In the Wilson cloud chamber photograph the path of the alpha particle is a thick and continuous line whereas that of the beta particle is a thin and broken line. How can this be explained?

Answer 1

To explain the observation asked about in the Wilson cloud chamber photograph, let's take just a moment to talk about what a cloud chamber is. In a cloud chamber, we see a supersaturated environment where alcohol or some other substance is present as a vapor in air. The substance appears in the air in such high quantity that it is ready to rain within the chamber. (In some chambers, the tiniest droplets are actually forming, and it is raining extremely small droplets!) The inside of the chamber is undisturbed by any outside forces (save gravity and the radiation moving through it).

When particles or rays of high energy move through the chamber, the atoms of air along the path of travel are ionized. Electrons are torn out of the orbitals of these atoms, and these ionized atoms will form places where that supersaturated vapor can condense. Tiny droplets of liquid will form along the path of travel of the radiation, and this "track" can be seen from outside the chamber. The condensation is actually a little "cloud" inside the chamber, hence the name cloud chamber. That said, let's see what happens when radiation passes through it.

The alpha particle is really a helium-4 nucleus. It is a pair each of protons and neutrons, and it is relatively heavy as particulate radiation goes. When an alpha particle moves through the chamber, it slams into air atoms all along its path of travel. There is a lot of energy exchange, and there will be a lot of ionized atoms and a lot of condensation along the course the particle takes. This heavy track of condensate will be easy to spot. Compare that to the track of a beta particle, which could be an electron or positron. The electron or positron is extremely small and light compared to the alpha particle, and it will be much, much less likely to interact with an atom along the path that it travels. This will result in much less ionized air along its track, and much less cloud formation along that path. The observer will see a clear difference between the two particle tracks as he peers into the chamber during the experiment.

We've left out a couple of things as regards the magnetic field that is usually incorporated into the cloud chamber, and also what we might expect as regards the tracks of other forms of radiation that might pass through. But the large difference in the sizes of the alpha and beta particles will result in marked differences in their ability to ionize the atoms of air along their respective routes through the chamber. And this will cause a large difference in the size of the cloud tracks that appear in their wakes, and will allow the observer to clearly distinguish between the two particles as they pass through the chamber. Links can be found below to related questions which might allow the investigator to gain further insight into the particles and the operation of the cloud chamber.