Did you ever wonder how physicists observe the world of the atoms? Mind you, particles, like the electron for example, are simply too small just to see them under a magnifying glass. Well, scientists have developed many complicated instruments to detect them. Some of those instruments are described on the page so you can learn a bit how to "spy on atoms".
The Geiger-Mueller counter tube
It was developed by Geiger and Mueller in 1913. It consists of a metal, cylindrical tube with a thin wire inside. The wire is a positive electrode, and the whole tube is a negative one. The tube is filled with some gas under a strong pressure. And there is a thin "window" at one end of the tube, through which particles enter.
The scintillation counter
In this kind of counters particles go through special substance exciting its atoms. A moment later the excited atoms emit photons because of getting back to their ground states. The photons fall on a special system called the photomultiplier. This system takes the advantage of photoelectric effect. The initial pulse of photons is changed here into an electrical one that is registered after some amplification. Such counters are used for example for detecting uncharged particles (f.e. neutrons).
The nuclear emulsion
As you maybe remember on the previous page it was said that Becquerel discovered radioactivity placing some uranium compound on a photographic film and the film got blackened. Today we know that the blackening was caused by particles emitted by uranium and passing through the film. In 1910 S. Kinoshita and M. Reinganum used photographic emulsion for registering charged particles. In the forties the technique was improved by L.F. Powell. On thephotographic emulsion paths of particles can be very precisely registered and so many phenomena of the world of the atoms can be studied.
A charged particle going through the nuclear emulsion causes ionisation of its atoms. This is the cause of the change of the chemical properties of the substance. The dark track appears.
The cloud chamber
It consists of a hermetic chamber filled with some gas. The temperature inside the chamber is a bit lower than the temperature of the gas liquefaction. But to condense any gas something more than only proper temperature is needed - a piece of dust or a charged particle for example. (That is why sometimes, small flecks are spread around from planes to cause rain in the areas of drought). So if there is a particle going through the cloud chamber, then it leaves some droplets behind on its path. And scientists can observe these droplets of the condensed gas.
The bubble chamber
The bubble chamber was developed in 1952 by D. A. Glaser. It is based on a similar idea as the cloud chamber. But this one is a chamber filled not with gas but with a liquid of a bit higher temperature than the boiling point. But with boiling it is just like with condensation - something more than only proper temperature is needed. So boiling is caused by a particle going through the liquid. Small bubbles appear behind the particle as it goes through the chamber. And they take pictures of those "bubble paths".
In the Wilson chamber and in the bubble chamber a magnetic field is used that/, which curves the trajectories of the charged particles. Thanks to that the kind of the particles can be determined.
The spark chamber
This instrument consists of a large number of wires or plates, which are situated parallel with each other. They are convertible charged (+ -+ - + -...). And so between each pair the electric voltage subsists. If an electron or ion appears between the plates, then it causes the ionisation of the atoms of the gas – it is an avalanche process. The voltage between the plates is high enough to make the proceeding ionisation cause a spark-over between the plates. The spark can be registered. And so a charged particle moving through the spark chamber leaves a track of sparks behind.
