Hans Geiger and Ernest Rutherford invented the original Geiger counter in 1908 to detect alpha particles. Geiger and Walther Mueller revised in 1928 to detect other forms of radiation as well. The Geiger counter sensor is a central anode of metal wire surrounded by a thin metal cathode tube filled with neon, argon and a halogen gas that detects radiation by the amount the gas is ionized inside the tube.
1 Turn on the Geiger counter to apply an electrical load to the anode cable. The counter will click or blink around 10 to 20 times per minute as it detects background radiation.
2 Pass the sensor, called a Geiger-Mueller tube, over the material to be evaluated with the fine mica window facing the material. The radiation of the material, if any, will pass through the window and ionize the gas inside the tube.
3 Study of the reading, either of a meter with needles, and LED indicator or audible click. If this is higher than the level of background radiation, the material is radioactive.
4 Count the number of clicks or blink or read the attached meter to determine how radioactive material is.
Tips and warnings
- By replacing the gas in the sensor with boron trifluoride and adding a plastic moderator, the Geiger counter can be used to detect neutrons.
- Be sure to use corresponding radiation protection when using the Geiger counter. Alpha particles (helium nuclei) are low-energy radiation that can be stopped by several inches of air, sheets of paper or layers of clothing. Beta particles (high energy electrons) are more powerful, capable of penetrating aluminum sheets up to three millimeters thick. Gamma particles (high-energy photons) can penetrate several centimeters of lead and require thick lead shielding to be stopped.
- All Geiger counters experience a small amount of “dead time” between the ionizing particles the gas in their sensor, usually measured in microseconds. While there is a mathematical formula to compensate for downtime, in most cases downtime can be ignored, except when treated with high-energy radiation.
- Geiger counters can only detect the presence and intensity of radiation. To determine the energy levels of the particles, use a proportional counter.
- Geiger counters can not accurately measure the presence of radon gas in a house. For this, the purchase of a radon detector with an activated carbon filter.
Experiments with the Geiger counter
A Geiger counter is a portable electronic device that detects ionizing radiation with a small high voltage vacuum tube. X-rays and radioactive decay products, such as alpha particles, beta particles and gamma rays, will sound a Geiger counter. The amount of radiation is recorded as a number on a screen and, as a series of audible clicks. You can use one to conduct experiments with radiation and radioactivity safely.
Every place on Earth has a small amount of natural radiation that comes from the sky and minerals in the ground. Scientists call this background radiation, and it can be measured with a Geiger counter. Simply turn the Geiger counter away from any strong radiation source. The background radiation will be measured in microsieverts units per hour or millirems per hour. Since the radioactive decay has a random character, you will see the measured radiation increase and decrease slightly over time.
Prepare a solution of pure water and 0.9 percent sodium chloride (table salt) by weight. Obtain a radioactive decay kit from a scientific source house that has a cesium-137 generator. These are safe enough for sending via postal mail. Fill the syringe of the generator with 5 milliliters of salt water, screw it to the capsule and pump the water, although in a small beaker. Immediately start measuring the radioactivity of the water with the Geiger counter. Record the radioactivity rate measured in each minute, and repeat for 20 minutes. Graph the results on a graph, with the rate on the Y axis and minutes on the X-axis. You can pour the water down the drain, as it will have very low radioactivity after 20 minutes.
Shielding against radiation
Get small “button” type radioactive sources from a scientific supply house. Have a variety of useful materials, such as paper, wood, steel and, if possible, lead. Using a Geiger counter, measure the radiation from one of the 8-inch button sources and write down the results. Place a piece of wood or metal between the source and the counter. Some materials will greatly reduce radiation, others to a lesser degree. Carry out this experiment with each of your radiation sources.
Part of the background radiation, called cosmic rays, comes from the stars and the sun. Its intensity changes with altitude. If you live in a mountainous area, get a receiver of the global positioning system (GPS). With the Geiger counter, record background radiation at ground level as in the previous section. Drive on a mountain road and take outdoor background radiation readings at 20-meter height intervals using GPS to measure altitude. Keep in mind that the higher you ascend, the more radiation you receive. The atmosphere acts as a natural shield.