The lifetimes of ‘free’ positrons have been Investigated (at room temperature) in. the gases N₂, O₂, CO₂, He, A, and Kr by recording the time distributions of the annihilation events. Whereas all the polyatomic gases exhibited pure exponential decay spectra, the annihilation spectra for the noble gases were more complex exhibiting an initial ‘shoulder’ followed by an exponential decay. An analysis of the results suggests that in all cases the exponential decay corresponds to the annihilation of positrons in thermal equilibrium with the gas molecules. The application of a DC electric field was found to produce marked changes in the shape of the annihilation spectra in argon. The annihilation spectrum in argon (at 10.5 atm) was investigated in detail. At zero electric field the initial portion of the spectrum is characterized by a relatively low non-constant annihilation rate, and is attributed to annihilation of positrons which are slowing down in the energy interval between the positronium formation threshold (8.9 eV in argon) and thermal energy. In this energy interval the positron can lose energy only by making elastic collisions with the atoms of the gas. A velocity-dependent annihilation rate over this energy range is suggested to account for this ‘shoulder’ in the annihilation spectrum. The time-width of this shoulder is found to be 340 nsec atm. The rest of the spectrum exhibits an exponential decay as observed in the polyatomic gases. Further investigations using applied DC electric fields (in the range between 0 to 122 V cm⁻¹atm⁻¹) revealed an increase in the mean lifetime of the ‘free’ positrons with increasing electric field. For a field of 790 V/cm and a pressure of 10.5 atm this lifetime has increased by a factor of 2. A diffusion equation governing the behavior of positrons in the noble gases has been derived and solved numerically for different cases of the elastic scattering cross section, the direct annihilation cross section, and the positronium formation cross section. A good fit to the experimental data for argon was obtained using the values 20(v₀/v)¹‧⁵πa₀², 2.25 x 10⁻⁴(v₀/v)¹‧⁵πa₀², and 5.0 x 10⁻⁵πa₀² respectively for these cross sections, where v is the velocity of the positron and v₀ its velocity at thermal energies (25°C). The form of the positronium formation cross section was taken as a step-function whose magnitude was adjusted to yield results consistent with the increase in positronium formation measured by Harder et al (1956). Further calculations are required in order to test the uniqueness of this set of cross sections. Similar experiments in helium revealed only slight evidence for the presence of a shoulder and only a small dependence of the mean lifetime on the applied electric field. This suggests that the annihilation cross section is closely approximated by an inverse velocity dependence. Preliminary investigations in krypton indicates a 'shoulder' width about § as great as in argon. The annihilation rate of the ‘free’ positrons (in the exponential region) was found to be directly proportional to the gas density for all the gases investigated except CO₂, where the annihilation rate can be represented by the equation λa = (0.852p – 0.040p²) x 10⁷/sec where p is the density of the gas expressed in terms of the density at 1 atm and 25°C. A comparison of these results to those obtained from the Dirac cross section (assuming all the electrons of the atom participate) reveals that the observed rates in N₂, O₂, CO₂ (at low pressures), He, A, and Kr are larger than the Dirac values by factors of 2.14, 1.59, 2.64, 1.91, 1.63, and 1.67 respectively. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A new type of fast "time-sorter" has been developed, employing a germanium diode as the detecting element. This instrument converts time delays between coincident events into a pulse amplitude distribution, which is then analyzed by a "kick-sorter", enabling a complete coincidence resolution curve to be recorded simultaneously. The resolution curves obtained with this apparatus were found to have half-widths of about 1 milli-microsecond, comparable with those obtained by delayed coincidence circuits. Using this time-sorter, the lifetimes of positrons in Aluminium and Mica were then studied. The positron life-time in Aluminium was obtained by measuring the centroid shifts between the coincidence resolution curves resulting from the observation of a Na²² source embedded in Al, and an assumed "prompt" source of cascade gamma rays, As⁷⁶. This latter source was employed for the comparison because the energies of its coincident gamma rays are similar to the 1.28 Mev and 511 Kev. gamma rays of Na²². This eliminates the problem of centroid shifts due to inequalities in pulse-height and rise-time between the two sources. The positron life-time in mica was obtained by comparing the centroid shifts of the resolution curves resulting from the annihilation of positrons in mica and in Aluminium. The measured positron lifetime in Aluminium is (1.6 ± 0.4) x 10[power -10] sec, and the lifetime in Mica is longer than this value by (0.7 ± 0.4) x 10[power -10] sec. