Some aspects of the interactions of orthopositronium with perfect and defect surfaces of insulating materials

Dauwe, C.; Waeyenberge, Bartel Van; Segers, D.; Hoecke, Toon Van; Kuriplach, J.

doi:10.1007/bf02056374

Cited by 12 publications

(11 citation statements)

References 9 publications

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“…The cooling of Ps in gases, powders, and porous materials has been studied for many years using a variety of experimental methods [57][58][59][60][61][62]. A model developed by Nagashima and co-workers to describe Ps energy loss by collisions (with gas atoms or grains in powders) has been able to successfully describe Ps cooling rates in materials with mean free paths ranging from 5 to 70 nm [63].…”

Section: B Ps Coolingmentioning

confidence: 99%

Positronium cooling in porous silica measured via Doppler spectroscopy

Crivelli²,

et al. 2010

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We have measured the kinetic energy of positronium (Ps) atoms emitted into a vacuum from a porous silica film subsequent to positron bombardment, via the Doppler spread of the linewidth of the Ps 1 3 S-2 3 P transition. We find that the deeper in the target film that positrons are implanted the colder is the emitted Ps, an effect we attribute to cooling via collisions in the pores as the atoms diffuse back to the film surface. We observed a lower limit to the mean Ps kinetic energy associated with motion in the direction of the laser, E x = 42 ± 3 meV, that is consistent with conversion of the confinement energy of Ps in the 2.7-nm-diameter pores to kinetic energy in vacuum. An implication is that a porous sample would need to be composed of pores greater than around 10 nm in diameter in order to produce thermal Ps in vacuum with temperatures of less than 100 K. By performing Doppler spectroscopy on intense pulses of Ps we have experimentally demonstrated the production of many excited-state Ps atoms simultaneously, which could have numerous applications, including laser cooling and fundamental spectroscopic studies of Ps and the production of antihydrogen.

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Section: B Ps Coolingmentioning

confidence: 99%

Positronium cooling in porous silica measured via Doppler spectroscopy

Crivelli²,

et al. 2010

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“…Ascribing the SLP components to the distortion of the exponential shape of the long-lived components due to positronium thermalization was hard to verify. The classical ETLA model of Ps thermalization [10] did not seem to be suitable for describing thermalization in small pores; the ETLA model fitting implemented in LT did not show acceptable results. In order to minimize large dispersion of the results present when MELT is used, mean lifetimes (without distribution) were fitted by LT. To achieve correct results for pore related lifetimes, the SLP components had to be taken into account.…”

Section: Intensitymentioning

confidence: 78%

Temperature dependence of positronium lifetime in cylindrical pores

Zaleski¹,

Goworek²,

Kierys³

2007

Phys. Status Solidi (c)

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Positron Annihilation Lifetime Spectroscopy (PALS) results for ordered mesoporous silica sieves (MCM-41 and its fibre form MSF) are used to verify predictions of the Extended Tao-Eldrup (ETE) model. Cylindrical pores (R ~ 1-2 nm) in these materials have well defined geometry and size, thus the structure of MCM-41 and MSF reproduces the model assumptions. Positron lifetimes were measured in the range 100-500K. In MCM-41, due to low intensity of pore related ortho-positronium component, it is difficult to determine the component's lifetime precisely. Accuracy of the lifetime fitting is better for MSF, where temperature dependence is in a good agreement with the ETE model results. For comparison, the ETE model predictions fit the lifetime of the pore related component in the Vycor sample only for T > 200 K. Below this value the lifetime exceeds the model predictions by over 30%.1 Introduction The ortho-positronium (o-Ps) lifetime in dielectrics is related to the size of electronfree volume. In the case of mesoporous materials (pore size 2-50 nm) at room temperature, the relation is approximated by the extended version of the Tao-Eldrup model (ETE), which takes into account population of excited levels of Ps trapped in the potential well [1]. Pores are usually described as infinite long cylinders that should be accounted for by assuming proper geometry in the model calculations. The correctness of the pore size estimation based on the ETE model for cylindrical geometry is confirmed by experimental results for Vycor glasses and silica gels at room temperature [2]. In Vycor glasses and silica gels, pores form a complicated and usually interconnected structure. Describing the pore shape as infinite long cylinders in particular provides only a significant approximation. Although, such an assumption is sufficient when distinct differences between o-Ps lifetimes in pores of various sizes are investigated.In addition to the relationship between o-Ps lifetime and pore size, the ETE model predicts positron lifetime dependence on temperature. In order to verify the effects of temperature change on the lifetime, highly porous samples with well-defined pore shape and size are preferred. These requirements are fulfilled by mesoporous silica sieves, designated as M41S [3]. An interesting member of this group is MCM-41, which has a hexagonal arrangement of uni-sized cylindrical pores. Unfortunately, the pores in MCM-41 are short (sizes of the porous grains are tens to hundreds of nanometer), resulting in Ps escape from the pores and low intensity of the pore-related lifetime component [2]. More suitable materials are mesoporous silica fibres (MSF), where similar to MCM-41, a honeycomb-like structure of the pores forms long rods which have a diameter of a few micrometers [4]. Moreover, it is possible to produce

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“…It could suggest, that the third component does not correspond to the o-Ps annihilation inside any free volume, but it is the artificial one, being the result of a non-exponential decay of the longest-lived component, due to o-Ps thermalization -such explanation was proposed e.g. in [20,21].…”

Section: Methodsmentioning

confidence: 99%

Aerogel IC3120 Under High Pressure Exerted Mechanically and by Nitrogen

Gorgol

Zgardzińska

2017

Acta Phys. Pol. A

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Highly porous IC3120 silica aerogel was subjected to the high pressure up to 450 MPa while the positron annihilation lifetime spectra were collected. The pressure was delivered to the investigated samples in two ways: by pistons and by gas (nitrogen) penetrating the aerogel. The evolution of all PALS parameters was discussed. With the increase of the pressure, shortening of ortho-positronium lifetimes (different, depending on the pressure method introduced) was observed. Similarity between the dependence of the longest-lived o-Ps component lifetime on the pressure, for the sample affected with nitrogen molecules and pure nitrogen was observed. It suggests, that the nitrogen fills the largest free volumes of the aerogel. The pressure exerted mechanically causes much smaller decrease of free volume available for positronium. The comparison of PALS results with the electron microscopy images, obtained after removing the pressure, confirmed that more intense and lasting changes were caused by affecting the aerogel with the pistons.

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Some aspects of the interactions of orthopositronium with perfect and defect surfaces of insulating materials

Cited by 12 publications

References 9 publications

Positronium cooling in porous silica measured via Doppler spectroscopy

Positronium cooling in porous silica measured via Doppler spectroscopy

Temperature dependence of positronium lifetime in cylindrical pores

Aerogel IC3120 Under High Pressure Exerted Mechanically and by Nitrogen

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