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Conduction processes in liquids Maybank, John

Abstract

In the work described in this thesis attempts have been made to obtain information on three aspects of the behaviour of liquid argon as an ionization counter. Ionization was produced by alpha particles from a source deposited on the negative electrode of a small parallel-plate chamber in which pure argon was liquefied. The current pulses resulting from movement of the liberated electrons in the field applied to the plates were analysed electronically. Firstly, it was desired to determine the time taken by the electrons originating from distinct ionization events to traverse measured electrode separations and be collected by the positive electrode. These transit times depend upon the electron mobility in liquid argon, defined as the velocity of the electrons per unit field. From this mobility, the mean free path and collision cross-section of the electrons with respect to argon atoms can be calculated. Some estimates of transit times and mobilities, and resulting values of mean free path and cross-section are given. However, it appears that limitations of the electrode spacing and the applied field cause the transit time to be so short as to necessitate the use of wide band amplifiers with, consequently, high noise levels. Secondly, the causes of variation in size of current pulses with angle of emission of the initiating alpha particles were investigated. The effect of greatest interest was that, due to the geometry of the chamber, as from it a determination was made of the range of alpha particles in liquid argon. For 5.3 MeV alpha particles, the weighted centre of ionization was found to be 0.006 cms. from the source, implying a particle range of about 0.009 cms. The number of electrons contributing to current pulses was also found to be a function of the angle of emission, on account of a variable degree of recombination with the positive ion column. This number was determined, and even under the most advantageous conditions fell considerably short of the probable total number liberated. This fact imposes a serious limitation on the potentialities of liquid argon as a useful counter.

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