Physics Photo of the Week

Discusion by Jesse Chen*

This project deals with the construction and usage of cloud chambers.

The history of cloud chambers started with Charles Thomson Rees Wilson, a Scottish physicist. Wilson utilized pressure to cool water vapor inside a sealed chamber, and as charged particles pass through the contraption, the water vapor is attracted to the particles¹ and a trail is formed as an indirect method of detecting particle tracks.

For the experiments Wilson conducted, the man was awarded the 1927 Nobel Prize for physics. This primitive apparatus had significantly advanced understanding of particle physics; as direct observation was impossible, the tracks that the particles left behind allowed scientists to measure various variables concerning the particles, such as velocity and mass².

Furthermore, the interactions between two or more particles could be detected through the trails. For example, if low energy cosmic rays are present in the chamber, they will exhibit a very jagged line. This is due to the cosmic ray bouncing off various atoms inside the chamber, and its trail reveals its path as it is pushed around³.

For the actual construction of a modern chamber, several materials are required. Firstly, the containment may just be an airtight glass jar with a metal lid. For the vapor and condensation, isopropyl alcohol may be used. To chill the vapor, dry ice (frozen carbon dioxide) will be used to create an environment suitable for particle track detection. Dark, light-absorbing materials, such as a dark cloth or any other medium that does not reflect light, will then be used to make the entire process more visible. A strong light source, possibly from a projector or even a bright table lamp, can be used to illuminate the process. Finally, a radiation source might be procured in order to hasten the detection process; perhaps a sliver of thorium may be used for its radioactive properties in showing the process faster⁴.

The process will begin with an empty glass jar that has been cleaned. Then, a piece of the dark cloth will be saturated in alcohol, and then placed at the inside side of the metal lid. The metal lid is chosen for its excellent thermodynamic conductive properties; as the jar is sealed tightly and then placed in the dry ice, the metal will act as a heat sink and help lower the overall temperature of the alcohol inside the jar. Then, by inducing heat on the bottom side of the jar that now is facing up, the alcohol in the absorbent material will evaporate faster, but then sink as it rapidly cools, thus creating the conditions necessary to observe tracks as the clouds near the bottom. This is essentially the same process that Wilson utilized; however, because he did not have dry ice readily available, he used pressure difference to create the same vaporous atmosphere inside the jar.

Then, as the jar and its contents cool after a short period of time, a strong light should be used to detect the faint trails that will be produced as a result of the particles’ passage and collisions within the environment of the jar. This will also contrast with the dark material prepared at the bottom of the jar, and make any trails very visible.

Finally, if a radiation source emitting alpha or beta particles is not available, natural cosmic rays produced from distant space sources such as novas may be viewed instead. These particles may be as a result of direct emissions, or they may be by-products as original particles interact with atmospheric particles, and create reactions that scatter its decay products towards the ground. As mentioned before, the cloud chamber only detects the effects of subatomic particles, and not the particles themselves. An example would be an alpha particle interacting with the vaporous atmosphere inside the jar, and ionizes the vapor particles. Other particles will then be attracted to these charged ions, and their gathering creates the distinct telltale trail of a passing ray.

However, as these events are random and the various angles of impact, from atmospheric entry to entering the cloud chamber may not be perfect, a constant stream should not be expected.⁵

Thus, the need for a localized, dependable, and safe source of radiation must be employed to fully appreciate the cloud chamber’s usage. Putting the thorium previously mentioned near the jar does this; as it radioactively decays, it will serve as a concentrated source of particles and create a large number of particle trails. Thorium is mentioned here due to its availability in Coleman lanterns6, and its relative safety as a solid metal in relations to other radioactive sources that may create harmful particulates that may be inhaled. Such damage will come from the radiation of the dust, and the same alpha and beta particles whose paths were detected in the chamber are able to penetrate interior cells, and damaging DNA inside the cells, causing mutations.