Positronium formation in sol–gel-prepared silica-based glasses: temperature and positron-irradiation effect
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Positronium
Annihilation
Silica glass
The two quantities which can be observed by annihilation of an individual positron in condensed matter are the positron age τ which is the time interval between implantation and annihilation of the positron, and the momentum p of the annihilating positron-electron pair. Correlated measurements of these quantities (Age-Momentum Correlation, AMOC) are an extremely powerful tool for the study of reactions involving positrons. It not only provides the information obtainable from the two constituent measurements but allows us to follow directly, in the time domain, changes in the positron-electron momentum distribution of a positron state (e.g., positronium slowing-down, cf. Sect. 3.3) or transitions between different positron states (e.g., trapping of positrons, cf. Sect. 3.2, chemical reactions of positrons and positronium, cf. Sec. 3.1, or self-localization of positronium from a metastable positronium state in liquid rare gases, cf. Sect. 3.4). So far AMOC measurements have only been performed at MeV positron beams [1,2]. It is highly recommended that the advantage of the beam-based AMOC technique should be applied to keV positron beams, too.
Positronium
Annihilation
Momentum (technical analysis)
Metastability
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في هذا البحث تم دراسة آليّة الفناء البوزيتروني للتغيُّرات التي تطرأ على بعض المواد وذلك من خلال دراسة أنواع مختلفة من الفناء البوزيتروني مثل الفناء الحر والفناء المقيَّد وفناء البوزيترونيوم بنوعيه البارا والأورثو بوزيترونيوم وتبيّن لنا أن فناء الأورثو بوزيترونيوم يمر بمرحلة الفناء الحر والذي يُسمى فناء الالتقاط pick- off. وتم حل معادلات فناء البوزيترون بطريقتين حسابيّاً وبرمجيّاً (باستخدام البرنامجين الحاسوبيّين scilab- 6.0.1 وMathematica12) وفق شروط محددة تم اعتبار أنه في اللحظة لم يكن البوزيترونيوم (بنوعيه البارا والأورثو) قد تشكل بعد، وتم دراسة جميع المراحل التي يتم فيها الفناء، ففي المرحلة الأولى دُرِسَ تفكك البوزيترون الحر وفي المرحلة الثانية تفكك البارا بوزيترونيوم وفي المرحلة الثالثة تفكك الأورثو بوزيترونيوم، ووُجِدَ أن معامل التناقص في عدد البوزيترونات الحرة يُعبِّر في الوقت ذاته عن احتمال تشكل البوزيترونيوم، ودُرِسَ طيف الفناء لكل من البوزيترون والبوزيترونيوم بنوعيه البارا والأورثو ومن خلال حل معادلات الفناء تم تحديد حالة البوزيترون التي يكون عليها ضمن شروط مختلفة وحسب عمر كل حالة، وهذا يستدعي دارسة طيوف الفناء لمواد مختلفة للاستفادة من ذلك في كشف العيوب التي تعاني منها المادة وتحسين بنيتها.
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Positronium
Annihilation
Positron emission
Antiparticle
Positron Lifetime Spectroscopy
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By combining angular correlation results of the positron annihilation quanta with lifetime results it is found that a weak but long-lived (599\ifmmode\pm\else\textpm\fi{}10 ps) lifetime component can be correlated with the para-positronium peak in angular correlation measurements when taking into account both pickoff annihilation and conversion of positronium. Conversion has not been considered in earlier works.
Positronium
Annihilation
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The study of positronium and positron reactions provides an alternative means to investigate hot radical reactions in the radiation spur. Positron and positronium react with radicals or others in the radiation spur created by the energetic positron itself. The positron may compete with other electron acceptors for electrons to form positronium. The positron also may be captured by a positron acceptor. The positronium formed may react with a radical or others to form a positronium compound. These reactions can be investigated by using the methods of positron annihilation, e.g. lifetime, angular correlation, and Doppler broadening measurements.
Positronium
Annihilation radiation
Positron Lifetime Spectroscopy
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Positronium
Thermalisation
Antiparticle
Positron emission
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Current knowledge of positron-surface interactions is summarized with applications to surface science. The review interlinks four areas of positron physics which have developed almost independently, but which are possibly equally relevant to the study of surfaces: (1) the trapping at surfaces or the escape as positronium of positrons implanted in small solids, (2) the interaction of positrons with crystal defects and voids in solids, (3) the emergence of positrons from positron beam moderators, and (4) the formation of positronium in ground and excited states by positrons backscattered from solid surfaces. The positron method offers a potentially very sensitive technique for the systematic study of surfaces.
Positronium
Antiparticle
Positron emission
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The total cross sections for positron and positronium scattering from H2O molecules have been measured for incident energies between 7 and 417 eV, and 10 and 100 eV, respectively. The experimental system has been characterized with respect to its angular acceptance of both scattered positrons and positronium in order to correct the data for forward-scattering errors once differential cross sections become available. The present data are compared with previous results for electron and positron total cross sections.
Positronium
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The effects of the positron (e(+)) annihilation in e(+) scattering by the H atom are included directly in the Hamiltonian as an absorption potential, and hence the finite lifetimes Gamma(ns) of positronium (Ps) in states ns are automatically taken account of. The Schrödinger equation is solved using the hyperspherical close-coupling method. The annihilation and Ps formation are shown to be inseparable near the Ps(1s) threshold E(th); Ps formation constitutes just an indirect pathway to annihilation. The annihilation cross section, which would diverge near E(th) if Gamma(1s) were infinite, connects smoothly across E(th) to the cross section for Ps formation, which is meaningful only at energies well above E(th).
Positronium
Annihilation
Hamiltonian (control theory)
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Detailed theoretical studies of annihilation in low-energy positron - atom scattering reveal a very pronounced enhancement of the annihilation rate in the vicinity of the positronium formation threshold, providing qualitative confirmation of the predictions of a recent model in which the enhancement is attributed to virtual positronium formation. The present investigations relate specifically to atomic hydrogen but similar results have also been obtained for helium. From investigations of annihilation at energies above the positronium formation threshold, we conclude that no clear distinction can be made between direct annihilation and annihilation following positronium formation.
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Annihilation
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