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Radon in the Environment |
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Measurement of alpha particle radiation using CR-39 Alpha particle radiation is the major source of natural radiation in our environment. It is derived from the radioactive decay of the colorless, tasteless, inert gas, radon (222Rn) and is second only to smoking as the leading cause of lung cancer in the US.
Alpha Particles
In the ScienceWorks lab we will use Americium-241(Am-241) as an active source of alpha particles to study their properties and demonstrate how to record their presence. An alpha particle is released from a single Am-241 atom with a known energy of 5.6 MeV. Because an alpha particle is relatively big in nuclear terms, it is very likely to interact with any matter it encounters. When it does so, it dislodges electrons from the shells of these atoms and produces a positive ion. It takes approximately 35 eV to ionize an atom. Thus, one Am-241 emitted alpha particle will produce 150,000 ions. These ions allow a current to flow in the smoke detector's ionization chamber. Origin of Americium-241
Americium (pronounced, "am-uh-RISH-ium" and named after America) was the first man-made element. Glenn Seaborg and colleagues produced it by neutron bombardment of Plutonium in 1944 while working on the Manhattan Project at the University of Chicago. It is a shiny, malleable metal, all isotopes of which are unstable. It has atomic number 95. Currently, Americium-241 is produced by beta decay of spent Plutonium-241 in nuclear reactors. It has a half-life of 432 years. The US Atomic Energy Commission in 1962 first offered americium dioxide for sale for $1500 per gram, and this price has remained essentially unchanged. Its major use is as a source of alpha particles in the ionization chambers of common smoke detectors, and one gram supplies enough Americium-241 for 5000 smoke detectors. Americium-241 is a potentially dangerous isotope if it is taken into the body in soluble form as it can concentrate in the skeleton and is both an alpha and a gamma emitter- potentially causing DNA mutations. However, the dioxide in smoke detectors is insoluble and even if ingested would pass harmlessly through the gastrointestinal tract without delivering a significant dose of radiation. CR-39
CR-39 is an allyl glycol carbonate plastic that has the unique properties of being inert to light, X-ray, and gamma and beta radiation, but reactive with alpha particles. When an alpha particle strikes the plastic's surface it produces a path of damage to a depth of approximately 40 nm. If the plastic is then submerged in 6.25 N NaOH at 98º C for 45 minutes, a track will be etched in the plastic that is visible under a microscope at 10x -20x power. This allows individual alpha particle impacts to be counted. As each alpha particle arises from the disintegration of an individual radon atom, the gas concentration can be derived. Thus, in a high school lab, radon concentration measurements from the local environment can be accurately made. However, such measurements require weeks of exposure and are more suited to experiments performed locally as part of a school project. Thus, in the ScienceWorks lab, we use a more active source of alpha particles, Americium-241. This metal is utilized in the ionization chambers of smoke detectors. |
Facts used to generate hypotheses for CR-39 experiments
Because alpha particles interact so readily with other atoms, they do not penetrate matter (even air) very deeply.
"Tracks" produced in CR-39 plastic are the product of a single alpha particle striking the surface. The number of tracks is proportional to the number of atoms disintegrating per unit time.
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Interesting Links to sources of information about radon, alpha particles, and natural radiation
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