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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• 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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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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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