Annihilation of Positrons in Argon I. Experimental

Abstract

carried out MPHF and optimized-spin-function studies on Li( 2 S) ^nw*,> 1 J L. 1 r , D = 9.3 Amagats E / D = 0.0 J 2.74nsec/Channel \

Free positron annihilation in gases under the influence of a static electric field

“…The larger errors at the lower density are a consequence of the greatly reduced rate of data acquisition which, at = 1 gas event/s, necessitated runs of greater than 600 ks duration. As shown in figure 4 the results obtained for the lowest Ar density agree reasonably well with the earlier measurements of Orth and Jones (1969) and Lee and Jones (1974).…”

“…The difference between the observed and the calculated behaviour of ~, p is probably due to the fact that, in the experiment, e+ can be slowed down by inelastic collisions to any energy below the positronium formation threshold (17.8 eV) such that they can have initial energies both above and below the position of the Ramsauer minimum (21.5 eV), whilst Shizgal and Ness (1987) assumed the initial e+ energy distribution to be a delta function at 1 eV. No theoretical calculations of T~ in Ar have been performed for E,, # 0, but the present values support the observations of Orth and Jones (1969) who found that ~, p was not markedly changed on the application of an electric field. The nearly constant value of ~~p is not expected on the basis of the He and Ne work of Shizgal and Ness (1987) who in both cases found rapid shortening of the shoulder as E,, is increased to 10 V cm-' amagat-'.…”

“…The values of zefi obtained in He at densities of 35.7 and 3.5 amagat for densitynormalised electric field strengths in the range 0 s < 12 V cm-' amagat-' are shown in figure 3, together with the theoretical curves of Campeanu and Humberston (1977) and Shizgal and Ness (1987). In figure 4 the zeff values measured in 9.9, 14.9 and 20.0 amagat of Ar are presented for 0 6 ep < 65 V cm-I amagat-' and a hand-drawn line through the data points of Orth and Jones (1969) and Lee and Jones (1974) is also included. The experimental errors associated with these earlier measurements of Zff were, typically, A I .…”

“…Studies of the effect of an electric field on positron annihilation in room-temperature Ar have previously been performed by Orth and Jones (1969) and Lee and Jones (1974) at densities below 10 amagat and, again, Zeff was observed to decrease as was increased from zero to =lo0 V cm-' amagat-'. As no experiments have been the density-normalised electric field strength, c p .…”

“…It is apparent from figure 3 Tefl values for positrons in Ar against density-normalised electric field strength at 9.9 amagat ( O ) , 14.9 amagat (U) and 20.0 amagat (A). The full curve was drawn through the data points of Orth and Jones (1969) and Lee and Jones (1974).…”