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The angular correlation between annihilation quanta due to the decay of positrons in eleven different materials has been measured with anthracene scintillation counters. The materials studied were, the pure elements, Mg, S, Cu, Sn, Pt, and Hg, and the chemical compounds LiCl, KCl, CsCl, KF, and KI. Calculations were performed to determine the effect of the geometry on the shape of the angular correlation curves. The effects of scattering were also investigated. The apparatus was tested by means of measurements on the γ-rays of Co⁶⁰ and Zn⁶⁵. The centre of mass momentum distributions of annihilating pairs in copper, platinum and possibly mercury were found to be exponential in character and the mean centre of mass momentum obtained for copper was found to be in reasonable agreement with the results obtained by DeBenedetti and coworkers and by P. E. Argyle. The momentum distributions for the other materials were not found to be of a simple exponential form. The results suggest that there is a relation between the shape of the distribution curve and the outer electronic structure of an atom. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Theories of P.S.I.* produced by electrons passing through gases show that one would expect the number of primary ion pairs formed to vary with the electron density in the gas and with the mean ionization potential of the gas. to vary inversely as the square of the velocity of the electron at low energies reaching a minimum value approximately at a kinetic energy equal to the rest energy and to be independent of the sign of the beta particle producing the ionization. It is consequently of interest to obtain measurements of the P.S.I, of beta particles at energies at which it is a minimum in various gases which are used for filling counters and ionization chambers including hydrogen, helium, neon, xenon, methane, chlorine and other quenching vapors such as methyl chloride which may become of practical concern for counters designed to operate over a wide temperature range. Present data on P.S.I. of electrons and mesons is limited to but few gases and the results are not in good accord. As mentioned, the P.S.I, is independent of the sign of the charge of the ionizing particle and, since all the indirect evidence and the principle of conservation of charge indicates the equality of the charge on the positron and negatron, it would be expected that the values of the P.S.I, would be identical for the two particles. However while it is known that the value of e/m for both particles is identical to within 2%, there is some possible theoretical indication that their masses might be slightly different. *The primary specific ionization will hereafter be denoted by P.S.I. Moreover the only direct estimation of the value of e⁺ is the measurement of P.S.I. from the original cloud chamber observation of a cosmic ray positron by Anderson¹ which established the charge equality of e⁺ and e⁻ to within 20%. Thus it appeared of some fundamental importance to compare with precision the P.S.I, of both positive and negative electrons of similar velocities in a gas of high atomic number such as neon and a gas of low atomic number such as helium. Finally it was hoped that the apparatus set up would be suitable for an investigation of the relation between the P.S.I. and velocity of the electron especially in the relativistic region in which the data is very meagre. This will need high energy beta sources (e.g. B¹²) which will "become available when the U.B.C. electrostatic generator is in operation. Apparatus has been set up to determine the P.S.I. of electrons in various gases by determining the inefficiency of a Geiger counter filled with gas. To eliminate the difficulty, of a variable path length of the particle through the ordinary cylindrical counter, rectangular and square envelope counters with thin windows have been constructed. The presence of a large thin window, even when a conducting surface, has been found to affect the spread of the discharge and results in the appearance of two size pulses analogous to the effect of an insulating bead oh the centre of the wire. Thus, contrary to the usual theory of Geiger operation, it seems that the spreading of the discharge does involve a cathode mechanism in this design. In order to select beta particles of homogeneous energy from the radioactive source, a small wedge shaped magnetic spectrograph has been built. The counter inefficiency is determined by passing the beta particles through the inefficient counter into a 100% efficient counter operating at normal pressures, and recording the coincidence rate as a fraction of the "efficient counter" rate. Co⁵⁶ was chosen as the most suitable positron emitter, and has been prepared in the Berkeley cyclotron by the Fe⁵⁶(d,2n)Co⁵⁶ reaction. RaE has been used as a negatron emitter. