Cloud chamber
Animated version
In physics, a cloud chamber is a particle detector that is used to detect ionizing radiation and, for some particles , also makes their path visible. Today, cloud chambers are almost exclusively used for demonstration purposes. In the past, cloud chambers were important scientific instruments for researching the rays emitted by radioactive substances. So was Charles Thomson Rees Wilson for developing the expansion cloud chamber (also Wilson cloud chamber) in 1927 awarded the Nobel Prize in Physics.
Cloud chamber in general
A cloud chamber is usually filled with an oversaturated mixture of air and alcohol ( ethanol or isopropanol ). When an energetic, charged particle passes through the gas, it generates numerous ions through impact ionization , which individually act as condensation nuclei for the formation of the finest droplets. In their entirety, they form a visible trail, a contrail.
By deflecting the particle by means of a suitable electric or magnetic field , statements can be made about the mass , charge and energy based on the resulting trajectories (see Fig.) , And thus ultimately about the type of particle and its formation process. In simple cloud chambers there is usually a strong permanent magnet at the bottom of the chamber, which uses the Lorentz force to force the charged particles onto a spiral path (the curvature increases because the particle is slowed down by the impact).
Even without a preparation, there is a certain amount of alpha and beta radiation in our environment, which can be made visible with the cloud chamber:
- Alpha particles produce thick, almost straight tracks only a few centimeters long. Although they are in principle deflected by magnetic fields due to their charge, their orbital radius is usually several meters due to the high mass of the α-particles, so that their orbits appear practically straight.
- Beta particles create thin, curved tracks a few centimeters long. Often there are also kinks in the web. Since beta particles are nothing more than electrons , they have a low mass and are therefore easily deflectable.
- Beta-plus particles (positrons) , like the “normal” negatively charged beta particles, create thin, curved tracks, which, however, are now bent in the same direction as with α-particles (see Fig.). Since positrons hardly occur in natural ambient radiation, radioactive preparations such as sodium-22 are required to generate them.
- Gamma radiation does not produce any traces in the cloud chamber, or only with a very low probability: Since gamma radiation itself is uncharged, it can only be detected indirectly, for example by generating charged particles again in secondary processes ( photo effect or Compton effect ). However, since the density of the air-alcohol mixture in a cloud chamber is quite low, the probability of such secondary processes in the case of the cloud chamber is also low.
- Neutron radiation also produces no traces in the cloud chamber, or only with a very low probability: Since it is uncharged like gamma radiation itself, it can only be detected indirectly in this case.
Types of cloud chambers
Depending on the type of production of the supersaturated air-alcohol mixture, a distinction is made between non-continuous and continuous cloud chambers:
Non-continuous cloud chamber / expansion cloud chamber
The Wilson cloud chamber (named after its inventor Charles Thomson Rees Wilson ) generates supersaturation through rapid expansion. By pulling out a piston (see figure opposite) the volume of air in the cloud chamber increases, the pressure and thus the temperature decrease. As a result, the steam is oversaturated and you only need small condensation nuclei to create a trail of fog. Since the air only cools for a short time, the expansion mist chamber is only able to generate fog trails for about a second. You can only take a short "snapshot" and then pull out the piston again after a pause.
Continuous cloud chamber / diffusion mist chamber
The diffusion mist chamber ( invented by Alexander Langsdorf in 1936) generates the supersaturation by cooling the base plate to approx. −30 ° C. About 10 cm above the floor there are heating wires that keep the air-alcohol mixture in the upper area at a temperature of approx. +15 ° C. There is thus a temperature gradient between the floor and the ceiling and an oversaturated layer is created just above the floor, in which the creation of traces of fog is possible. The diffusion mist chamber can remain in operation for many hours. Traces of fog that form on the ions tend to disappear again and make new traces more visible when the free ions of the old traces of fog are repeatedly "sucked off" by a "suction voltage" between floor and ceiling. Such an "ion absorber" is useful, but not absolutely necessary.
Web links
- Typical cloud chamber shots. LEIFI
- Original of the Wilson cloud chamber. In: njsas.org (English)
- Cloud Chamber - Fundamentals of Particle Physics. In: solstice.de
Individual evidence
- ↑ Introduction to Nuclear Physics in the Google Book Search
- ↑ Progress in Nuclear Physics, Volume 3 in the Google Book Search