Positron impact ionisation phenomena

A magnetically guided beam of nearly-monoenergetic slow positrons has been used to study positron impact ionisation phenomena in gases. A novel hemispherical scattering cell incorporating an efficient ion extraction and detection system has been developed and has been utilised throughout this work. The energy spectra for the electrons ejected around 0° relative to the incident beam, following positron impact ionisation of Ar, have been measured by a time-of-flight method and a retarding electric field analyzer. The angular acceptance of the electron detection system has been estimated and used to compare the measured spectra with the double differential cross-sections calculated by Mandal et al (1986), Sil et al (1991) and Schultz and Reinhold (1990). The importance of the electron-capture-to-the-continuum process is discussed in this context and found to be minor at small forward angles, in contrast to the case of heavy positively charged projectiles. The apparatus was modified to produce a pulsed beam of slow positrons and utilised to measure in detail the total ionisation cross-section (Qt+) for a variety of atomic and molecular targets. For Ar, He and H2, Qt+ which includes contributions from Ps formation, has been subtracted from corresponding total cross-sections, in order to deduce the behaviour of the elastic scattering cross-section (Qel) in the vicinity of the Ps formation threshold (Eps). Here a small change in the gradient of Qel, has been found. The energy dependencies of the Qt+ for He, Ne and Ar, close to Eps have been interpreted in terms of threshold theory. In the case of Ar the outgoing Ps appears to be predominantly s-wave in character. For He and Ne the analysis suggests that the Ps contains significant contributions from a number of partial waves. In the case of O2, structure in Qt+ has been found, which is attributed to coupling between two inelastic channels, namely Ps formation and excitation to the Schuman-Runge continuum

Pay Your Debts

Debt is duty, and laws do not make or unmake duty. They simply define duty, and often do that very imperfectly. What you owe to another that other has a right to have; for obligations \u27on one side involve correspondent rights on the other. Abridged from a sermon published in The Standard.https://place.asburyseminary.edu/ecommonsatsdigitalresources/1171/thumbnail.jp

Near-threshold ionization of He and H-2 by positron impact

The single ionization cross sections for He and H-2 by positron impact have been measured in the first few eV above threshold and found to exhibit a different energy dependence from the corresponding electron results. If the data, between 1 and 3 eV above threshold, are fitted by a power law, exponents of 1.99 +/- 0.19 and 1.70 +/- 0.11 are obtained for He and H-2, respectively. This agrees qualitatively with extensions of the Wannier theory in that the exponent is larger than for electron impact. The quantitative disagreement with the predicted value of 2.65 might indicate that the range of validity of this theory is smaller than expected

Vacancy Clusters at Nanoparticle Surfaces

The authors detect vacancy clusters at Au nanoparticle surfaces using a combination of positron lifetime spectroscopy, 1- detector, and 2-detector measurements of Doppler broadening of annihilation radiation. Gold nanoparticles are formed by MeV implantation of gold ions into MgO (100) followed by annealing. Clusters of two Mg and two O vacancies (v{sub 4}) are attached to the gold nanoparticle surfaces within the projected range (R{sub p})

The lives and deaths of positrons in the interstellar medium

We reexamine in detail the various processes undergone by positrons in the ISM from their birth to their annihilation using the most recent results of positron interaction cross sections with H, H2 and He. The positrons' lives are divided into two phases: the 'in-flight' phase and the thermal phase. The first phase is treated with a Monte Carlo simulation that allows us to determine the fraction of positrons that form positronium and annihilate as well as the characteristics of the annihilation emission as a function of the medium conditions. The second phase is treated with a binary reaction rate approach, with cross sections adopted from experimental measurement or theoretical calculations. An extensive search and update of the knowledge of positron processes was thus undertaken. New reaction rates and line widths have been obtained. We investigate the treatment of the complicated interactions between positrons and interstellar dust grains. New reaction rates and widths of the line resulting from the annihilation inside and outside of the grain have been obtained. The final results of our calculations showed that dust is only important in the hot phase of the ISM, where it dominates all other processes. Combining the new calculations, we have constructed annihilation spectra for each phase of the ISM, considering various grain contents, as well as an overall combined spectrum for the ISM as a whole.Comment: 16 pages, 6 figures. accepted in Astronomy and Astrophysic

The culture of institutional governance at a university in Laos : an ethnographic exploration

This paper reports on an ethnographic exploration of the culture of institutional governance at one of the five public universities in Laos. Drawing on documentary materials, on-site observations made over an extended period, and semi-structured interviews conducted with 31 academic managers from across all management levels at the site institution, the paper seeks to throw light on the institution’s embedded practices of internal governance, as well as on the beliefs, values and aspirations associated with those practices. A constructivist and interpretive methodology was employed to generate data. The picture to emerge is one of a university hemmed in by State controls and ideology, in which there is an overwhelmingly bureaucratic and managerial culture, and in which a governance structure that could potentially support institutional and academic autonomy does not do so. Resource constraints also contribute to the institution’s limited capacity to exercise autonomy