About Geiger Counters
Nuclear radiation is a normal part of our planet and the cosmos. It is also a by-product of man-made actions and activities. The unfortunate catastrophe in Japan has created a new sense of urgency for personal radiation measurement to an entirely new generation of post-Cold War individuals.
Against this backdrop we will provide a fundamental description of how a Geiger counter detects nuclear radiation. First a few words on radiation, and a particular type called ionizing radiation.
Radiation is all around us. The sun radiates energy, radio stations radiate electromagnetic waves and radiators in apartment buildings radiate heat to keep residents warm in cold weather. However, nuclear radiation is a different type of radiation known as ionizing radiation.
Ionizing Radiation - Four Primary Types
Ionizing radiation alters the structure and, by association, the electric charge of individual atoms by a process known as ionization. The substances that produce ionizing radiation are said to be radioactive.
There are four primary types of ionizing radiation – alpha, beta and gamma radiation as well as x-radiation
Generally, an alpha particle can travel no more than one to three inches in the air before stopping, and can be stopped by a piece of paper. However, they can pass through a thin mica window. As we will see later, this property of mica is very important in radiation measurement.
Beta radiation can pass through a sheet of paper and some clothing, but not through thin metal or glass. Beta particles can damage skin, however, both alpha and beta radiation are most harmful when inhaled or ingested.
Gamma radiation occurs naturally in nature and is almost identical to x-rays. Generally, they can travel into and often times through anything. Gamma particles can be shielded with several feet of water or concrete, several inches of steel, or a smaller amount of lead.
X-radiation is man-made radiation with a number of valuable uses in medicine and dentistry. X-rays are electromagnetic radiation of the same nature as light and radio waves, but they are also ionizing. They can penetrate a variety of materials, including body tissue. The shielding techniques for x-rays are similar to those for gamma radiation. This is why the x-ray technician will wear a lead apron when taking x-rays.
The Geiger–Müller Tube
Having discussed the nature of radiation and the four primary types of ionizing radiation, we will now move to discussing how a Geiger counter operates. Please see the figure below for the following discussion.
The key element within a Geiger counter is a Geiger tube that consists of two electrodes separated by a mixture of gases. High voltage is applied to the electrodes which creates an electrical field within the chamber.
When radiation passes through the chamber and ionizes the gas, a pulse of electrical current is generated. This detection will typically result in a flashing of an LED and sounding of a “beep” on most Geiger counters. The pulses are electronically processed by the instrument and the radiation level measurement is displayed. The mica window on the tube allows alpha particles to pass so that they can be detected.
Importance of Alpha Detection
Some Geiger tubes only detect beta and gamma radiation leaving users blind to the presence of alpha particles.
Detecting alpha radiation is very critical because as radioactive substances decay they oftentimes decay into elements that produce alpha radiation. If these elements find their way into the food supply, individuals risk consuming food that contain alpha particles.
This is one of the key reasons that, at present, demand for high-quality, US-made, alpha-detecting Geiger counters is at record levels in Japan.
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