A particle cloud chamber is a device that makes visible to the human eye the presence of certain particles and cosmic rays. This paper will discuss the process in which a successful chamber prototype was developed. This paper will also discuss how the chamber utilizes the properties of a supersaturated environment to detect these particles. Finally, plans for the future development of a larger model to study the charge and energy of particlesthrough the use of a magnetic field is presented.
The purpose of a particle cloud chamber is to detect various types of particles that are present or pass through the chamber viewing area. Though there are many different types of chambers to chose from, it was decided that a continuous, dry ice-based model be built. In the dry ice-based model a supersaturated cloud is formed at the bottom of the alcohol filled chamber. The dry ice creates a temperature gradient which causes the supersaturation. Many different variations were tested and built until a stable prototype was completed. With the prototype complete, a larger scale model will go under production boasting a much larger viewable area. With this model it will be possible to accurately study the affects of a magnetic field to determine particle charge and energy.
Cloud chambers are very interesting in that they demonstrate first hand radiation trails emanating from an alpha source. “Video Analysis of Cloud Chamber Phenomena”8 states that the best way to view this phenomena is to incorporate a camcorder. The article argues that camcorders increase the amount of people that can view the experiment without overcrowding. Also this technique provides increased resolution and magnification, thus making beta tracks visible. In this article, Jason Cassidy concludes that the use of a camcorder is more convenient, practical and the cloud chamber a more powerful tool.
In the “Sourcebook on Atomic Energy”3 the author concludes more saturation and pressure results in increased frequency of radiation trails. Other ways of increasing the frequency of trails are to use methyl alcohol and having a large difference in the temperature gradient. Although these factors increase activity, at best the sensitive region is only three inches deep. Nevertheless those three inches are all one needs to conduct the experiments. The author also states that cloud chambers are still widely used today for studying high-energy particles obtained from accelerators.
John Timothy and Mary Ann Sankey7 review cloud chambers, pointing out the construction and experimentation involved. They state that chambers are relatively cheap and easy to produce. A typical petri dish combined with black tape and felt could produce a crude chamber. In addition to some dry ice, an alpha source, and a lamp are required to conduct the experiments. In closing the author points out that the cloud chamber makes the nuclear world seem more real to students for they can view it, first hand, indirectly.
Why indirectly? As charged alpha particles pass through the supersaturated environment, the particle ionizes the gas around it, thus creating a vapor trail. So what is seen is really alcohol vapor ionized by the disturbance of an alpha particle. Some people say that this phenomena displays radiation itself; they can say that, but similar to electrons, we can only see where the radiatoin has been as stated by William B. Fretter6 in “Cosmic Ray Detection.”
Cloud chambers date back quite considerably in history. As stated in “Cloud Chamber,”2 C.T.R. Wilson built the first cloud chamber in 1894. He realized that the charged particles would react with the nuclei of the condensed gas. As a result, ions would form a trail where the particle had been. Demonstrating radiation first hand was a revolutionary development which broadened the way people thought. Later in 1910, Wilson constructed a new, cylindrical, flat-topped expansion chamber. He hypothesized that ions would condense in droplets along trajectories of these charged particles.
Newer methods of supersaturation include use of dry ice as a cooling agent to cause more alcohol to diffuse into the chamber. This was developed in 1939 by A. Langsdorf2 to create a continuous nuclear-sensitive environment. His idea was to have vapor from the top of the chamber diffuse down a temperature gradient causing the lower section to become supersaturated.2 This new chamber was remarkably better than the Wilson Chamber in which tracks could only be seen for 10-60 seconds after the chamber expanded. The author concludes that with the development of large particle accelerators, diffusion chambers were unable to keep pace with rate at which particles could be produced, and were gradually replaced by bubble chambers.
Victor Vranz Hess,1 who discovered cosmic rays, theorizes that theses rays emanate from outer space. Although that theory eliminates possibilities here on Earth the galaxy still remains a massive place. Enrico Fermi,1 suggested that positively charged particles collide with gas in the Milky Way sending them flying at incredible speeds. Most theories believe that their origins lay in quasars, sourcing from the high frequency radio waves.1 Still others believe they emanate from supernovas, or that they originate at the beginning of the universe.1 In many cases these cosmic rays still exist and carry a charge that can cause vapor trails in cloud chambers.1
The particle cloud chamber that was developed has a few essential parts: a chamber, dry ice, felt, methyl alcohol and a radiation source (preferably alpha rays). The initial chamber consisted of a 10 gallon tank and a wooden base. The wooden base holds dry ice and a partition is made of sheet metal and is taped over with black tape on the viewing side to provide a contrast from the expected white condensation streaks. This first model failed but its design flaws were recognized and were revised in the second model.
