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Geiger counters 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 milliroentgens per
hour (mR/hr), microsieverts (µSv/hr), or millisieverts (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.
How to Determine Alpha, Beta, or Gamma source.
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 above background annually.
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 to 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.
Gamma and X-Rays: 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.
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