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COPYRIGHT 1994 Poptronix, Inc.
Build this radon monitor to detect a possible health threat in your home and, while doing it, learn more about radioactivity.
THIS TWO-PART ARTICLE DISCUSSES the design, construction, and use of a simple, inexpensive environmental radon gas detector that you can build. It is called the beverage can environmental radon monitor or BERM because its ionization chamber sensor is made from a readily available aluminum beverage can. You will be given a choice of methods for measuring and recording events or rates that can be translated into units of radon density.
Most people are exposed to environmental radon in excess of the natural rate because of the time they spend indoors. This first article explains what radon is, why it is a health hazard, and the importance of knowing the level of radon in the rooms of your house where you spend most of your time while indoors. It also includes the information needed to build the ionization chamber, its amplifier circuitry, and alternative circuits for charging the chamber's internal high-voltage capacitor to 500 volts.
The second part of this article covers pulse-rate measurement, instrument calibration, and the conversion of pulse rates to radon density units. The article also offers alternative methods and circuits for performing these functions.
Even if the BERM is only crudely calibrated, it can warn you of unsafe radon levels in your home. However, when properly calibrated, it can give readings that compare favorably with these obtained from professional radon monitoring instruments costing thousands of dollars. Constructing the BERM will give you "hands on" experience in measuring a common form of radioactivity, and give you a better understanding of how it produces isotopes, subjects not easily grasped in lectures or from reading.
The cost of parts to build the BERM, exclusive of a power supply, is typically less than $20. Because most of the components are readily available, you might be able to reduce even that modest cost by making use of parts you already have on hand. You will need the standard electronic technician's set of hand tools as well as such basic electronic test equipment as a two-channel oscilloscope and either an analog or digital multimeter.
What is radon?
Radon is a natural, inert, radioactive gas emitted from the earth. Odorless, colorless, and invisible, it is a byproduct of the radioactive decay of uranium. Because it is inert and does not chemically bond to elements, it is released from the soil into the atmosphere. Radon is emitted almost everywhere on earth, but some geographical regions have higher concentrations than others, depending on the local geology and soil porosity.
Radon becomes a health problem when it decays and produces other short-lived isotopes called daughter products or progeny. These chemically active isotopes are usually formed as charged particles (ions). They bond readily to other substances such as dust and smoke particulates. Table 1 lists a portion of the decay chain of radon 222 and its short-lived progeny.
When radon decays, it releases alpha particles with an energy of 5.5 million electron volts (5.5 MeV). That would seem to be a large amount, but alpha particles travel only 4 to 7 centimeters (1.5 to 2.5 inches) in air before dissipating their energy in the ionization of air molecules. A piece of paper or even human skin is thick enough to stop alpha particles. Direct exposure to radon, unlike direct exposure to beta particles, gamma rays, X-rays, or even ultraviolet light, poses little risk for humans.
The health threat from radon is indirect. Energetic alpha particles can cause chromosomal damage to the thin layers of lung tissue when humans breath air contaminated by radon and its progeny. That damage is a potential cause of lung cancer, especially when coupled with the effects of cigarette smoke in the lungs.
There are several different forms of radon, but radon 222 is the most prevalent form, and is of the most concern to health researcher. The number 222 refers to its isotope number. The alpha particles emitted by radon and its progeny are helium nuclei.
Most of the radon 222 that is inhaled is either exhaled directly or it diffuses into the bloodstream where its alpha emission does little detectable damage. However, radon's short-lived progeny such as polonium 214 and polonium 218 are more likely to emit alpha particles that are capable of damaging sensitive human tissue.
The alpha particles from the decay process of polonium 218 have 6.0 MeV of energy while those from poloniun have 7.7 MeV, both higher than the 5.5 MeV of radon 222. For this reason, researchers believe that they are the agents primarily responsible for inducing lung cancer in situations where radon 222 is present in amounts considered to be above the safe level.
Radon has been a constituent of the air for millions of years. We became aware of its existence only when instruments were developed that could detect and measure it. Its presence is of concern because of the alarming statistics on death due to lung cancer. Its presence has long been considered a contributing...
Read the full article for free courtesy of your local library.
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