Page 355 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch13_323-350.indd Page 332 9/3/10 6:14 PM user-f465
tiL12214_ch13_323-350.indd Page 332 9/3/10 6:14 PM user-f465 /Users/user-f465/Desktop

A Closer Look

How Is Half-Life Determined?
t is not possible to predict when a radio- isotope has its own decay constant that can The half-life of uranium-238 is therefore
Iactive nucleus will decay because it is a be measured. For example, a 238 g sample of __
6

random process. It is possible, however, to uranium-238 (1 mole) that has 2.93 × 10 half-life = a mathematical constant
decay constant
deal with nuclear disintegration statistically, disintegrations per second would have a
0.693

since the rate of decay is not changed by any decay constant of = __

–18
external conditions of temperature or pres- radioactive decay decay rate 4.87 × 10 l∙s
=

17
sure, or any chemical state. When dealing __ = 1.42 × 10 s

with a large number of nuclei, the ratio of constant number of nuclei
6
2.93 × 10 nuclei∙s
the rate of nuclear disintegration per unit = __ This is the half-life of uranium-238 in

23
of time to the total number of radioactive 6.02 × 10 nuclei seconds. There are 60 × 60 × 24 × 365, or
7
nuclei is a constant, or = 4.87 × 10 l/s 3.15 × 10 , seconds in a year, so
–18
17
1.42 × 10 s
radioactive decay decay rate __ 9

__ The half-life of a radioactive nucleus is 7 = 4.5 × 10 yr
=

constant number of nuclei related to its radioactive decay constant by 3.15 × 10 s∙yr
The half-life of uranium-238 is thus 4.5
The radioactive decay constant is a specific __
a mathematical constant

half-life = billion years.
constant for a particular isotope, and each decay constant
radiation passes through the window, however, it ionizes some
of the gas atoms, releasing free electrons. These electrons are
accelerated by the field between the wire and cylinder, and the
accelerated electrons ionize more gas molecules, which results
in an avalanche of free electrons. The avalanche creates a pulse
of current that is amplified and then measured. More radiation
means more avalanches, so the pulses are an indirect means
of measuring radiation. When connected to a speaker or ear-
phone, each avalanche produces a “pop” or “click.”
Some materials are phosphors, substances that emit a flash
of light when excited by radiation. Zinc sulfide, for example,
is used in television screens and luminous watches, and it was
used by Rutherford to detect alpha particles. A zinc sulfide atom
gives off a tiny flash of light when struck by radiation. A scintil-
lation counter measures the flashes of light through the photo-
FIGURE 13.10 This is a beta-gamma probe, which can electric effect, producing free electrons that are accelerated to
measure beta and gamma radiation in millirems per unit of time. produce a pulse of current. Again, the pulses of current are used
as an indirect means to measure radiation.
Wire Geiger tube RADIATION UNITS
(positive charge) (negative charge)
You have learned that radioactivity is a property of isotopes with
Power unstable, disintegrating nuclei and radiation is emitted particles
supply (alpha or beta) or energy traveling in the form of photons (gamma).
+ +
Radiation can be measured (1) at the source of radio activity or
(2) at a place of reception, where the radiation is absorbed.
– The activity of a radioactive source is a measure of the
– Argon gas Window number of nuclear disintegrations per unit of time. The unit of
(transparent
to radiation) activity at the source is called a curie (Ci), which is defined as
10
Counter 3.70 × 10 nuclear disintegrations per second. Activities are
usually expressed in terms of fractions of curies, for example,
a picocurie (pCi), which is a millionth of a millionth of a curie.
Activities are sometimes expressed in terms of so many pi cocuries
FIGURE 13.11 The working parts of a Geiger counter. per liter (pCi/L).
332 CHAPTER 13 Nuclear Reactions 13-10