[Historical Photographs] First Published Photographs of Alpha Particle Collisions

P. M. S. Blackett published in 1923 the curious case of "forked tracks" when alpha particles are allowed to travel in atmosphere as seen in a cloud chamber.

Alpha particles typically travels unimpeded under normal circumstances and leaves thick straight tracks. Occasionally some alpha particles comes close enough with the nucleus of an atom in air, causing it to deflect elastically, the "collision" process ionises and transfers momentum to the target nucleus causes it to leave tracks in the cloud chamber as well.

It is this reasoning that the forked track was interpreted as collisions of alpha particles with nucleus of an atomic nuclei.  Most of the nucleus the alpha interacted with are those of nitrogen or oxygen atoms as they both constitutes about 99 % in air.

Using a stereoscopic pair of cameras, he was able to work out the precise scattering angles of both incident and recoil particles; from the ratio of the angles he's able to confirm the masses of the particles involved. 

In many of the following photos, the shorter Y-shaped branch is caused by the massive recoil nucleus as it has a shorter range and stronger ionisation while the longer Y-branch is the deflected alpha track.  

The following were some photographs and captions from his paper:

Original caption: This shows the collision of an alpha particle with an oxygen atom, φ = 76° 6',  θ = 45° 12'

Author's note: Both left and right tracks are essentially the same. They are taken by two cameras oriented perpendicularly to the cloud chamber (stereo photography) in order to accurately measure the scattering angle. Mathematical groundwork from the paper suggest that if the recoil particle is more massive than the incident particle, the scattering angle between both particles will always add up more than 90° - which is evident in this photo.

Original caption: Collision with an atom of hydrogen. φ =  9° 21', θ = 65° 39'

Author's note: The hydrogen recoil nucleus is essentially a proton and is 4 times less massive compared to the incident alpha particle, so it is basically "kicked forward", and the sum of scattering angle is less than 90°.

Original caption: Collision with a helium atom. Although they do not appear so, the two parts of the track are nearly equally inclined to the stem. The sum of the two angles is 89° 45' - it should be 90°. (The curvature of some of these tracks is a very interesting phenomenon, now under investigation.)

Original caption: Two rather unusual forks due to collisions with air atoms.

REFERENCE

P.M.S. Blackett, "The Study of Forked alpha-ray Tracks", Proc. Roy. Soc. A., 103, p.p. 78, (1923)