Papers by Keyword: Positronium

Abstract: The 2D Positronium (2D Ps) atom confined in the 2D cave has been considered and its properties were compared with the 3D Positronium located in the infinity square well potential. Basing on the solution of Schrödinger equation for the 2D hydrogen atom the wave function of the 2D Ps was given. It allows us to calculate, for instance the angular correlation of the annihilation radiation (ACAR) of such a system. It was shown that the ACAR is much broad than ACAR for the 3D Ps and that for the Ps in the bubble model.

Abstract: Positron lifetime spectroscopy was employed for investigation of the size distribution of free volume holes in Nafion N-1110 membrane. Size distribution of free volume holes was determined in the as-received and dried membrane and in the membrane immersed in water at various temperatures. It was found that absorbed water causes reduction of the mean size of free volume holes and increase of their dispersion and concentration. This indicates that free volumes are split by swollen ionic clusters absorbing water. The kinetics of water uptake and the amount of absorbed solvent was determined by in-situ measurement of the weight increase.

Abstract: The detection of electron anti-neutrinos is generally carried out by searching for the coincidence of the reaction products, neutron and positron, in liquid scintillators. However, in these last a positron may form positronium (Ps) with an electron of the medium; efficiency of the process may be high. Furthermore, the triplet ground state sublevel (o-Ps) has lifetimes of a few ns. These features introduce distortions in the time distribution of the emitted photons, which is essential for position reconstruction and pulse shape discrimination algorithms in anti-neutrino experiments. This drawback can be favorably exploited by using o-Ps as a probe to detect anti-neutrinos in the scintillator. We report results of positron annihilation lifetime measurements in some organic liquid scintillators used for neutrino experiments. The o-Ps induced distortion of the scintillation photon emission time distribution may enhance the anti-neutrino detection.

Abstract: Positron chemistry refers to chemical processes of high-energy positrons injected into molecular substances, the most interesting of which is the formation of positronium (Ps), the hydrogen-like bound state between a positron and an electron. Ps is formed predominantly by fast intra-track radiation chemical processes. In polymers it tends to be localized in intra/inter-molecular open space in the sparsely packed amorphous structure. Whilst short-lived singlet para-positronium (p-Ps) undergoes self-annihilation, the positron in long-lived triplet ortho-positronium (o-Ps) annihilates with one of the spin opposite electrons bound in the surrounding polymer molecules. This process is called pick-off annihilation. The pick-off annihilation lifetime reflects the polymer chain packing through the size of the volume, where Ps is localized. Positrons are used to probe the amorphous structure of various polymeric systems. In this article, basic concepts and experimental techniques of positron chemistry in polymers as well as applications to the characterization of functional polymeric materials are overviewed.

Abstract: This article deals with the insight of using the positron (the simplest antimatter) as an entity that non-destructively probes material structure to the extent of atomic size defects, also describes the tools that have been in practice in recent times and in the front line activities. It also guides the reader on the use of (monoenergetic) slow positron beams that are currently available to study surface/ near surface structural details of various advanced materials. In addition, the bound state of electron and positron (positronium) is touched upon along with various conjectures for harnessing such species and utility of such light quasi-stationary states have been included. A brief mention has been made on the application of positrons towards medical diagnostic aspects and its recent importance in an astrophysical context.

Abstract: Positronium is an ideal system for the research of Quantum Electrodynamics (QED), especially for QED in bound state. The discrepancy of 3.9σ was found recently between the measured HFS values and the QED prediction of O(α3). It might be due to the contribution of unknown new physics or systematic problems in the all previous measurements. We propose a new method to measure HFS directly and precisely. A gyrotron, a novel sub-THz light source is adopted with a Fabry-Pérot cavity with high finesse and an efficient transportation system in order to obtain sufficient radiation power at 203 GHz. The present status of the optimization studies and the current design of the experiment are described.

Abstract: The ground state hyperfine splitting of positronium, , is sensitive to high order corrections of QED. A new calculation up to O( ) has revealed a 3.9 discrepancy between the QED prediction and the experimental results. This discrepancy might either be due to systematic problems in the previous experiments or to contributions beyond the Standard Model. We propose an experiment to measure employing new methods designed to remedy the possible systematic errors which may have affected the previous experiments. Our experiment will provide an independent check of the discrepancy. The prototype run has been performed and a result of = 203.3804 0.0022 (stat., 11 ppm) 0.0081 (sys., 40 ppm) GHz has been obtained. Compensation magnets to obtain O(ppm) magnetic field uniformity has been developed. The final run will start soon and a measurement with a precision of O(ppm) is expected within a few years.

Abstract: The modification of the extended Tao-Eldrup model accounting ortho-positronium quenching in air is presented. Taking into account quenching by oxygen molecules adsorbed on the surface of porous material gives reasonable agreement between results of the model and the experimental positron annihilation lifetime spectroscopy data. Pore size distributions calculated using this model from the spectra for MCM 41 mesoporous sieve obtained in air, oxygen or vacuum are compared and discussed taking into account effect of ortho-positronium migration from small open pores to the larger ones. The rates of ortho-positronium quenching by air (47.2 µs-1 MPa 1), oxygen (220 µs-1 MPa-1) or nitrogen (1.7 µs-1 MPa-1) obtained from pressure dependences of the lifetimes observed in MCM 41 agree reasonably with the experimental results of other authors, if the correction for oxygen adsorbed on the surface is applied.

Abstract: Positron lifetime and Doppler broadened annihilation line at wide temperature range 15-300 K was measured. The phase transition at melting temperature and its temperature shift in undercooled water was observed. The change in temperature dependence of long lifetime component in region 150 K was observed. The lifetime distributions (by program MELT) were calculated in full temperature range with an accent for the phase transition temperature range.

Abstract: Temperature dependence on S(t), positron age dependent shape parameter of annihilation gamma-rays, in water was observed. S(t) at the ortho-Ps pick off annihilation time region were higher at higher temperatures. It is indicating that there are more reactions of ortho-Ps at higher temperature. Hence it was successfully indicated that the longest lifetime, i.e. ortho-Ps component, on positron annihilation lifetime spectra for water could be shorter at higher temperatures. The radical reactions, such as PsOH formation or ortho-Ps oxidation by hydroxyl radicals, are spin dependent reactions. The geminate pair of ortho-Ps and hydroxyl radical in water can give spin dependent reaction and the enhancement of S(t) with certain time interval, i.e. quantum beats, were expected. It was observed even for water at 11°C. It is probably indicating the hyperfine coupling constants of hydroxyl radicals in water.