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The feasibility of using positrons to probe plasmas is investigated. Analysis of the y rays resulting from the annihilation of positrons with plasma electrons may yield information about the momentum distribution, temperature and number density of these electrons. A number of 'positron probe1 success criteria are introduced and a wide range of positron-plasma systems are evaluated in light of these criteria. Of special importance are positron annihilation time (Ta) and thermalization time (Tt) calculations, which indicate that the most important success criterion, Tt < Ta, is satisfied by the class of plasmas characterized by kTe > 10 eV. Two potential fusion plasmas, namely Tokamak and laser compression plasmas, belong to this class (where some positron diagnostic techniques may be practical). Detailed fractional thermalization calculations for positrons from a ²²Na source in hypothetical, fully ionized H₂ plasmas of these types indicate that most positrons will thermalize and annihilate while the plasma environment is in existence. Positron sources, including pair creation, which is predicted to occur in both Tokamak and laser compression plasmas, and y ray detectors are discussed. Annihilation y ray counting rates are estimated under a variety of circumstances for both NaI(Tl) and Ge(Li) detectors to prepare the way for the assessment of specific positron diagnostic techniques. Four such techniques are evaluated with reference to both Tokamak and laser compression plasmas. It is concluded that diagnostic techniques involving measurements of shift and broadening of the annihilation line appear most promising in both cases. The measurement of positron lifetimes may be possible in some Tokamak plasmas, but is not feasible in laser compression plasmas. 'Slow' positron probing of plasmas would be ideal, but slow positron sources of sufficient intensity do not yet exist. Angular correlation measurements do not appear feasible at this time. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Thesis--Ph. D., University of Hong Kong, 1978.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Thesis (Ph. D. in Physics)--University of California, Berkeley, Jan. 1961. / Includes bibliographical references (leaves 64-69).

The annihilation of positrons in Argon has been investigated as a function of Argon density and applied electric field using the technique of lifetime measurements. Lifetime spectra were analyzed using the maximum likelihood method of curve fitting. Results obtained at zero electric field yielded a linear dependence on density for the direct annihilation rate of (5.6 ± 0.1) x 10⁶sec⁻1 amagat⁻¹, with some evidence of non-linearity at densities greater than 10 amagats. The density dependence of the long-lived component of the time spectra indicated a zero density intercept of (7.2 ± 0.4) x 10⁻⁶sec⁻¹ in agreement with the theoretical value of the free orthopositronium annihilation rate (7-2. x 10⁶ sec⁻¹ ). In addition an orthopositronium quenching rate of (0.29 ± 0.04) x 10⁶ sec⁻¹ amagat⁻¹ was obtained from the linear dependence of the orthopositronium annihilation rate on density. The electric field dependence of the direct annihilation rate and orthopositronium formation have been measured and are used to provide an internally consistent picture of the behaviour of positrons in a gas under the influence of an applied electric field. Furthermore, these results have been compared with theoretical results for the direct annihilation rate obtained from one parameter representations of the effective positron-Argon atom interaction. It is shown that, while such potentials are successful in describing the low-energy elastic-scattering of electrons from noble gas atoms, they are inadequate for the case of positrons. However, consideration of the way in which the direct annihilation rate changes as a function of electric field leads to an upper limit of 15π₀² for the momentum-transfer cross-section for positrons in Argon at thermal energies. Such an estimate is shown to be independent of any assumption concerning the effective positron-Argon atom interaction. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Lifetime techniques have been used to measure the direct annihilation rate in Argon gas as a function of temperature. In order to accomplish this, a pressure chamber capable of holding 10 amagats of gas at 300°C was designed and constructed. The temperature of the Argon gas was varied between 140°K and 480°K. The results were collected at specific temperatures for gas densities of 8.0 and 10.2 amagats. The direct annihilation rate was found to be a decreasing function of temperature meaning it is a decreasing function of velocity. The direct annihilation rate decreased by 30% over the range investigated. Various functions in temperature were fitted to the results by the least squares technique. The shoulder width-density product was found to be constant with temperature indicating that the shoulder annihilations take place at velocities significantly greater than thermal velocities. The statistics on the ortho-positronium lifetime were not sufficiently good to see a temperature variation of this component. A comparison of an electric result with these results gave a position - Argon atom scattering rate of 9.0xl0¹¹secˉ¹amˉ¹. This was assuming that the scattering rate was independent of velocity. / Science, Faculty of / Physics and Astronomy, Department of / Graduate