The Nature of the Process of Ionization of Gases by Alpha Rays

Ionization of gases by $\alpha$-rays; valency of the ions.—By catching an ionized molecule upon an oil drop at the instant of ionization and then measuring the charge thus added to the drop, it has been directly proved that the ionization of an air molecule by either $\beta$-, $\gamma$-, or X-rays uniformly consists in the detachment of one electron. In the case of ionization by $\alpha$-rays, the authors show by an extended discussion of previous work bearing on the subject that while some experiments indicate the possibility of the formation of multi-valent ions, none of them is conclusive. Therefore, the droplet method was used to supply direct evidence. The necessary modifications of apparatus are described and the experimental procedure. Uncovered radium furnished the $\alpha$-rays. The gases tested were air, carbon dioxide, carbon tetrachloride, methyl iodide, and mercury dimethyl; so that $\alpha$-rays were shot through atoms of H, C, O, N, Cl, I and Hg, which vary in atomic weight from 1 to 200. Experimental results. Altogether 2,900 ions were each caught on an oil drop at the instant of ionization, and each had its charge determined. Of these captures, only 5 corresponded to a double charge and no ion was caught which carried three or more elementary charges. How many of the 5 double charges obtained were due to doubly-charged ions and how many to simultaneous catches of two singly charged ions, is unknown. In any case the results prove that, at least 99 times out of 100, ionization by an $\alpha$-ray, in the case of each of the gases and vapors studied, consists in the detachment of a single electron from a molecule. The observations for 170 captures by three droplets in three gases are given in detail. The paper ends with a discussion of the circumstances under which multi-valent ions may possibly be produced by $\alpha$-rays.