If one has the patience looking into a diffusion cloud chamber containing an alpha source for longer than a few minutes, it will take an average about 10 to 15 minutes to observe something strange:
Alpha particles moving in a cloud chamber normally produces thick, straight tracks. On rare occasions, you may observe a Y-shaped track due to alpha "collisions" with an atomic nucleus. Depending on the medium which the alpha particle travels, the chances of collision varies. These collisions are mostly elastic, losing less than 1 % of its kinetic energy.
If we assume the collision event is elastic; by conservation of linear momentum, measuring the scattering angle of both tracks at the collision centre allows us to predict the mass of the particle which the alpha particle collided.
Such was the experiment conducted by P. M. S. Blackett back in the early 1920's. Here's the photograph from his paper:
In these two pictures, the cloud chamber was filled with air, and the alpha particle collisions were probably nuclei of nitrogen or oxygen. Since they are more massive than the alpha particles, the emerging tracks are more than 90 degrees, and some alpha particles may backscatter (shown in (b)) as was demonstrated by Rutherford's gold-foil experiment.
Exact measurements of scattering angles using stereoscopic cameras determines the mass ratio as 1:4 thus confirms the analysis of the event.
Reference
P.M.S. Blackett, Proc. Roy. Soc. A, 103, 78, (1923)
No comments:
Post a Comment