7D30.50 Wilson Cloud Chamber
SUMMARY: To observe particle tracks from a radiation source and also mainly muons from secondary cosmic rays that have interacted with the earth's atmosphere.
DESCRIPTION: As shown in the above photo, a cloud chamber is saturated with alcohol and cooled with dry ice. An alpha source is placed inside. A power supply is used to apply a high voltage onto a conducting ring around the top of the chamber and a plate on the bottom. The voltage will stratify the layer of super saturated alcohol vapor and remove the buildup of excess charge inside the chamber. A light source is directed through an illumination window on the side and a camera is focused from above.
Two types of tracks can be seen: Alpha tracks, caused by the emission of radon gas from a piece of pitchblende and spiral muon tracks, which come from cosmic rays.
|Radium Source||Slanty Room (066A) Chemicals Cabinet, Radioactive Sources Box|
SETUP NOTES: 3 hours.
The discussion below is borrowed from University of British Columbia+:+
Pitch Blend, Fe-55 (Half life: 2.737 years, electron capture decay), Cs-137 (HL: 30.08 years, Beta-), C-14 (HL:5700 years, Beta-)
Cloud Chamber Demonstration
A cloud chamber is a device that is used to show the tracks of radioactive particles. It is impossible to see the radioactive particles, however, it is possible to see a vapor trail.
A simple cloud chamber consists of a few basic parts:
- a sealed environment
- an alcohol source that allows easy evaporation
- a cold plate
Much of the initial work on self diffusing cloud chambers was done by Langsdorf (Review of Physical Instruments, 1939) and can be found in an old but very thorough article. The basic principles are actually quite simple. Lightweight methyl alcohol vapor saturates the chamber. The alcohol naturally falls as it cools down. The cold condenser provides a steep temperature gradient. The result is a supersaturated environment. The alcohol vapor condenses around ion trails left behind by the traveling particles. The result is cloud formation. This can easily be seen in the cloud chamber by the presence of droplets falling down to the condenser. As particles pass through the chamber they leave ionization trails and because the alcohol vapor is in a supersaturated it condenses onto these trails. The result is large cloud trails that follow the path of the particles. These cloud trails are readily visible, especially when a radioactive source is placed in the chamber. Since the tracks are emitted radial outward from the source, it is very easy to see their point of origin.
Chambers have what is called a sensitive area. This is an area just above the cold condenser plate that is sensitive to radioactive tracks. At this height, most of the alcohol has not yet condensed into clouds. This means that the ion trail left by the radioactive particles provides an optimal trigger for condensation and cloud formation. This sensitive area is increased in height by having a steep temperature gradient, little convection, and very stable conditions.
A strong electric field is often used to draw cloud tracks down to the sensitive region of the chamber. Typically around 100v/cm, it also serves to increase the sensitivity of the chamber. As radioactive particles pass above the sensitive area of the chamber, they too leave behind an ion trail.. The voltage in effect serves to draw these tracks down to the sensitive area of the chamber, where the condensation they cause can be seen. While tracks from sources can still be seen without a voltage supply, background tracks are very difficult to observe. In addition, the voltage can also serve to prevent large amounts of "rain" from obscuring the sensitive region of the chamber. This rain is caused by condensation forming above the sensitive area of the chamber. This means that ion trails left by radioactive particles are obscured by constant precipitation. The black background makes it easier to observe cloud tracks. When a white light illuminates the droplets from the side, they become very visible against the dark background.
This chamber was originally used by T.R. Wilson around 1911 to study cloud formation. He also assumed that radioactive particles could leave cloud tracks in these chambers.
In order to see tracks a fluorescent lamp was placed at a low angle on the chamber's side. Illumination must come from the sides of the chamber.
Before tracks can be visible, a tangential light source is needed. This illuminates the white droplets against the black background. Drops should be viewed from a horizontal position. If the chamber is working correctly, tiny droplets should be seen condensing. Often this condensation is not apparent until a shallow pool of alcohol is formed at the condenser plate. The tracks become much more obvious once temperatures and conditions have stabilized in the chamber. This requires the elimination of any significant drift currents. These may occur due to poor chamber sealing. A simple silicon seal was placed on the chamber door as a gasket. Optimum temperatures for our chamber were around -50 degrees Celsius.
