The Geiger Counter
Geiger Counters are instruments that can detect and measure radioactivity. H. Geiger and E.W. Muller invented the Geiger counter in 1928. To be considered a true Geiger Counters, the instrument must contain a Geiger counter (GM) table as its radioactivity sensor. Geiger counters are useful in performing experiments with radioactivity and nuclear energy. You could even go prospecting for uranium, if you desire.
The Geiger Mueller (GM) tube hasn't changed much since it was invented in 1928. The operating principle is the same. A cutaway drawing of a GM tube is shown in figure 1. The wall of this particular GM tube is a thin metal cylinder (cathode) surrounding a center electrode (anode). It is constructed with a thin Mica window on the front end. The thin mica window allows the passage and detection of alpha particles. The tube is evacuated and filled with Neon, Argon plus Halogen gas.
It is interesting to see how the GM tube detects radioactivity. In the figure shown, a 500-volt potential is applied to the anode (center electrode) through a ten mega-ohm current limiting resistor. To the cathode of the tube a 460-k ohm resistor is connected.
In the initial state the GM tube has a very high resistance. When a particle passes through the GM tube, it ionizes the gas molecules in its path. This is analogous to the vapor trail left in a cloud chamber by a particle. In the GM tube, the electron liberated from the atom by the radioactive particle and the positive ionized atom both move rapidly towards the high potential electrodes of the GM tube. In doing so, they collide with and ionize other gas atoms. This creates a small conduction path allowing a momentary surge of electric current to pass through the tube.
This momentary pulse of current appears as a small voltage pulse across R2. The halogen gas quenches the ionization and returns the GM tube to its high resistance state making it ready to detect radioactivity.
Count Rate Vs Dose Rate
Each output pulse from the GM tube is a count. The counts per second give an approximation of the strength of the radiation field. Below is the GM tube’s response to a cesium-137 source, shown in figure 2
Every style of GM Tube will have its own unique count rate vs dose rate. The graph above is for one style of GM Tube.
Radioactivity is the spontaneous emission of energy from the nucleus of certain elements, most notably uranium. There are three forms of energy associated with radioactivity; alpha, beta and gamma radiation. The classifications were originally made according to the penetrating power of the radiation.
Alpha rays were found to be the nuclei of helium atoms, two protons and two neutrons bound to-gether. Alpha rays have a net positive charge. Alpha particles have only a weak penetrating ability; a couple of inches of air or a few sheets of paper can effectively block them.
Beta rays were found to be electrons, identical to the electrons found in atoms. Beta rays have a net negative charge. Beta rays have a greater penetrating power than Alpha rays and can penetrate 3mm of aluminum.
Gamma rays are high-energy photons. This has the greatest penetrating power being able to pass through several centimeters of lead and still be detected on the other side.
Images Digital Geiger Counters are sensitive to all three types of radioactivity.
Finding Radioactive Sources
Uranium ore from a mineral or a rock store should also emit sufficient radiation to trigger the counter. Uranium ore is available on Amazon.com as well as Images Website:
If you need a radioactive source that outputs a know radiation type and energy level you can purchase a radioactive isotope. A radioactive isotope is a more reliable source of radioactivity because you have an idea of the radiation type being emitted and its energy value.
Small amounts of radioactive isotopes are available for sale encased in 1 inch diameter by ¼" thick plastic disks. The disks are available to the general public license exempt. This material outputs radiation in the micro-curie range and has been deemed by the Federal government as safe. To purchase a radioactive isotope go to the Images SI Inc. website. They have a large variety of radioactive isotopes available for purchase.