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  • Writer's pictureKristjan Salev


Updated: Feb 4, 2023

How does a Geiger counter work

The Geiger counter is a useful gadget that helps detect ionizing radiation. It was developed in 1908 by German physicist Hans Wilhelm Geiger. Although the original version of this device was only able to detect one type of radioactive particle (alpha particles), the version that was developed in 1928 was also able to detect other types of radioactive particles. A device that was once quite large and expensive has become significantly smaller and more accurate as electronics have evolved. At present, anyone can buy an affordable device that fits in their pocket and can measure radioactivity whenever it is needed. Let's explore, what are the main parts of this device and how it works.

How does a Geiger tube work?

The main component of the Geiger counter is a tube filled with some inert gas. This can be, for example, helium or neon. In a normal state, the gas particles have a neutral charge. After ionization, however, there remains a free positively charged gas atom or molecule and a free negatively charged electron. There is a positive electrode inside the tube, called the anode. A negative electrode, called a cathode, is formed from the wall of the tube. There is an electric field between these two electrodes.

Geiger tube

Now, when the radioactive radiation ionizes the gas particles, and the positively and negatively charged particle is separated from each other, the negatively charged electrons move to the anode and the positively charged particles to the cathode. Since there is a strong electric field between the anode and the cathode, the electrons receive enough energy to further ionize the gas, and thus an avalanche of free electrons is released, all of which move to the anode.

Geiger tube and ionizing radiation

The Geiger tube is connected to the electrical circuit. When the free electrons pass through the circuit, the counter registers an electric current. This indicates that a radioactive particle has entered the Geiger tube. As the electrons pass through the circuit and join the positively charged particles in the wall of the tube, they again become neutrally charged molecules, and the gas returns to its initial state. The whole process repeats when the next radioactive particle enters the tube.

How Geiger counter detects radiation

The counter registers these electrical impulses, and in this way, we get an idea of how strong the radiation field is. Most Geiger counters can detect beta, gamma, and x-ray radiation. More expensive Geiger counters can also detect alpha radiation.

How are alpha, beta, and gamma radiation produced?

Radioactivity happens when unstable nuclei disintegrate. When that happens, they emit nuclear fragments and particles. Alpha particle is a helium nucleus, consisting of two protons and two neutrons and a beta particle is an electron (or positron). X - rays and gamma rays are photons of pure energy.

Alpha particles are heavier and less dangerous. They have a very limited ability to penetrate the material and can already be blocked with a sheet of paper. However, they can be dangerous if inhaled or swallowed with food.

Beta particles are lighter and can penetrate the skin and several other materials, but can be stopped, for example, with a thin sheet of metal or plastic.

Gamma rays and x - rays have a greater penetrating effect. If gamma photons are emitted from the excited nucleus, then the x-ray photons come from outside the nucleus, when electrons orbiting the nucleus rearrange within an atom. They both have the ability to penetrate the human body. Gamma rays have even more energy than x-rays and require about 6.5 feet of concrete or about 1.5 feet of lead to stop them.

aplha, beta, and gamma radiation penetration ability

What does CPM mean on a Geiger counter display?

If a radioactive particle enters the Geiger tube, it is counted. CPM on the digital display of your Geiger counter means, how many radioactive particles per minute have entered the Geiger tube (counts per minute). In some counters it is also possible to display counts per second (CPS) but to get counts per second, one can just divide the CPM value by 60.

Why does a Geiger counter need to be calibrated?

CPM or CPS readings only give us the number of radioactive particles counted in the tube over some time, not the rate of emission from the source of the radiation. For example, a larger Geiger tube may count more radioactive particles than a smaller tube over the same period. That is why all Geiger counters must be calibrated. Using a known radiation source (usually Cesium - 137), a conversion factor is found. That allows to convert the CPM value to another unit - microsieverts per hour (uSv/h) or milliroentgens per hour (mR/h).

Geiger counter units in display

What are the units on the Geiger counter display?

The effect of radiation on the body is better described by the equivalent dose in units of sieverts (Sv) which is the unit of the health effect of ionizing radiation. Roentgen (R) is the unit of measurement of exposure to ionizing radiation.

We are surrounded by both natural and artificial background radiation. The maximum annual equivalent dose should not exceed 50 millisieverts. According to National Council on Radiation Protection and Measurements (NCRP), the average annual radiation dose per person in the U.S. is 6.2 millisieverts.

In the image above, the detector displays 0.08 uS/h. If, for example, we were to be constantly exposed to such radiation for one year, our annual radiation dose would be 0.7 mSv ( There are 24 x 365 = 8760 hours per year, so 8760 x 0.08 = 700.8 microsieverts per year, which is equal to 700,8/1000 = 0.7 millisieverts per year).

Where does natural radioactivity come from?

Natural radioactivity can be found and measured anywhere. The Earth beneath our feet contains various radioactive elements, such as natural uranium or thorium. As plants absorb radioactive material from the ground, it inevitably enters the food chain. In most cases, however, this is not a problem, because humans have lived on this planet for thousands of years and the organism is adapted to such natural radioactivity. The largest source of natural radioactivity is however radon. Radon is a gas that emanates from the ground and can give an annual radiation dose of up to 1.26 uS.

Another common source of natural radioactivity is cosmic radiation. Most of the harmful effects of this kind of radiation are eliminated by the Earth's atmosphere and magnetic field. However, when the atmospheric layer above the head is thinner, for example, when flying on an airplane, cosmic radiation also has an increasing effect on our bodies.

Sources of artificial radioactivity

As man learned the secrets of the atomic world and began to use nuclear energy, sources of artificial radiation were created. Nuclear weapons testing, some accidents in power plants, and storage of industrial nuclear waste have made some areas quite contaminated. More than 30 years after the Chernobyl accident, radioactivity levels in the vicinity of the power plant can still be measured at up to 1.2 uS/h. In one of the most polluted places on Earth - in Semipalatinsk polygon in Kazhakstan, where nearly 500 nuclear bombs were detonated during the Soviet era, the Geiger counter registers background radiation that exceeds the safe levels hundreds of times.

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