Cloud chamber

As a fog chamber, a particle is called in physics, is used to detect ionizing radiation or nuclear reactions and thereby makes visible the way for some particles. Cloud chambers are used today mainly for demonstration purposes.

Cloud chamber generally

A cloud chamber is usually filled with a supersaturated air - alcohol mixture (ethanol or isopropanol). When a high-energy, charged particle passes through the gas, it generated by impact ionization numerous ions that act individually as condensation nuclei for the formation of very fine droplets. In their totality they form a visible trace, a vapor trail.

By deflecting the particle by a suitable electric or magnetic field can use the resulting trajectories (see figure) statements about the mass, charge and energy, and ultimately about the nature of that particle and its evolution are made. In simple mist chambers to most is a strong permanent magnet at the bottom of the chamber, which forces the charged particles by means of the Lorentz force on a spiral path ( the curvature increases because the particle is decelerated by the shocks).

Even without preparation is in our environment a certain amount of alpha and beta radiation, which can be made ​​visible with the cloud chamber:

• Alpha particles produce thick, nearly straight traces of only a few centimeters in length. Although they are indeed distracted principle of magnetic fields due to their charge, is their orbital radius due to the high mass of the α - particles are usually several meters so that their orbits appear practically straight.
• Beta particles produce thin, curved tracks with a few centimeters in length. Often kinks in the web. Because beta particles are nothing more than electrons, they have a low mass and are therefore easily distracted.
• Beta -Plus - particles ( positrons) produce as well as the "normal" negatively charged beta particle thin curved tracks, but they are now bent in the same direction as with α - particles ( see Fig.) Since positron radiation in the natural environment, however, unlikely to occur, one needs for their production of radioactive preparations such as sodium 22nd
• Gamma radiation generated in the cloud chamber no or only a very low probability traces: Since gamma radiation is itself uncharged, it can be detected only indirectly, about the fact that they again charged particles produced in secondary processes ( photoelectric effect or Compton effect). However, since the density of the air - alcohol mixture is quite low in a cloud chamber, also the probability of such secondary processes in the case of the cloud chamber is low.
• Neutron radiation is also generated in the cloud chamber no or only a very low probability traces: Because it is like the gamma radiation itself uncharged, can they be detected only indirectly in this case.

Types of cloud chambers

Depending on the type of generation of supersaturated air - alcohol mixture is made between non-continuous and continuous cloud chambers:

Non - continuous cloud chamber / expansion cloud chamber

• Radioactive source
• Lighting
• Saturated water vapor Alcohol
• Piston
• Observation window

The Wilson cloud chamber (named after its inventor, Charles Thomson Rees Wilson, 1869-1959 ) created the supersaturation by the rapid expansion. By pulling out of a piston ( see adjacent figure) increases the volume of air in the cloud chamber, the pressure and thus the temperature decreases. Thus, the vapor is supersaturated and it takes only small condensation nuclei to produce a mist trail. As the air cools only a short time, the expansion cloud chamber is only about a second able to produce cloud tracks. One can thus produce a short "snapshot" and must then again after a pause, pull out the plunger.

Continuous cloud chamber / diffusion cloud chamber

The diffusion cloud chamber produces the supersaturation by cooling the bottom plate to approximately -30 ° C. There are about 10 cm above the floor heating wires that hold the air - alcohol mixture in the upper area at a temperature of 15 ° C.. Between floor and ceiling, there is thus a temperature gradient and there is just above the ground, a supersaturated layer in which the generation of mist traces. The diffusion cloud chamber can remain in operation for many hours. Cloud tracks that form the ion, rather disappear again and let new tracks are more visible when the free ions of the old cloud tracks are a " driving potential " between floor and ceiling again and again " sucked ". Such an " ion sucker " is useful, but not absolutely necessary.