An investigation of the production of positive and negative electron pairs in a cloud chamber

In the work described below a cloud chamber has been employed to study the production of positive and negative electrons by gamma rays, the pairs originating in the gas of the chamber. The total energy calculated from the curvature of the tracks is therefore unaffected by any primary absorptions, and measurements of direction of emission are little affected by scattering. Similar experiments have recently been performed by Immelman (1936) in krypton and by Klarmann and Bothe (1936) in xenon and krypton. We have obtained 127 pairs in 2412 exposures with argon or methyl iodide vapour in the chamber and a Ra Th source. During the course of the work certain developments in cloud-chamber technique, described below, were made and a new instrument was devised for measuring the tracks. Apparatus and Technique The cloud chamber used was one constructed some years ago by Professor Blackett and Mr E. P. Hudson. It was of the horizontal type and made of stainless steel. The closely fitting piston had a diameter of 20 cm., the inner diameter of the glass top was 20·6 cm. and the normal depth 6·5 cm. After some trials a suitable lubricating and sealing oil for the piston and cylinder was found to be a mixture in equal parts of castor and olive oils. The glass parts were sealed with “Apiezon Q” compound. The driving mechanism is similar to one already described (Blackett 1929). In this work the usual cycle of operations could be completely suspended at any point in the cycle for any length of time by automatically stopping the main motor and restarting it again by a switch controlled by an accessory slow-running motor. The normal period of the cycle was 20 sec., stopping at the end of the slow expansion, and the idle period was about 40 sec., making approximately 1 min. between expansions.