Geiger counter: how do they detect and measure radiation?

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A Geiger counter, also known as a Geiger-Muller tube, is an inexpensive and useful instrument used to quickly detect and measure radiation.

There are two types of radiation, non-ionizing and ionizing. Non-ionizing radiation such as microwaves has enough energy to shake atoms apart, but not enough to knock them off electrons and change their composition. Ionizing radiation, on the other hand, can strip atoms of their electrons, in a process called ionization. As a result, a pair of ions are formed – a positively charged atom and a negatively charged electron.

A Geiger counter uses the natural process of ionization to detect and measure radiation. The device houses a stable gas in its chamber. When exposed to radioactive particles, this gas ionizes. This generates an electric current which the meter records over a period of 60 seconds.

When ionization occurs and current is produced, a speaker clicks and a reading is given – often in millisieverts (mSv). There are several types of radioactive particles that cause ionization, known as alpha, beta, or gamma radiation. However, Geiger counters cannot differentiate between different types of radiation.

Exposure to ionizing radiation can be harmful to human health. When this type of radiation comes into contact with DNA molecules in living cells, its energetic nature can disrupt, damage or alter the DNA. Short exposure to some forms of radiation, such as X-rays for medical examinations, does not cause immediate health risks. However, prolonged exposure can lead to DNA mutations and produce cancers. Geiger counters are therefore a valuable tool for evaluating a potential source of radioactivity.

Types of ionizing radiation

Positively charged alpha radiation particles contain two protons and two neutrons, like the nucleus of a helium atom. They are heavy and slow, and can be blocked by a piece of paper or a thin layer of skin. This makes them significantly less dangerous than other types of radiation.

Beta radiation particles are high-energy electrons (or sometimes the counterparts of electrons, called positrons). They are relatively light particles, about one thousandth the mass of a proton. Natural sources of beta radiation are radioactively decaying elements, such as uranium or actinium.

Gamma radiation, also known as gamma rays, is a form of electromagnetic radiation similar to X-rays. It emits the most energetic photons (particles of electromagnetic radiation) in the electromagnetic spectrum. Gamma rays are very penetrating and can easily pass through the body and cause damage.

This vial of uranyl nitrate contains radium, one of the most radioactive elements on the periodic table.  This would certainly send any Geiger counter into a clicker slump.

This vial of uranyl nitrate contains radium, one of the most radioactive elements on the periodic table. This would certainly send any Geiger counter into a clicker slump. (Image credit: Getty)

Creation of the counter

The Geiger counter was conceptualized and designed by German physicists Hans Wilhelm Gieger and British physicist Ernest Rutherford, in 1908. Their initial creation could only detect alpha particles.

The pair used their counter to study alpha particles and in 1911 published the results of several groundbreaking experiments, such as the gold foil experiencewho finally revealed the nucleus of atoms to the world.

Between 1925 and 1928, Geiger and his doctoral student Walter Muller improved the sensitivity of the counter to detect all types of ionizing radiation. The design of the Gieger-Muller counter remains relatively unchanged in the Gieger counters used today.

Ernest Rutherford and Hans Geiger, nuclear physicists

German physicist and inventor of the Geiger-Muller tube, Dr Hans Wilhelm Geiger, with fellow physicist Ernest Rutherford. (Image credit: Getty Images/Science & Society Picture Library)

Additional Resources

Learn more about radiation at Centers for Disaster Control and Preventionfind out how radiation therapy can be used to treat cancer, via Cancer.gov and learn what radiation that you might encounter in everyday life.

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