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NRC Science 101: The What and How of Geiger Counters

Joe DeCicco
Senior Health Physicist
Source Management and Protection Branch
 

In earlier Science 101 posts, we talked about ionizing radiation and different types of radiation. In this post, we’ll look at the Geiger counter, an instrument that can detect radiation.

science_101_squeakychalkJust to recap, the core of an atom (the nucleus) is surrounded by orbiting electrons, like planets around a sun. The electrons have a negative charge and usually cancel out an equal number of positively charged protons in the nucleus. But if an electron absorbs energy from radiation, it can be pushed out of its orbit. This action is called “ionization” and creates an “ion pair”—a free, negatively charged electron and a positively charged atom.

Humans cannot detect creation of an ion pair through their five senses. But the Geiger counter is an instrument sensitive enough to detect ionization. Most of us have heard or seen a Geiger counter. They are the least expensive electronic device that can tell you there is radiation around you—though it can’t tell you the original source of the radiation, what type it is or how much energy it has.

How does it work? A Geiger counter has two main parts—a sealed tube, or chamber, filled with gas, and an information display. Radiation enters the tube and when it collides with the gas, it pushes an electron away from the gas atom and creates an ion pair. A wire in the middle of the tube attracts electrons, creating other ion pairs and sending a current through the wire. The current goes to the information display and moves a needle across a scale or makes a number display on a screen. These devices usually provide “counts per minute,” or the number of ion pairs created every 60 seconds. If the loud speaker is on, it clicks every time an ion pair is created. The number of clicks indicates how much radiation is entering the Geiger counter chamber.

You hear a clicking sound as soon as you turn on the speaker because there is always some radiation in the background. This radiation comes from the sun, natural uranium in the soil, radon, certain types of rock such as granite, plants and food, even other people and animals.

The background counts per minute will vary; the needle will move or the number will change even when there is no know radiation source nearby. Many different things cause this fluctuation, including wind, soil moisture, precipitation (rain or snow), temperature, atmospheric conditions, altitude and indoor ventilation. Other factors in readings include geographical location (higher elevations give higher counts), the size and shape of the detector, and how the detector is built (different chamber material and different gases).

geigercounterDepending on the elevation and the type of Geiger counter, a typical natural background radiation level is anywhere from five to 60 counts per minute or more. Because background radiation rates vary randomly, you might see that range standing in one spot. It is important to understand that the Geiger counter indicates when an ion pair is created, but nothing about the type of radiation or its energy.

Other types of instruments can provide an exposure rate (expressed as milliroentgen per hour or mR/hr). These counters must be calibrated to read a particular type of radiation (alpha, beta, gamma, neutron, x-ray) as well as the amount of energy emitted. The reading will only be accurate for that type of radiation and that energy level. And these instruments need to be calibrated regularly to be sure they are providing correct information over time.

For more sophisticated environmental radiation readings, check out the Environmental Protection Agency’s nationwide system, RadNet. Using equipment far more sensitive than a Geiger counter, it continuously monitors the air and regularly samples precipitation, drinking water and pasteurized milk.

Over its 40-year history, RadNet has developed an extensive nationwide “baseline” of normal background levels. By comparing this baseline to measurements across the U.S. states in March 2011, following the accident at the Fukushima reactors in Japan, the EPA was able to detect very small radiation increases in several western states. EPA detected radiation from Japan that was 100,000 times lower than natural background radiation—far below any level that would be of concern. And well below anything that would be evident using a simple Geiger counter, or even Geiger counters spread across the country.

If RadNet were to detect a meaningful increase in radiation above the baseline, EPA would investigate immediately. With its nationwide system of monitors and sophisticated analytical capability, RadNet is the definitive source for accurate information on radiation levels in the environment in the U.S.

By the way, the Geiger counter is also called a Geiger-Mueller tube, or a G-M counter. It was named after Hans Geiger, a German scientist, who worked on detecting radiation in the early 1900s. Walter Mueller, a graduate PhD student of Geiger’s, perfected the gas-sealed detector in the late 1920s and received credit for his work when he gave his name to the Geiger-Mueller tube.

