Showing papers on "Positronium published in 1967"

Positron Lifetimes in Metals

Abstract: Positron decay curves in various metals have been measured, using careful sample preparation techniques and a delayed-coincidence system with 0.30-nsec (full width at half-maximum) prompt time resolution and ${10}^{4}$:1 peak-to-background ratio. Evidence was found that the annihilation in pure metals under ideal conditions is characterized by a single exponential decay, the second lifetime usually observed being an artifact of sample preparation. In the case of Mg with ${\mathrm{Na}}^{22}$ produced in situ by the ($p, \ensuremath{\alpha}$) reaction, a single exponential decay over approximately four decades was observed. In a plot of annihilation rate versus conduction electron density, the observed rates for the nearly-free-electron metals fell close to the theoretical curve of Kahana, approaching the spin-averaged positronium rate at low densities. Compared with these rates, annihilation rates at a given valence-electron density were found to be higher in the transition metals and lower in Be, Bi, and in the semiconductors Si and Ge, in agreement with qualitative ideas about the one-positron many-electron system in real metals. Within the experimental error (\ifmmode\pm\else\textpm\fi{}2.5%), no difference in lifetime was observed between intrinsic and heavily $n$-type Si, and similarly for Ge.

Positronium formation by positron impact on hydrogen

Abstract: The cross section for positronium formation by positron impact on hydrogen is calculated in the two-state approximation, taking account of the polarization of the hydrogen and positronium atoms in each channel. It is found that the polarization forces dominate the cross section near the threshold for positronium formation and evidence is presented for the existence of a positronium-proton virtual bound state that gives rise to a resonance in elastic e+-H scattering just below the formation threshold.

Positron annihilation in copper, silver, gold and thallium halides

Abstract: A complex decay was found for positrons annihilating in Cu, Ag, Au and Tl chlorides, bromides and iodides The time spectra are characterized by two components, with lifetimes about 2·10−10s and 4·10−10s Their relative intensities, when summed, become nearly equal to 100% of the total number of counts, thus proving that none of these components can arise from the formation and decay of a positronium atom in ortho- and para-states

Effect of paramagnetic impurity on the long lifetime of positron in liquid ammonia.

Abstract: We have computed the paramagnetic quenching effect of lithium and sodium in liquid ammonia on the long-lifetime component of positron annihilation. The triplet-to-singlet conversion cross sections, calculated in the Born approximation, turn out to be 5.9×10 -14 cm 2 for Li and 7.2×10 -14 cm 2 for Na for thermalized positronium. Reasonable agreement with the meagre existing experimental data is obtained.

Zeeman Effect in Positronium Annihilation at Low Temperatures.

Abstract: The annihilation of positronium is discussed as a means of obtaining a precise measurement of the Compton wavelength of the electron. In particular the spectral line shape of the annihilation radiation is calculated for positronium thermalized in helium gas cooled to the region of its critical point with an applied magnetic field sufficient to yield a three photon to two photon branching ratio of unity for theS0 component. Under such conditions the rapid thermalization of the positronium and the long half-life of two photon decay out of the predominantly triplet state make possible the production of rather narrow spectral distributions. An exact solution of the Zeeman effect in positronium is included. This study suggests that a wavelength determination beyond the one part per million level is attainable within the range of present technology.

Mechanisms of energy transfer in dielectrics.

Abstract: : A study of the processes determining moderation of electrons at energies below the internal ionization threshold of a dielectric is made. The probe used is the positron through its annihilation characteristics under a static electric field. These characteristics are related to the cross sections for electronic scattering and energy loss. Measurement on the positron annihilation spectrum of the insulators polyethylene and Teflon are given. As part of the work, we have improved the performance of subnanosecond delayed coincidence apparatus. In related work, data on the positron annihilation spectra of powders of small particle size is used to calculate positronium diffusion constants in the substances. Such powders can serve as high yield sources of free ortho-positronium and of gamma rays with continuous energy spectra. Investigation indicates a correlation between defect concentration and intensity of the longlived component of the positron annihilation spectra in alkali halides. The method of positron annihilation can provide an attractive complement to studies on color centers. (Author)

Comparison of the interactions of positronium and other free radicals with cobalt complexes in aqueous alcohol solutions

Abstract: 1. The rate constants of the interactions of positronium and a number of radicals (α,α′-diphenyl-β-picrylhydrazyl; 2,2,6,6-tetramethyl-4-oxopiperidino-1-oxy, and the bis-diphenylchromium cation) with various additives in aqueous alcohol solutions were measured and compared. 2. Radicals, and especially positronium, are very sensitive to the presence of unpaired spin density (approximately one hundredth of one unpaired electron) and interact complexly with a paramagnetic additive — with the first coordination sphere of the “quenching” ion. 3. In the investigated interactions of positronium and radicals, steric hindrances and kinetic factors are important.