Alpha particles are the most massive of the three ionizing radioactive particles. It is a Helium nucleus. It consists of two protons and two neutrons. Unstable elements can be thought of as having too much positive charge for the nucleus to hold together. This means that the nucleus is unstable. To become stable, alpha particles are emitted. This can be represented by the following equation
where A is the number of protons and neutrons, and Z the number of protons, an element X has.
While alpha particles have a lot of mass as compared to the other two particles, alpha particles react readily, and have very little penetration power. Thus when we are exposed to alpha particles, most of the particles react with our outer dead skin layer. During the late 1800's miners were subject to very high rates of lung cancer. Much of this cancer was caused by alpha particles.
Gamma radiation is a very high energy photon. Everyday light that we see can be thought of in two ways, as a particle or as a wave. When we think of it as a particle, the particles are called photons. Photons also serve to carry the electrostatic force.
Often when a nucleus undergoes alpha decay, it is left in an excited state. As the nucleus returns to its ground state, a high energy photon is emitted. This is called gamma radiation.
Of the tree types of radiation, gamma particles have the most penetrating power. These particles can easily pass through a person, or a thin sheet of lead. They are stopped by thick lead shielding.
Beta radiation is composed either of high energy electrons, or the anti-matter counter part, positrons. Positrons are exactly the same as electrons, except that they have one positive charge rather than a negative charge. Free electrons of much, much lower energy, and thus danger, are produced by TV's and computer screens. Inside a TV, electrons are fired out towards the screen. There they react to produce the colors we see. Standing in front on a TV, we are directly in the line of fire of these electrons. Some new computer monitors advertise "low radiation" screens. In addition after market screen covers are advertised as not only protecting your eyes, but shielding.
The high energy and hence danger of beta particles is a result of the reactions that cause their emission. Usually elements that have too many neutrons to be stable emit beta rays. Ordinary beta decay occurs when a neutron is lost and a proton is gained. This results in the emission of a high energy electron, and another particle called the anti-neutrino. The anti-neutrino is just the anti-matter counterpart of the neutrino, one of a long list of elementary fundamental particles.
As cloud chambers show, radiation is always around us.. It is possible to see examples of the natural background radiation by looking at a cloud chamber with no radioactive source in it. Sensitive chambers show a continual barrage of particles. A person is struck by a radioactive particle, once a second for each square centimeter on their body. Cloud chambers make it extremely easy to visualize the radioactive tracks that are continually around us. It is impossible to escape the effects of radiation. The very potassium in our body is a significant source of radiation. It is, however, possible to minimize our exposure to radioactive particles. As one can see from figures representing the sources of our radiation exposure, the largest sources are relatively impossible to control. Natural deposits in the soil contribute a large fraction of our annual radiation dose. Secondary cosmic radiation produced in the atmosphere from high energy stellar cosmic particles also continually douse us with radiation. It is really only the very minor sources that we can control. These minor sources are usually the ones being publicly touted as lethal. However few stop to realize that radiation from radon gas in one's basement is a higher source of radiation than that received by a nuclear power plant next door.
Large sources of extra radiation can come from unexpected sources. Pilots and Stewardess receive at least equal radiation dosages as any worker in a nuclear power plant. The higher in elevation one is, the less atmosphere one has to be shielded from radioactive cosmic rays. This has become quite a serious issue for members of the airline industry. The most sane way to understand radiation is to be able to put it into perspective. Radiation is not merely a byproduct of modern technology. Cloud chambers would have shown the same background radiation levels 100 years ago as they would today.
Links on Radiation and Cloud Chambers
Supersaturated Environments : a company that sells large cloud chambers.
Radioactive decay https://en.wikipedia.org/wiki/Radioactive_decay
Demonstration Cloud Chamber at the Museum of Technology in Berlin
Wilson's original cloud chamber from 1911: