Chapter 9

The Nucleus, Radioactivity, and Nuclear Medicine

Denniston Topping Caret

5th 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

9.1

Nat

ural

Rad

ioac

tivi

ty

B115

atomic symbol

atomic number number of protons

mass number number of

protons and neutrons

Nuclear Symbols

B115

9.1

Nat

ural

Rad

ioac

tivi

ty Writing Nuclear Symbols

• 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

9.1

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ural

Rad

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ty

9.1

Nat

ural

Rad

ioac

tivi

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

á á He He 42

42

2 42

+

9.1

Nat

ural

Rad

ioac

tivi

ty Alpha Particles

• Alpha particle () - 2 protons, 2 neutrons

• Same as He nucleus (He2+)

• Slow moving, and stopped by small barriers

• Symbolized in the following ways:

â â e 01-

01−

9.1

Nat

ural

Rad

ioac

tivi

ty Beta Particles

• 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:

9.1

Nat

ural

Rad

ioac

tivi

ty Gamma Rays

• Gamma rays () - pure energy (electromagnetic radiation)

• Highly energetic

• The most penetrating form of radiation

• Symbol is simply…

9.1

Nat

ural

Rad

ioac

tivi

ty Properties of Alpha, Beta, and Gamma Radiation

• 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

9.2 Writing a Balanced Nuclear Equation

• Nuclear equation - used to represent

nuclear change

• In a nuclear equation, you do not balance the elements, instead...– the total mass on each side of the reaction

arrow must be identical– the sum of the atomic numbers on each side of

the reaction arrow must be identical

9.2

Wri

ting

a B

alan

ced

Nuc

lear

Equ

atio

ns He Th U 42

23490

23892 +→

238 = 234 + 4

92 = 90 + 2

mass number

atomic number

Alpha Decay

• Upon decomposition, nitrogen-16 produces oxygen-16 and a beta particle

• In beta decay, one neutron in nitrogen-16 is converted to a proton and the electron, the beta particle is released

e ON 01-

168

167 +→

9.2

Wri

ting

a B

alan

ced

Nuc

lear

Equ

atio

nsBeta Decay

• Gamma radiation occurs to increase the stability of an isotope

– The energetically unstable isotope is called a metastable isotope

• The atomic mass and number do not change

• Usually gamma rays are emitted along with alpha or beta particles

Tc Tc 9943

m9943 +→9.

2 W

riti

ng a

Bal

ance

d N

ucle

ar E

quat

ions

Gamma Production

• To predict the product, simply remember that the mass number and atomic number are conserved

• What is the identity of X?

e XU 01-

23992 +→

93

239Np9.

2 W

riti

ng a

Bal

ance

d N

ucle

ar E

quat

ions

Predicting Products of Nuclear Decay

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

9.3

Pro

pert

ies

of

Rad

iois

otop

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

9.3

Pro

pert

ies

of

Rad

iois

otop

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

9.3

Pro

pert

ies

of

Rad

iois

otop

es

9.3

Pro

pert

ies

of

Rad

iois

otop

esDecay Curve for the Medically Useful Radioisotope Tc-99m

9.3

Pro

pert

ies

of

Rad

iois

otop

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

9.4 Nuclear Power

Energy ProductionE = mc2

• Equation by Albert Einstein shows the connection between energy (E) and mass (m)

• c is the speed of light • The equation shows that a very large amount of

kinetic energy can be formed from a small amount of matter– Release this kinetic energy to convert liquid water into

steam– The steam drives an electrical generator producing

electricity

9.4

Nuc

lear

Pow

er

• Fission (splitting) - occurs when a heavy nuclear particle is split into smaller nuclei by a smaller nuclear particle

•Accompanied by a large amount of energy

•Is self-perpetuating

•Can be used to generate steam

energy n 3 Ba Kr U U n 10

14156

9236

23692

23592

10 +++→→+

Nuclear Fission

9.4

Nuc

lear

Pow

erFission of Uranium-235

• Chain reaction - the reaction sustains itself by producing more neutrons

9.4

Nuc

lear

Pow

erRepresentation of the “Energy Zones” of a Nuclear Reactor

• A nuclear power plant uses a fissionable material as fuel– Energy released by the fission heats water– Produces steam– Drives a generator or turbine– Converts heat to electrical energy

• Fusion (to join together) - combination of two small nuclei to form a larger nucleus

• Large amounts of energy is released

• Best example is the sun

• An Example:

• No commercially successful plant exists in U.S.

energy n He H H 10

42

31

21 ++→+

9.4

Nuc

lear

Pow

erNuclear Fusion

9.4

Nuc

lear

Pow

erBreeder Reactors

• Breeder reactor - fission reactor that manufactures its own fuel

• Uranium-238 (non-fissionable) is converted to plutonium-239 (fissionable)

