Page:Popular Science Monthly Volume 87.djvu/121

Rh It will be seen that the number of throws varies from minute to minute. This is to be expected since the chance of an alpha particle entering the opening is governed by the ordinary laws of probability. It will be seen that two throws, marked by asterisks, are much larger than the others. These were due to the passage of two alpha particles through the opening within a short interval. This was readily seen from the motion of the spot of light reflected from the electrometer needle. As the needle was moving slowly near the end of its swing caused by one alpha particle, a second impulse due to the entrance of another was communicated to it.

By this method, the number of alpha particles expelled from one gram of radium per second was determined. Of course only a minute fraction of the alpha particles was actually counted, but the total number was deduced on the assumption, verified by experiment, that the alpha particles on an average were expelled equally in all directions. In this way, one gram of radium in equilibrium was found to expel the enormous number of $$1.36 \times 10^{11}$$ alpha particles each second.

Another interesting result followed from these experiments. It has long been known that the alpha particles produce a marked phosphorescence in crystalline zinc sulphide. When examined by a lens, the light is found not to be uniform but exhibits a very beautiful scintillating effect. By counting the number of scintillations due to the alpha particles, it was found that each scintillation was produced by the impact of a single alpha particle. It is thus seen that two distinct methods, one electrical and the other optical, are available for detecting and counting single alpha particles, i. e., single atoms of matter. This is only possible because the atoms are in swift motion and expend their great energy of motion in ionizing the gas or in producing luminosity in zinc sulphide.

Still another simple method was devised later. Kinoshita first showed that a single alpha particle produced a detectable effect on a photographic plate which was observable under a microscope. A number of experiments have been made by Reinganum, Makower, and Kinoshita to examine the effect of single alpha particles on a photographic plate. If a fine needle point coated with a trace of radioactive matter rests on the surface of the film, the plate on development shows a number of distinct trails radiating from the active point. Each of these trails results from the action of a single alpha particle. A beautiful photograph of this kind (magnification about 300) obtained by Kinoshita is shown in Fig. 4. It appears that each alpha particle makes a certain number of the grains, through which it passes, capable of development.

The use of an ordinary electrometer is not very suitable for counting alpha particles by the electric method, since the time of swing of the electrometer needle is fairly long, and accurate counting can be made