10 responses to “NRC Science 101: The What and How of Geiger Counters

  1. CaptD July 17, 2014 at 4:34 pm

    Here is a ☢ Monitor (made in CA) that not only measures ☢ but also uploads the data to the Web and it is low cost! They make models that stand alone and also that connect to your smart cell phone!

    http://iradgeiger.com/

  2. richard123456columbia July 16, 2014 at 7:12 pm

    I have taken snap shots of RadNet with readings over 70CPM (Beta Gross Count Rate)

  3. Jason Kennerly July 16, 2014 at 12:35 pm

    I say we educate the public. Let them have cheap geiger counters in retail stores.

    Then those who refuse to be educated, let them live with the health consequences of not enough potassium in their diets and without the benefit of smoke detectors!

    Evolution can fix this problem of radiation fear, just like it did for fire. A human infant is the only baby mammal that will stare, fascinated, at a tiny flame.

  4. richard123456columbia July 15, 2014 at 4:04 pm

    Your statement : “EPA detected radiation from Japan that was 100,000 times lower than natural background radiation—far below any level that would be of concern. And well below anything that would be evident using a simple Geiger counter, or even Geiger counters spread across the country.”
    I watched the readings in Vancouver Canada after 311 and it rose to 50cpm and then was shut down. Why was it shut down and how high did it go?

    • Moderator July 16, 2014 at 10:27 am

      The EPA RadNet system only monitors radiation in the U.S. Without knowing more about the meter, the type of energy it was detecting and normal background rates in that area, it is impossible to say what a 50 cpm reading indicates. You would need to contact whoever was operating that meter for answers to your questions.

      For more information on RadNet’s operations immediately following Fukushima, check out this link: http://www.epa.gov/japan2011/index.html

      Joe DeCicco

    • Anonymous July 30, 2014 at 4:01 pm

      With respect to determining what oversight for releases to the environment is needed, why would you assume that the public has any more ability than the people who are trained and paid to do so. We can’t even agree on taxes, debt ceilings, illegal immigration, who’s in the 1%, saturated fats, gluten, the Kardashians, Rick Dees’ weekly top forty picks, etc., etc.. What will happen is the public will band together and select someone to represent them on the issue…oh wait, we already have that. It’s called congress. If you don’t like how congress is handling things, vote someone else in. Making vague statements about how the unicorns and the butterflies will step in and save us from the evil government doesn’t accomplish anything except wasting another soap box.

  5. Nissan July 15, 2014 at 1:29 pm

    I think, the investigation of the EPA should be done immediately. And the discovery of this tool (instrument) is really exciting news for the community.

  6. Garry Morgan July 15, 2014 at 12:23 pm

    Good article, thank you. We use Geiger Counters and Radionuclide analyzers frequently, here is why.

    Rad Net does not cover all areas nor areas near nuclear facilities. Real time monitoring is lacking in the United States beyond the confines of the nuclear facility itself. Unfortunately the real time monitors at the nuclear facilities do not provide real time information to citizens. Nuclear operators quarterly reports with the publication of a required annual report document is not sufficient to disclose radionuclide releases in a timely fashion. It has been observed that off site and on site monitors are either missing or non-functioning.

    Our monitoring indicates radiation from nuclear facilities is not confined to the NRC’s 10 mile, 25 mile or even 50 mile Emergency Planning Zones. In the Tennessee River Valley we have discovered increased ionizing radiation levels up to 80 miles downwind during fuel movement operations and reactor shutdowns where there is an atmospheric emissions release.

    Your secrecy is your worse enemy, unless you have something to hide. The NRC should require ALL nuclear facility operators to provide ALL monitoring in real time and make it available to the public in real time. Unless the emphasis is on money before human health.

    The excuse for not providing real time monitoring according to some NRC staff, “…the public will not understand the data and it may cause fear.” Ladies and gentleman of the NRC, you are failing in your mission if you assume the public will not understand the data you should be providing. I’ll assure you, if you provide the real time monitoring of nuclear facilities on a public website someone will insure the public will definitively understand the data being provided. You should not make faulty assumptions. Maybe the public will understand, is that the nature of your fear?

    Make Radiation Visible – http://www.makeradiationvisible.org/

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