• Plutonium-239 undergoes fission to produce energy

9.5 Radiocarbon Dating

• Radiocarbon dating - the estimation of the age of objects through measurement of isotopic ratios of carbon– Ratio of carbon-14 and carbon-12

• Basis for dating:– Carbon-14 (a radioactive isotope) is

constantly being produced by neutrons from the sun

H C n N 11

146

10

147 +→+

9.5

Rad

ioca

rbon

Dat

ing

• Living systems are continually taking in carbon

– The ratio of carbon-14 to carbon-12 stays constant during its lifetime

• Once the living system dies, it quits taking in the carbon-14

– The amount of carbon-14 decreases according to the reaction: e N C 0

1-147

146 +→

• The half-life of carbon-14 is 5730 years

– This information is used to calculate the age

Radiocarbon Dating

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

9.6

Med

ical

App

lica

tion

s of

R

adio

acti

vity • Based on the fact that high-energy

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

9.6

Med

ical

App

lica

tion

s of

R

adio

acti

vity

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

9.6

Med

ical

App

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tion

s of

R

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vity

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

9.6

Med

ical

App

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tion

s of

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vity

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

• Tracer in the liver

• Used in the diagnosis of Hodgkin’s disease

Au n Au 19879

10

19779 →+

Ga p Zn 6731

11

6630 →+

9.6

Med

ical

App

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s of

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Examples of Artificial Radioactivity

9.6

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ical

App

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s of

R

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vity

Preparation of Technetium-99m

• Some isotopes used in nuclear medicine have such a short half-life that they need to be generated on site

• 99mTc has a half-life of only 6 hours

e Tc Mo 01-

99m43

9942 +→

9.7 Biological Effects of Radiation

Radiation Exposure and Safety

The Magnitude of the Half-Life

• Isotopes with short half-lives have one major disadvantage and one major advantage– Disadvantage: larger amount of radioactivity per

unit time– Advantage: if accident occurs, reaches

background radiation levels more rapidly

9.7

Bio

logi

cal E

ffec

ts o

f R

adia

tion

Shielding

• Alpha and beta particles need a low level of shielding: lab coat and gloves

• Lead, concrete or both are required for gamma rays

Distance from the Radioactive Source

• Doubling the distance from the source decreases the intensity by a factor of 4

Radiation Exposure and Safety

9.7

Bio

logi

cal E

ffec

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f R

adia

tion

Radiation Exposure and Safety

Time of Exposure

• Effects are cumulative

Types of Radiation Emitted

• Alpha and beta emitters are generally less hazardous then gamma emitters

Waste Disposal

• Disposal sites are considered temporary

9.8 Measurement of Radiation

Nuclear Imaging

• Isotope is administered

• Isotope begins to concentrate in the organ

• Photographs (nuclear images) are taken at periodic intervals

• Emission of radioactive isotope creates the image

9.8

Mea

sure

men

t of

Rad

iati

on

• Computers and television are coupled

• Gives a continuous and instantaneous record of the voyage of the isotope throughout the body– Gives increased sensitivity– CT scanner is an example

Computer Imaging

9.8

Mea

sure

men

t of

Rad

iati

onThe Geiger Counter

• Detects ionizing radiation

• Has largely been replaced by more sophisticated devices

9.8

Mea

sure

men

t of

Rad

iati

onFilm Badges

• A piece of photographic film that is sensitive to energies corresponding to radioactive emissions

• The darker the film, when developed, the longer the worker has been exposed

9.8

Mea

sure

men

t of

Rad

iati

onUnits of Radiation Measurement

The Curie

• The amount of radioactive material that produces 3.7 x 1010 atomic disintegrations per second

• Independent of the nature of the radiation

The Roentgen

• The amount of radiation needed to produce 2 x 109 ion pairs when passing through one cm3 of air at 0oC

• Used for very high energy ionizing radiation only

9.8

Mea

sure

men

t of

Rad

iati

onUnits of Radiation Measurement

9.8

Mea

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men

t of

Rad

iati

on

Rad - Radiation absorbed dosage

• The dosage of radiation able to transfer 2.4 x 10-3 cal of energy to one kg of matter

• This takes into account the nature of the absorbing material

Units of Radiation Measurement

9.8

Mea

sure

men

t of

Rad

iati

onThe Rem

• Roentgen Equivalent for Man

• Obtained by multiplication of the rad by a factor called the relative biological effect (RBE)• RBE = 10 for alpha particles

• RBE = 1 for beta particles

• Lethal dose (LD50) - the acute dosage of radiation that would be fatal for 50% of the exposed population

– LD50 = 500 rems

Units of Radiation Measurement