Spur Contribution to Positronium Formation in Molecular Media.

Positrons, when slowing down in molecular substances, can capture an electron to form positronium (Ps) with experimentally discernible annihilation characteristics. Positronium formation is usually attributed to electron capture by positrons in a kinetic energy range, called the Ore Gap, near the ionization potential of the molecules, of width comparable to 6.8eV, the Ps ionization potential. This thesis investigates experimentally to what extent Ps can also be formed by the combination of a thermalized positron with one of the thermalized electrons which constitute with the geminate ions the "spur" produced by the positron during the last ionizing collisions with the molecules. A method was developed for the purpose of increasing systematically the subexcitation stopping power in the medium and, hence, to decrease the ranges and the mobilities of epithermal positrons and electrons, and to increase the rate of dissipation of the 6.8eV necessary for Ps formation to occur. This was achieved through the introduction of increasing concentrations of dipoles in the form of (-CO-) groups into a nonpolar (-CH(,2)-) liquid. The objective was to initially enhance the probability of Ps formation in spurs at small dipole concentrations and, then, diminish the spur contribution when the positron mobility becomes so small that all thermalized positrons annihilate before encountering a spur electron for Ps formation. Positron lifetime measurements on dilute solutions of various ketones in n-hexane show that the Ps yield reaches a maxium at a CO concentration of 2 x 10('-3) (CO/CH(,2)). The maximum heightens with temperature (200(DEGREES) K to 320(DEGREES) K) and shrinks under the influence of electric fields up to 56kV/cm. In this frame of reference, the observed change of the maximum with temperature is related to increases of the dipole relaxation rate with temperature and, hence, of the dipole subexcitation stopping power, while electric fields retard the electron-positron approach. Analysis of the data leads to the conclusion that 37% of all positrons form Ps in pure n-hexane of which 35% derive from the Ore gap and the remaining 2% from formation in spurs. As the maximum Ps yields is reached in a solution of one ketone molecule per 100 n-hexane molecules, the fraction of positrons forming Ps in spurs has risen to 9%. We conclude that spurs can contribute up to 20% to the total Ps yield in such systems. The dependence of the spur contribution on experimental parameters elucidates the subexcitation process underlying transient electronic effects induced by ionizing radiation in molecular insulators.