Geiger Counters

Geiger-Müller Tube based detectors can detect the four main types of ionizing radiation: alpha, beta, gamma, and x-rays. Some detect only gamma and x-rays. Our instruments are calibrated to Cesium 137, but also serve as excellent indicators for many other sources of ionizing radiation. Gamma and x-rays are measured in milli-Roentgens per hour (mR/hr), micro-Sieverts (µSv/hr), or milli-Sieverts (mSv/hr). Alpha and beta are measured in counts per minute (CPM) or counts per second (CPS).

The window of the GM tube is very thin mica. This mica window is protected by a screen. Some levels of alpha, low energy beta, gamma, and x-rays that cannot penetrate the plastic case or the side of the tube can be sensed through the window.

Try not to touch the instrument to any suspected radioactive substance.

Although some beta and most gamma radiation can go through protective gear, try to avoid skin contamination and ingestion. When you leave a radioactive area, remove any protective outerwear and dispose of it properly. If you think you have been contaminated, as an additional precaution, shower and consult a physician.

To determine whether the radiation detected is alpha, beta, or gamma, hold the instrument toward the source.

Alpha: If there is no indication through the back of the case (the side of the tube), position the window close to but not touching the source. If there is an indication, it is alpha, beta, or low energy gamma. If a sheet of paper placed between the window and the source stops the indication, it is most likely alpha. To avoid particles falling into the instrument, do not hold the source above the window.

Beta: Place a piece of aluminum about 1/8 inch (3 mm) thick between the instrument and the source. If the indication stops, decreases, or changes, it is most likely beta radiation. Most common isotopes emit both beta and gamma radiation. This is why the indication would decrease or change but not stop.

The non-occupational dose limits set by the government is 100 mR per year above background per year.

It is up to the individual to decide what a safe radiation level is. It will be different depending on the individual and their knowledge of radiation and its affects. Radiation levels will vary according location and circumstances. As an example; if your background level is 25 CPM (counts per minute) where you live, when you fly in an airplane at 30,000 feet your rate meter may measure 200 CPM (.2 mR) for 2 to 5 hours. That is 8 times your normal background radiation on the ground, but it is only for a limited amount of time.

When measuring radiation in an emergency response situation, it is good to have something to compare your readings to. Taking a background radiation level reading in your area before a radiation event will help you determine if you have an elevated level of radiation and whether or not to stay in that location. Background radiation is naturally occurring radiation that is always present. It includes; high energy gamma rays from the sun and outer space and alpha, beta, gamma radiation emitted from elements in the earth. Using a rate meter, you can determine your normal background radiation levels.

For a good emergency response kit for radiation we recommend a general purpose Geiger counter (like the Monitor 4), a carbon fiber dosimeter (such as the PEN200) and a Charger to reset the dosimeter. There are electronic dosimeters, however, if you were in the blast zone of an atomic bomb the pulse of the bomb would make most electronic equipment inoperable. The carbon style dosimeters will still operate.

Gamma: If there is an indication of radioactivity, it is most likely gamma or high energy beta. Low energy gamma and x-rays (10-40 keV) cannot penetrate the side of the GM tube, but may be detected through the window.

If you perform the alpha/beta test above and there is no change or only a very slight change in the indication, the source is emitting primarily gamma radiation.

Scintillation Detectors

Some forms of radiation are very difficult or impossible for a Geiger tube to detect. Tritium, for example, is a byproduct of a nuclear reactor and is used in research. The beta emissions from Tritium are so weak that there are very few instruments that are capable of detecting it. More sophisticated equipment is needed for the measurement of environmental samples, such as radioactivity in milk, produce, soil, etc., unless you are looking for gross contamination. Scintillation Probes, like geiger counters, have the ability to detect radiation, but are much more sensitive than their GM counter parts. They are made of a crystal or a plastic that generates a photon of light when hit by a particle is coupled with a photo multiplier tube and both enclosed in a light tight package. The output pulse is proportional to the energy of the input particle. Today, they are used in a wide variety of radiation detectors, survey meters, and spectrum analyzers requiring identification and detection of specific isotopes at lower levels. Spectrum Analyzers come in stand alone units that require a specific scintillation probe to function as well as pc peripherals that can use various probes and all the variations in between.

Ionization Chamber

An insulating chamber with some gas the ionizes under the influence of strong radiation. Many of the surplus civil defense survey meters (often erroneously called geiger counters) use this type of detector. They are used in air duct monitoring and for calibration of laboratory sources.

Radiation Survey Meters | Geiger Counters | Radiation Detectors