A reciprocating expansion apparatus for detecting ionising rays

Abstract

The present investigation was undertaken for the purpose of obtaining a very sensitive method of detecting ionising rays, such as X-rays and rays from radio-active substances. An electroscope or electrometer, in combination with an ionisation chamber, even if the required sensitiveness were obtained, would not distinguish between the effects of a single ray and of two or more which entered the chamber simultaneously. In order to attack certain problems relating to the structure of ethereal waves and other delicate questions, it is necessary to have some means of detecting and discriminating individual rays. Mr. C. T. R. Wilson’s beautiful method of making visible the tracks of ionising rays in gases affords us the most powerful means for that purpose, and the writer accordingly tried to obtain a modified form of the original apparatus, such as could be readily used for various experiments. One important quality such apparatus should possess is the ability to correlate the number of tracks which are registered by it with the time interval during which the rays which produce them are emitted. With the original method, the expansion takes place so suddenly that it is difficult to determine this interval with accuracy, although it must be remembered that such suddenness has the merit of yielding well defined tracks at the moment of expansion. Another point is that the possibility of registering tracks, by producing the appropriate supersaturation of the gas in the cloud chamber, should be available continuously for the few hours which is usually required for any series of observations.