found in crystalline solids. (C) 1998 Elsevier Science B.V. All rights reserved.

  62. Goworek, T. Ciesielski, K. Jasinska, B. Wawryszczuk, J., Positronium states in the pores of silica gel, Chemical Physics, 230(2-3), 1998, 305-315

    Abstract:
    The lifetime spectra of positrons annihilating in silica gels with pore radii from 2 to 10 nm were measured, and the results compared with the predictions made by the Tao-Eldrup model extended to include the annihilation of positronium from the excited states in the potential well. The long-lived parts of the spectra were analysed as a sum of 1-3 components with continuous lambda distributions (gaussian in the logarithmic scale). The Ip state population in Si40 is demonstrated; the decay constant of ortho-Ps in it agrees quite well with the model estimate and the growth of intensity with temperature resembles that expected from the Boltzmann distribution. In one-component fit the lifetimes at the maximum of lambda distribution at a room temperature for various pore radii were found also to be very close to the calculated lifetime averaged over all states of the positronium particle in the rectangular potential well. (C) 1998 Elsevier Science B.V. AII rights reserved.

  63. Goworek, T. Ciesielski, K. Jasinska, B. Wawryszczuk, J., Lifetimes of o-Ps in the pores of silica gel, Positron Annihilation, (), 1997, 296-298

    Abstract:
    An attempt to extend the popular Tao-Eldrup model (TEM) toward large voids is presented. The lifetimes of o-Ps in various states of the particle in the spherical and cylindrical wells have been calculated. The existence of o-Ps in excited well state("1p-like" state) is demonstrated.

  64. Jasinska, B. Koziol, A. E. Goworek, T., Free volumes in perfect molecular crystals and o-Ps lifetime, Positron Annihilation, (), 1997, 308-311

    Abstract:
    The size of free volume in molecular crystals was estimated from crystallographic data. The threshold value of free volume radius allowing Ps formation was found to be (0.12 - 0.13) nm. The lifetime estimates were compared with experimental data for 13 crystalline solids.

  65. Goworek, T. Ciesielski, K. Jasinska, B. Wawryszczuk, J., Positronium in large voids. Silicagel, Chemical Physics Letters, 272(1-2), 1997, 91-95

    Abstract:
    The lifetime spectra of positrons annihilating in a porous medium (silicagel) were measured as a function of temperature. In the low temperature limit the lifetime of o-Ps in the pores agrees rather well with the popular Tao-Eldrup formula. With the rise of temperature the second long-lived component grows; that component was identified as o-Ps annihilation from the Ip- like state in the potential well. The lifetimes of o-Ps in various states of the spherical well have been calculated. The experimental value of the lifetime for the Ip state is in good agreement with the model calculation. (C) 1997 Elsevier Science B.V.

  66. Goworek, T. Jasinska, B. Wawryszczuk, J. Ciesielski, K., Positronium formation in dihydroxybenzenes, Journal of the Chemical Society-Faraday Transactions, 93(8), 1997, 1573-1578

    Abstract:
    Lifetime spectra of positrons in solid dihydroxybenzenes have been investigated over a wide range of temperatures. The variation of Ps formation probability with temperature and relative permittivity is an argument in favour of spur model concepts in the description of Ps formation in solids. An attempt to estimate the average excess electron-positron distance in the spur is undertaken. Detailed analysis of the e(+) lifetime spectrum in hydroquinone indicates that the long- lived part of that spectrum can be a continuous distribution of lifetimes, not a discrete component.

  67. Goworek, T. Jasinska, B. Wawryszczuk, J. Ciesielski, K., Positrons in benzenediols, Journal of Radioanalytical and Nuclear Chemistry-Articles, 210(2), 1996, 441-449

    Abstract:
    Temperature dependence of positronium formation probability P was measured for catechol, resorcinol and hydroquinone. The variations of P with temperature and relative permittivity can be explained in the framework of spur reaction mode. It seems that the spur processes are important at the first stage of Ps formation, the final value of P depends also on free volume availability and trapping efficiency.

  68. Jasinska, B. Koziol, A. E. Goworek, T., Ortho-positronium lifetimes in nonspherical voids, Journal of Radioanalytical and Nuclear Chemistry-Articles, 210(2), 1996, 617-623

    Abstract:
    Experimental values of o-Ps lifetime were compared with model calculations, approximating the voids in a solid by spheres or cuboids. In the case of resorcinol the model of spherical voids is well consistent with the experimental data. For channel-like free volumes the formula, analogous to that for spheres, gives systematically the values of o-Ps lifetime too large. No positronium is found in anthracene, although the radius of free volume is larger than 0.97 Angstrom at which the Ps binding energy should be zero.

  69. Wawryszczuk, J. Goworek, T., Does the p-Ps lifetime agree with expectations from magnetic quenching?, Journal of Radioanalytical and Nuclear Chemistry-Articles, 211(1), 1996, 161-164

    Abstract:
    Magnetic quenching experiments with naphthalene give small value of density parameter eta. Corresponding lifetime of p-Ps should be equal or close to that of free annihilation. Contrary to expectations, we obtained the lifetime of short-lived component tau(1) = 180 ps (its intensity is larger than predicted by statistical weight rule). Generally, the experiments with the use of magnetic field give smaller eta value than these using zero-field methods.

  70. Duplatre, G. Kajcsos, Z. Goworek, T. Varga, L. Liszkay, L. Billard, I. Lazar, K., Magnetic quenching effects on long-lived positronium states in zeosil, Journal of Radioanalytical and Nuclear Chemistry-Articles, 211(1), 1996, 225-235

    Abstract:
    Positron annihilation data collected for zeosil (a zeolite-type solid allowing abundant Ps formation) are discussed, with particular attention payed to the longest-lived positronium states as observed in lifetime spectroscopy experiments. To unravel the nature of these components, additional information was sought by applying an external magnetic field and by recording the total energy distribution of the annihilation radiation. From both the lifetime and the extended Doppler spectra, it is shown that positronium presents a very long- lived component with a relative intensity of ca. 30% and a lifetime close to the intrinsic tripler lifetime (140 ns), therefore undergoing almost no pick-off annihilation. From the magnetic field experiments, the contact density parameter has a value of essentially 1, which is characteristic of an unperturbed triplet Ps state in vacuo. This positronium state therefore does not appear to interact with the medium.

  71. Goworek, T. Jasinska, B. Wawryszczuk, J. Ciesielski, K., Positrons in resorcinol, Journal of the Chemical Society-Faraday Transactions, 92(9), 1996, 1573-1578

    Abstract:
    The temperature dependence of parameters describing the positron lifetime spectrum in resorcinol has been investigated. It was found that the solid phase transition point is located near 353 K, but over several K the transition is very slow, its rate lies in the range of days. The dimensions of positronium traps (free volumes) were estimated from crystallographic data. In spite of the non-spherical shape of these free volumes the o-Ps lifetime and its temperature dependence calculated from the popular Tao-Eldrup formula are relatively consistent with the experimental values. The main features of the temperature dependence of the positronium formation probability can be explained in the framework of the spur model. The increase in probability in the region below the transition point may be the result of ion-molecule reactions in the spur, or of the dependence of the Ps trapping probability on the trap size.

  72. Goworek, T. Jasinska, B. Wawryszczuk, J., Positrons in benzenediols - Does the spur model work in a solid?, Abstracts of Papers of the American Chemical Society, 211(), 1996, 34-NUCL


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