Chapter 9
The Nucleus, Radioactivity, and Nuclear Medicine
Denniston Topping Caret
7th Edition
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
9.1 Natural Radioactivity
• Radioactivity - process by which atoms emit energetic particles or rays
• Radiation - the particles or rays emitted– comes from the nucleus
• Nuclear symbols - what we use to designate the nucleus– Atomic symbol– Atomic number– Mass number
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B115
atomic symbol
atomic number number of protons
mass number number of
protons and neutrons
Nuclear Symbols
B115
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• This defines an isotope of boron
• In nuclear chemistry, often called a nuclide
• This is not the only isotope of boron– boron-10 also exists
– How many protons and neutrons does boron-10 have?
• 5 protons, 5 neutrons
Three Isotopes of Carbon
• Each nucleus contains the same number of protons• Only the number of neutrons is different• With different numbers of neutrons the mass of
each isotope is different
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9.1
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ty Unstable Isotopes• Some isotopes are stable
• The unstable isotopes are the ones that produce radioactivity
• To write nuclear equations we need to be able to write the symbols for the isotopes and the following:
– alpha particles
– beta particles
– gamma rays
α α He He 42
42
2 42
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• Alpha particle () - 2 protons, 2 neutrons
• Same as He nucleus (He2+)
• Slow moving, and stopped by small barriers
• Symbolized in the following ways:
β β e 01-
01
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• Beta particles () - fast-moving electron
• Emitted from the nucleus as a neutron, is converted to a proton
• Higher speed particles, more penetrating than alpha particles
• Symbolized in the following ways:
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• Gamma rays () - pure energy (electromagnetic radiation)
• Highly energetic
• The most penetrating form of radiation
• Symbol is simply…
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• Ionizing radiation - produces a trail of ions throughout the material that it penetrates
• The penetrating power of the radiation determines the ionizing damage that can be caused
• Alpha particle < beta particle < gamma rays
9.3 Properties of Radioisotopes
Nuclear Structure and Stability
• Binding energy - the energy that holds the protons, neutrons, and other particles together in the nucleus
• Binding energy is very large
• When isotopes decay (forming more stable isotopes) binding energy is released
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Rad
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es Important factors for stable isotopes– Ratio of neutrons to protons
– Nuclei with large number of protons (84 or more) tend to be unstable
– The “magic numbers” of 2, 8, 20, 50, 82, or 126 help determine stability – these numbers of protons or neutrons are stable
– Even numbers of protons or neutrons are generally more stable than those with odd numbers
– All isotopes (except 1H) with more protons than neutrons are unstable
Stable Radioisotopes
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Rad
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esHalf-Life
• Half-life (t1/2) - the time required for one-half of a given quantity of a substance to undergo change
• Each radioactive isotope has its own half-life
– Ranges from a fraction of a second to a billion years
– The shorter the half-life, the more unstable the isotope
Half-Lives of Selected Radioisotopes
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esDecay Curve for the Medically Useful Radioisotope Tc-99m
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esPredicting the Extent of
Radioactive DecayA patient receives 10.0 ng of a radioisotope with a half-life of 12 hours. How much will remain in the body after 2.0 days, assuming radioactive decay is the only path for removal of the isotope from the body?
• Calculate n, the number of half-lives elapsed using the half-life as the conversion factorn = 2.0 days x 1 half-life / 0.5 days = 4 half lives
• Calculate the amount remaining10.0 ng 5.0 ng 2.5 ng 1.3 ng 0.63 ng 1st half-life 2nd half-life 3rd half-life 4th half-life
• 0.63 ng remain after 4 half-lives
9.6 Medical Applications of Radioactivity
• Modern medical care uses the following:– Radiation in the treatment of cancer– Nuclear medicine - the use of
radioisotopes in the diagnosis of medical conditions
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gamma rays cause damage to biological molecules
• Tumor cells are more susceptible than normal cells
• Example: cobalt-60
• Gamma radiation can cure cancer, but can also cause cancer
Cancer Therapy Using Radiation
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Nuclear Medicine
• The use of isotopes in diagnosis
• Tracers - small amounts of radioactive substances used as probes to study internal organs
• Nuclear imaging - medical techniques involving tracers
• Example:– Iodine concentrates in the thyroid gland
– Using radioactive 131I and 125I will allow the study of how the thyroid gland is taking in iodine
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Tracer Studies
• Isotopes with short half-lives are preferred for tracer studies. Why?
– They give a more concentrated burst
– They are removed more quickly from the body
• Examples of imaging procedures:
– Bone disease and injury using technetium-99m
– Cardiovascular disease using thallium-201
– Pulmonary disease using xenon-133
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Making Isotopes for Medical Applications
• Artificial radioactivity - a normally stable, nonradioactive nucleus is made radioactive
• Made in two ways:
• In core of a nuclear reactor
• In particle accelerators – small nuclear particles are accelerated to speeds approaching the speed of light and slammed into another nucleus
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