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Counters for the Detection of Radioactive Emissions

written by: Terry Ligard • edited by: Lamar Stonecypher • updated: 8/3/2010

Radioactive emissions can interact with atoms of other materials. The rate of nuclear decay can be derived by measuring the radioactivity of a sample, or its effects on these interactions over a period of time. Two devices that aid in this measurement are ionization and scintillation counters.

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    Radioactivity is a natural phenomena demonstrated by some elements, primarily those residing in the actinide series, and also isotopes of lighter elements. Radioactive emissions can also be artificially generated by bombarding the nuclei of an atom in a particle accelerator. The three main radioactive emissions are alpha, beta, and gamma radiation. Each has distinct properties, such as in velocity, the extent of penetration in matter, and the way in which they are affected by electric and magnetic fields.

    Due to their numerous benefits in industry, health and laboratory research, radioactive substances are growing in use. Therefore, it is also important that detailed profiles on these substances are kept, such as the extent of radioactivity. A common tool used to measure this is a counter.

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    Ionization Counters

    Ionization counters are used to measure radioactive emissions as these emissions ionize a gas. All of these counters follow a general mechanism in which the ionization of a gas produces free electrons and gaseous cations. These cations are attracted to electrodes which then conduct a current to a recording device.geiger counter 

    Today, the most widely used ionization counter is the Geiger-Müller counter, which detects and measures alpha, beta, and gamma radiation. It is composed of a tube full of a mixture of argon and methane. The tube itself is the cathode, and a thin wire running through the center of the tube is the anode. The radioactive sample to be tested emits radiation that enters the tube through a small window, which then strikes argon atoms, producing free electrons that are accelerated to the thin wire anode. While these electrons move towards the anode, they collide with other argon atoms, releasing even more electrons, in what is called the avalanche effect. The electrons create a current that is amplified and received at a recording device. This recording device can appear as a meter reading, a clicking sound, or both.

    Geiger-Muller counter interior 

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    Scintillation Counters

    A distinct property of radioactive emission is its ability to excite atoms. When excited, atoms try to return to a more stable composition by emitting energy in the form of light. A scintillation counter utilizes this property of radioactivity to measure the level of radioactive emissions. The light-emitting substance used is called a phosphor.

    Wallac scintillation counter 

    A part of a photomultiplier tube, an apparatus that amplifies the original electrical signal, is covered in a layer of the phosphor. When radioactive emissions strike the phosphor, photons of light are emitted, which then strike a cathode, emitting electrons by means of the photoelectric effect. Similar to the ionization counter, the electrons collide with other parts of the scintillation counter releasing even more electrons, and an overall current is sent to a recording device.scintillation counter interior 

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    Book: Chemistry: The Molecular Nature of Matter and Change by Martin S. Silberberg

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    Images Courtesy