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9
4.5 ✕ 10 yr 24.1 days 1.1 min
238 234 234 234
92 U α 90 Th β 91 Pa β 92 U
α
2.7 ✕ 10 yr
5
8.3 ✕ 10 yr 1.62 ✕ 10 yr
4
3
230 226 222
90 Th α 88 Ra α 86 Rn
α
3.83 days

3.05 min 26.8 min
218 214 214
84 Po α 82 Pb β 83 Bi

β
13.1 min

1.5 ✕ 10 sec 22 yr
–4
214 210 210
84 Po α 82 Pb β 83 Bi

140 days 5 days
206 210
82 Pb α 82 Po β

FIGURE 13.9 The half-life of each step in the uranium-238 radioactive decay series.

As an example of the half-life measure, consider a hypo- MEASUREMENT METHODS
thetical isotope that has a half-life of one day. The half-life is As Becquerel discovered, radiation affects photographic film,
independent of the amount of the isotope being considered, but exposing it as visible light does. Since the amount of film
suppose you start with a 1.0 kg sample of this element with a exposure is proportional to the amount of radiation, photo-
half-life of one day. One day later, you will have half of the origi- graphic film can be used as an indirect measure of radia tion.
nal sample, or 500 g. The other half did not disappear, but it is Today, people who work around radioactive materials or X rays
now the decay product, that is, some new element. During the carry light-tight film badges. The film is replaced periodically
next day, one-half of the remaining nuclei will disintegrate, and and developed. The optical density of the developed film pro-
only 250 g of the initial sample is still the original element. One- vides a record of the worker’s exposure to radiation because the
half of the remaining sample will disintegrate each day until the darkness of the developed film is proportional to the exposure.
original sample no longer exists. There are also devices that indirectly measure radiation by
The half-life of uranium-238 is 4.5 billion years. Figure 13.9 measuring an effect of the radiation. An ionization counter is one
gives the half-life for each step in the uranium-238 decay series. type of device that measures ions produced by radiation. A second
type of device is called a scintillation counter. Scintillate is a word
meaning “sparks or flashes,” and a scintillation counter measures
the flashes of light produced when radiation strikes a phosphor.
13.2 MEASUREMENT OF RADIATION
The most common example of an ionization counter is
The measurement of radiation is important in determining known as a Geiger counter (Figure 13.10). The working com-
the half-life of radioactive isotopes. Radiation measurement is ponents of a Geiger counter are illustrated in Figure 13.11.
also important in considering biological effects, which will be Radiation is received in a metal tube filled with an inert gas,
discussed in the section on radiation units. As is the case with such as argon. An insulated wire inside the tube is connected
electricity, it is not possible to make direct measurements on to the positive terminal of a direct-current source. The metal
things as small as electrons and other parts of atoms. Indirect cylinder around the insulated wire is connected to the nega-
mea surement methods are possible, however, by considering tive terminal. There is no current between the center wire and
the effects of the radiation. the metal cylinder because the gas acts as an insulator. When

13-9 CHAPTER 13 Nuclear Reactions 331