It was decided that due to flaws in the original large scale design that the most basic design be made before progressing to larger scale plans. The second model followed a glass jar design in which alcohol soaked felt is placed at the bottom region of the jar and the inside of the lid is covered with black electrical tape. The jar is then flipped over and placed on the dry ice. This model worked to a small degree but the visible area was small and produced very faint results.
With the production and subsequent failure of this model it was determined that the use of electrical tape was not as reliable when stretched out tightly on the lid of the jar. It was found that with a single use the tape lost all adhesive properties and peeled off. It was decided to follow an even simpler design of even smaller volume. This third model consisted of two small petri dishes taped together with the source and alcohol soaked felt placed within. A light shone in from the top and made visible the particle tracks against a bottom covered loosely with electrical tape. This model brought the first strong signs of success, but as time went on the taping around the edge of the dishes came apart as alcohol vapor condensed upon it and ruined its adhesivity.
The final prototype was built with all the flaws of the previous models in mind. Due to the lack of reliability that tape offered, the final model was constructed with hot glue holding together the two dishes. The failures of black electrical tape to remain on the chamber resulted in the final model using permanent ink to provide the contrasting background for the trails. Finally with the availability of the needle source we decided to bore a hole in the chamber wall just large enough for the source needle. The felt is hot glued within the chamber and the final result is a one piece chamber with no moving parts. In this final prototype a new flaw was discovered, the plastic which made up the petri dishes became brittle with use and small fractures have developed on the bottom surface of the chamber which comes in contact with the dry ice.
The procedure for viewing the trails of particles is simple. First an abundance of methyl alcohol is injected into the chamber and onto the felt within using a dropper. Next the radiation source is placed into the bore on the chamber’s side until the cork fits perfectly in place. The entire chamber is then placed on top of the dry ice. A piece of cloth may then be used to rub the top of the chamber giving the top of the chamber a negative charge (this increases track visibility). After about 15 minutes the chamber becomes supersaturated enough to view the particles leave the source.
The final chamber prototype is very stable. All the flaws of the previous models are addressed in this final design. The needle source which was purchased during the later part of the experiment brought more clarity to the experiment, showing the exact point at which the radiation emanates. In the smaller chamber types, trails were much more frequent and distinct. This is due to the overall size of the chamber and the relatively high levels of supersaturation the smaller chambers can successfully achieve. If at anytime a crack has formed in the chamber and a leak forms, the supersaturation of the chamber is ruined as the alcohol visibly dissipates out of the chamber. Also discovered while using the smaller types of chambers was that viewable levels of supersaturation were achieved earlier due to a smaller chamber volume. The viewable area of supersaturation rarely ever reached levels above three and one half inches from the chamber floor.
The phenomenaobserved in the particle cloud chambers that were developed are due to the unique properties of a supersaturated environment. Supersaturation is attained in the models constructed by using a steep temperature gradient, by placing dry ice under the chamber. The alcohol in the chamber it evaporates into vapor and when it reaches the bottom of the chamber will not condense easily due to lack of nucleation sites, in spite of the low temperature provided by the dry ice. The vapor continues to settle downward from the top of the chamber and thus a level of vapor saturation much higher than usual is formed toward the bottom of the chamber. As radiation particles pass through the chamber they knock ions off of the air molecules within the chamber. The supersaturated vapor within the chamber readily condenses on these ions in particle’s path. Thus we see the particles, at least indirectly, in the form of their trails. It has been determined that alpha rays have a higher visibility than beta rays due to their density. They appear as white streaks while the weaker beta rays appear simply as wispy, faint, and often crooked streaks.
A trial and error process has lead to the design of the most successful type of particle cloud chamber. All the refinements have led to a design that creates the highest level of supersaturation with the highest stability. The final chamber built clearly shows the tracks of particles and in one case possibly cosmic rays. The next step in our project is to use a magnetic field to study the charge and energy of the particles and the occasional cosmic rays. The ultimate goal of the project will be undertaken this summer when a new larger scale model will be constructed.