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A Closer Look

Carbon Dating
arbon is an element that occurs natu- carbon dioxide from the atmosphere, and The half-life of carbon-14 is 5,730
Crally as several isotopes. The most animals obtain carbon from the food they years. Therefore, a bone containing one-
common form is carbon-12. A second, eat. While an organism is alive, the propor- half the normal proportion of carbon-14
heavier radioactive isotope, carbon-14, is tion of carbon-14 to carbon-12 within its is 5,730 years old. If the bone contains
constantly being produced in the atmo- body is equal to its surroundings. When an one-quarter of the normal proportion of
sphere by cosmic rays. Radioactive ele- organism dies, the carbon-14 within its tis- carbon-14, it is 2 × 5,730 = 11,460 years
ments are unstable and break down into sues disintegrates, but no new carbon-14 is old, and if it contains one-eighth of
other forms of matter. Hence, radioactive added. Therefore, the age of plant and ani- the naturally occurring proportion of
carbon-14 naturally decays. The rate at mal remains can be determined by the ratio carbon-14, it is 3 × 5,730 = 17,190 years
which carbon-14 is formed and the rate at of carbon-14 to carbon-12 in the tissues. old. As the amount of carbon-14 in a
which it decays are about the same; there- The older the specimen, the less carbon-14 sample becomes smaller, it becomes more
fore, the concentration of carbon-14 on the is present. Radioactive decay rates are mea- difficult to mea sure the amount remaining.
earth stays relatively constant. All living sured in half-life. One half-life is the amount Therefore, carbon-14 is generally only
things contain large quantities of the element of time it takes for one-half of a radioactive useful for dating things that are less than
carbon. Plants take in carbon in the form of sample to decay. 50,000 years old.

The International System of Units (SI) unit for radioactiv- is the millisievert (mSv). Both the millirem and the millisievert
ity is the Becquerel (Bq), which is defined as one nuclear dis- relate ionizing radiation and biological effect to humans. The
integration per second. The unit for reporting radiation in the natural radiation that people receive from nature in one day is
United States is the curie, but the Becquerel is the internationally about 1 millirem (0.01 millisievert). A single dose of 100,000
accepted unit. Table 13.4 gives the names, symbols, and conver- to 200,000 millirems (1,000 to 2,000 millisieverts) can cause
sion factors for units of radioactivity. radiation sickness in humans (Table 13.5). A single dose of
As radiation from a source moves out and strikes a mate- 500,000 millirems (5,000 millisieverts) results in death about
rial, it gives the material energy. The amount of energy released 50 percent of the time.
by radiation striking living tissue is usually very small, but it can Another measure of radiation received by a material is the
cause biological damage nonetheless because chemical bonds rad. The term rad is from radiation absorbed dose. The SI unit
are broken and free polyatomic ions are produced by radiation. for radiation received by a material is the gray (Gy). One gray is
The amount of radiation received by a human is expressed equivalent to an exposure of 100 rad.
in terms of radiological dose. Radiation dose is usually writ- Overall, there are many factors and variables that affect the
ten in units of a rem, which takes into account the possible possible damage from radiation, including the distance from
biological damage produced by different types of radiation.
Doses are usually expressed in terms of fractions of the rem,
for example, a millirem (mrem). The SI unit for radiation dose
TABLE 13.5
Approximate single dose, whole-body effects of radiation
exposure
TABLE 13.4
Level Comment
Names, symbols, and conversion factors for radioactivity
0.130 rem Average annual exposure to natural background
Name Symbol To Obtain Multiply By radiation
0.500 rem Upper limit of annual exposure to general public
Becquerel Bq Ci 2.7 × 10 –11
25.0 rem Threshold for observable effects such as reduced
gray Gy rad 100
blood cell count
sievert Sv rem 100
100.0 rem Fatigue and other symptoms of radiation sickness
curie Ci Bq 3.7 × 10 10
200.0 rem Definite radiation sickness, bone marrow damage,
rem rem Sv 0.01 possibility of developing leukemia
millirem mrem rem 0.001 500.0 rem Lethal dose for 50 percent of individuals
rem rem millirem 1,000 1,000.0 rem Lethal dose for all

13-11 CHAPTER 13 Nuclear Reactions 333