Denniston Topping Caret 4 th Edition

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Chapter 10

The Nucleus, Radioactivity, and Nuclear Medicine

10.1 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

mass number

number of protons and neutrons

atomic number

number of protons 11 B 5 atomic symbol • This symbol is the same as writing boron-11.

11 B 5 • Remember for section 2.2, this defines an isotope of boron.

• In nuclear chemistry this is often called a

nuclide

.

• This is not the only isotope (nuclide) of boron.

–

boron

-10 also exists –

How many protons and neutrons does boron-10 have?

–

5 protons, 5 neutrons

• Some isotopes are stable • The unstable isotopes are the ones that produce radioactivity.

• To write nuclear equations (section 10.2) we need to be able to write the symbols for the

isotopes

and the following: – alpha particle – beta particles – gamma rays

Alpha Particles

•

Alpha particle

( a ) – 2 protons, 2 neutrons.

• Same as He nucleus (He 2+ ) • Slow moving, and stopped by small barriers.

• Symbolized in the following ways: 1 2 4 He 2  2 4 He α 2 4 α

Bata Particles

•

Beta particles

( b ) – fast-moving electron.

• Emitted from the nucleus as a neutron is converted to a proton.

• Higher speed particles, more penetrating than alpha particles.

• The symbol is…  0 1 e 0 1 β β 1

Gamma Rays

•

Gamma Rays

( g ) – pure energy (electromagnetic radiation.) • Highly energetic, the most penetrating form of radiation.

• Symbol is simply… 1 g

Properties of Alpha, Beta, and Gamma Radiation

1 •

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.

10.2 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 2

Alpha Decay

238 92 U  234 90 Th  2 4 He 238 = 234 + 4 mass number 92 = 90 + 2 atomic number

Beta Decay

16 7 N  16 8 O  0 1 e

Gamma Production

• 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. 99 m 43 Tc  99 43 Tc  g • Usually gamma rays are emitted along with alpha or beta particles.

Predicting Products of Nuclear Decay

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

239 U 92  X  1 0

e

•

What is the identity of X?

239 93 Np

10.3 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.

• 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 1 H) with more protons than neutrons are unstable.

Half-Life

3 •

Half-life

(t 1/2 ) - 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.

A 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 that radioactive decay is the only path for removal of the isotope form the body.

10.4 Nuclear Power

4

Energy Production

E = mc 2 • Equation by Albert Einstein shows the connection between energy (E) and the mass (m) • c is the speed of light • The equation shows that a very large amount of energy can be formed from a small amount of matter.

Nuclear Fission

•

Fission

(splitting) occurs when a heavy nuclear particle is split into smaller nuclei by a smaller nuclear particle.

0 1 n  235 U  92 236 U  92 92 36 Kr  141 Ba 56  3 0 1 n  energy • Accompanied by a large amount of energy.

• Is self perpetuating. • Can be used to generate steam.

•

Chain reaction

- the reaction sustains itself by producing more neutrons

• 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

Nuclear Fusion

• 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: 2 1

H

 3 1

H

 4 2

He

 0 1

n



energy

• No commercially successful plant exists in U.S.

Breeder 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.

10.5 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.

14 7 N  0 1 n  14 6 C  1 1 H 5

• 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: 14 6 C  14 7 N  0 1 e • The half-life of carbon-14 is 5730 years.

– This information is used to calculate the age.

10.6 Medical Applications of Radioactivity

6 • Modern medical care uses the following: – Radiation in the treatment of cancer.

–

Nuclear medicine

- the use of radioisotopes in the diagnosis of medical conditions.

Cancer Therapy Using Radiation

• 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.

7

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 131 I and 125 I will allow the study of how the thyroid gland is taking in iodine.

• 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

Making Isotopes for Medical Applications

•

Artificial radioactivity

- a normally stable, nonradioactive nucleus is made radioactive.

8 9 • 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.

Examples of artificial radioactivity: 197 79 Au  0 1 n  198 Au 79 • Tracer in the liver 66 30 Zn  1 1 p  67 31 Ga • Used in the diagnosis of Hodgkin’s disease.

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

• 99

m

Tc has a half-life of only 6 hours.

99 42 Mo  99m 43 Tc  0 1 e

10.7 Biological Effects of Radiation

Radiation Exposure and Safety

10

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

Shielding

• Alpha and beta particles need low level of shielding (lab coat and gloves.) • Lead, concrete or both required for gamma rays.

Distance from the Radioactive Source

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

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.

10.8 Measurement of Radiation

11

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.

Computer Imaging

• 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

The Geiger Counter

• Detects ionizing radiation • Has largely been replaced by more sophisticated devises.

Film 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.

Units of Radiation Measurement The Curie

• The amount of radioactive material that produces 3.7 x 10 10 atomic disintegrations per second.

• Independent of the nature of the radiation 12

The Roentgen

• The amount of radiation needed to produce 2 x 10 9 ion pairs when passing through one cm 3 of air at 0 o C.

• Used for very high energy ionizing radiation only.

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.

The 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 (LD 50 )

- the acute dosage of radiation that would be fatal for 50% of the exposed population.

• LD 50 = 500 rems.

The End Chapter 10