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Radiation Units

What is a
Gieger Counter?

Geiger counters detect some or all of the four major types Geiger counters detect some or all of the four major types of ionizing radiation, namely Alpha, Beta, Gamma, and X-rays. Both Gamma and X-rays are part of the electromagnetic spectrum, specifically at its high frequency, short wavelength end. That same spectrum also includes the more familiar ultraviolet light, visible light, infrared rays, microwaves, and radio waves, listed in order of decreasing frequency and increasing wavelength from Gamma and X-rays.

As the four major types of ionizing radiation go, Gamma and X-rays are very powerful and potentially very dangerous. They can pass through virtually anything, and are effectively shielded or absorbed only by materials of high atomic weight such as lead. Because of their penetrating ability, X-rays are used to see inside the human body, destroy cancer cells in radiation therapy, or analyze the internal structure of rocks and minerals, for instance.

Gamma rays are produced naturally by the sun and other bodies in outer space, their transmission to earth being known as "cosmic radiation". Certain minerals that make up part of the earth containing the radioactive elements Uranium and/or Thorium also emit Gamma rays. This cosmic radiation, along with these radioactive earth minerals, combine to produce the "background count" of a Geiger counter. That is, even when a Geiger counter is removed from a specific radioactive object, the meter will still register a background level or count of radioactivity. This might typically be in the range of 20 counts per minute, but will vary depending upon where it is on the earth. The background count should always be factored in or "subtracted" from the overall reading derived from a specific radioactive source. X-rays, being very similar to Gamma rays, are produced from man-made sources such as X-ray tubes, arcs, and lamps.

While not part of the electromagnetic spectrum, Alpha and Beta particles are the two other types of ionizing radiation detectable by Geiger counters. Alpha radiation consists of positively charged particles emitted from the nucleus of an atom in the process of decay. These particles are also very dense which, with their strong positive charge, precludes them from penetrating more than an inch of air or a sheet of paper. Because of this, Alpha particles are not a serious health hazard, except when they are emitted from within the body as a result of ingestion, for instance, when their high energy poses an extreme hazard to sensitive living tissue. Radioactivity of this nature is difficult to detect using a standard Geiger counter, but since most Alpha-emitting substances also produce Gamma rays, this can allow for detection in some cases.

Beta radiation consists of negatively charged particles emitted from an atom in the process of decay. These particles are relatively light and can penetrate somewhat better than an Alpha particle, though still only through a few millimeters of aluminum.

What household items can emit radiation?

Smoke Detectors - Contain an Alpha and Beta ray producing radioactive isotope of the element Americium that senses smoke. This is sealed, though, and may not give you any reading.

Lantern Mantels - Some of the mantels in camping lanterns, especially those from earlier years, are made of the radioactive element Thorium. Be careful not to inhale the mantle ash!

Watches - Some old watches and clocks, even gyroscopes, have dials painted in radium to make them glow in the dark. Radium emits Alpha and Gamma rays. More recently, Tritium, a radioactive isotope of Hydrogen, has been used to paint the dials, but it is too weak to penetrate the watch lens.

Camera Lenses - It has been reported that certain old Leica camera lenses colored yellow owe their hue to the presence of the radioactive element, Thorium.

Jewelry - Certain gems are irradiated by the radioactive element Radium, X-rays, or other particles to produce a color change. This can make the gem itself radioactive which can last for years in some cases. Also, cloisonne, an enameled variety of jewelry, is glazed with Uranium oxide.

Rocks and Minerals - Minerals such as uraninite and its pitchblende variety, also carnotite and thorianite, contain the radioactive elements Uranium and/or Thorium.

Pottery - Some older pottery such as the Fiesta Ware brand often found in antique stores is glazed with Uranium oxide. When broken, the dust can emit Alpha particles of radiation.

Furniture - More recently, certain metal objects in the home are reported to be partly made of spent radioactive material.

How much radiation exposure is too much?

The following data is from an industry source believed to be reliable. However, you should verify the following government data. Additionally, "acceptable" levels of radiation exposure are somewhat subjective, so we encourage you to research and make your own conclusions as to the maximum levels of radiation exposure which would be acceptable to you:

The US Government recommends that you do not exceed the following maximum dosages of radioactivity:

Occupational:        5R/yr = 5,000mR/yr
Non-Occupational:   .1R/yr (100mR/yr) + 100mR/yr (background)*
                  = 200mR/yr total

    *estimated annual exposure to background radiation

Assume for now that you classify yourself as non-occupational, i.e. you don't work in a field where you handle radioactive materials or are exposed to radioactivity. Then, if you are exposed to a "hot" rock reading 100mR/hr for 1 hour's length of time, you have already received your maximum recommended annual dosage of 100mR/yr.

This is also equivalent to the dosage you would receive by taking 100 airplane trips per year lasting 4 hours each in a plane flying at 30,000', since that dosage, attributable to cosmic radiation, is at the rate of 250 CPM or .25 mR/hr.