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.

NRC Joins Five Other Agencies in Reporting on Navajo Land Contamination

Maureen Conley
Public Affairs Officer
 

navajoThe government has made good progress in reducing risks from uranium contamination on Navajo land, five federal agencies told Congress in a report last week. EPA compiled the report with input from the NRC, the Department of Energy, the Bureau of Indian Affairs, the Centers for Disease Control and the Indian Health Service.

This report recaps work done since October 2007. At that time, Congress asked the agencies to develop a five-year plan to address the contamination, which dates back to the 1940s.

Demand for uranium skyrocketed near the end of World War II. The ore was needed for nuclear weapons manufacturing and later to fuel commercial power reactors. The Navajo Nation lands had large uranium deposits, but mining and milling then was not nearly as regulated as it is today. Mining companies left extensive contamination requiring cleanup.

In 1978 Congress passed a law to ensure that uranium mill waste (called tailings) would be safely managed into the future. Under that law, DOE is responsible for the long-term care and maintenance of four former mill sites: Tuba City, Ariz.; Shiprock, N.M.; Mexican Hat, Utah; and Monument Valley, Ariz.

The NRC oversees DOE’s work at those sites. For example, DOE is responsible for cleaning up contaminated groundwater at the sites. The NRC reviews those cleanup plans. DOE monitors disposal facilities for uranium mill tailings. The NRC observes DOE inspections at the sites. The NRC also reviews and comments on DOE’s performance and environmental reports.

While the NRC does not regulate mine cleanup, the agency will also be working closely with EPA, DOE, the New Mexico Environment Department, and the Navajo Nation during the cleanup of a contaminated mine site in Church Rock, N.M. This conventional strip mine operated from 1967 to 1982. EPA plans call for the mine waste to be disposed at the nearby Church Rock mill site, which must be done in compliance with NRC disposal regulations.

Over the past five years, NRC staff has met many times with members of the Navajo Nation. We will continue these oversight and outreach activities.