Collisions of α-particles with fluorine nuclei

Present knowledge concerning individual collisions of swift particles and nuclei is derived mainly from the researches of Blackett and his co-workers; it has reference to the collisions of α-particles with hydrogen, helium, nitrogen, oxygen and argon nuclei—and of H-particles with oxygen nuclei. Its content is briefly as follows: conservation of kinetic energy and momentum has been found to hold, to the accuracy of measurement, in all save obviously disintegration collisions (disintegration has been found, amongst the above, only for nitrogen), and an empirical relation between velocity and range has been derived for each of the five types of recoil atom in question. The present paper describes an investigation of the collisions of α-particles with fluorine nuclei by the same general method; it establishes the appropriate rangevelocity relation, though it does not afford any positive information concerning the inelastic (disintegration) collisions which from other evidence are known to occur. Less complete data establishing the range-velocity relation for carbon recoil atoms have been obtained as a bye-product in the course of the work. Fluorine has been employed in the present work very conveniently in the form of the saturated carbon compound, carbon tetrafluoride. In general the application of expansion chamber methods to fresh elements has been greatly retarded on account of the chemical activity of the simpler chemical compounds available. Of the lighter elements carbon has received less attention than might otherwise have been profitable presumably because it appeared unlikely that any disintegration effects would be observed, but, for the rest, boron, silicon, phosphorus, sulphur and chlorine, in order to be examined, must have been employed in the form of highly active gases. In such conditions technical difficulties arise in the construction of a suitable chamber and the finding of a convenient substitute to employ in place of the usual water vapour. With elementary fluorine these difficulties are doubtless effectively insuperable; with carbon tetrafluoride, on the other hand, they entirely disappear. This compound has been reported at various timesf in the past, but it remained for Lebeau and Damiens in 1926-30 and Ruff and Keim in the latter year, definitely to establish its properties, both chemical and physical. The most important, in view of its employment in the expansion chamber may be summarized as follows: boiling point, —130° C .; ratio of specific heats, Cp /Cv , 1·17; molecular stopping power, 2·75 (v. inf .); action on water, glass, copper, silver, lead, nil; very slow absorption by some oils. Except in a fully equipped laboratory the preparation of the gas cannot be easily undertaken, since elementary fluorine is required. Professor Lebeau, however, kindly undertook to provide a sufficiently large sample for the work to be described. For this most generous gift of 2 litres of the gas I wish here to offer my sincere thanks.