Kelter P., Mosher M., Scott A. Chemistry. The Practical Science

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more stable nuclei than their neighbors. Check the graph of

binding energy per

nucleon (proton or neutron in the nu-

cleus) versus the mass number shown in Figure 21.9 to see

that these elements sit at local maxima. The nuclides

and

Ca are “doubly magic”; they have magic numbers for

both protons and neutrons. In February 2000, a French re-

search team created another doubly magic nucleus, Ni-48,

which contains 28 protons and 20 neutrons.

When you examine a larger set of nuclides, other trends

appear. There are 279 stable isotopes, a few naturally

occurring radioactive elements, and hundreds of synthetic

isotopes. Many of these ﬁnd application in nuclear medi-

cine. Figure 21.10 sketches the band of stable isotopes for

each element with at least one stable isotope. Note the fol-

lowing points:

■

At higher atomic numbers, stable nuclei have increas-

ingly more neutrons than protons.

■

Some radioactive elements have too many neutrons

relative to the stable isotopes. These tend to decay by beta

emission, where a neutron changes into a proton and an

electron.

■

Some radioactive elements have too few neutrons relative

to the stable isotopes. These tend to decay by positron

emission, where a proton changes into a neutron and a

positron.

The plot ends at bismuth, Z

= 83, for no elements beyond

this have stable isotopes. Alpha emission is typical for heav-

ier elements that are unstable, simply because too many nu-

cleons are present, be they protons or neutrons. Table 21.7

lists some guidelines that are helpful in determining the

type of decay for a particular nucleus.

The radioactive decay can occur by the process of

positron emission, or

beta-plus emission,



. This process

does not occur naturally on Earth and was not discovered

until scientists began creating new isotopes in the laboratory. Physically this is

done by slamming high-energy particles, ranging from protons and neutrons to

atomic nuclei, into a target nucleus. This can result in changing the nucleus’s

mass number and giving it additional energy. In general, the laboratory process is

Nucleus + small particle → bigger nucleus

The product nucleus that is formed may undergo radioactive decay.

For example, in 1930 phosphorus-30 was synthesized in the laboratory by the

bombardment of an aluminum target with alpha particles,

He:

Al +

He →

P +

More modern work includes the formation of tiny amounts of superheavy ele-

ments, which we will deﬁne as those beyond atomic number 106. Recent (1999

and 2000) reactions include

208

Pb +

Kr →

293

118

Uuo +

1/2

< 1 msec)

249

Bk +

Ne →

267

107

Bh + 4

1/2

= 17 seconds)

918 Chapter 21 Nuclear Chemistry

Number of

rotons

(

)

20 40 60 80 100

100

120

140

160

Number of neutrons (A–Z)

1:1 neutron-to-proton ratio

Unstable region

(too many neutrons;

spontaneous beta

production)

Unstable region

(too many protons;

spontaneous positron

production)

Stable nuclides in the

zone of stability

FIGURE 21.10

The band of known isotopes for each element with at least

one stable isotope. n/p is the neutron-to-proton ratio. Note

that the n/p ratio is greater than 1 for most stable isotopes.

Video Lesson: The Stability

of Atomic Nuclei

Some nuclides are expected to be quite stable, others not so. The nature of the

nucleus becomes clearer when we study the half-lives of these nuclides.

EXERCISE 21.6 Ten Tin Isotopes

Tin has more stable isotopes than any other element. Explain why you might expect

this to be the case. Then examine the radioisotopes of tin and discuss their decay

modes.

Stable isotopes

112

Sn,

114

Sn,

115

Sn,

116

Sn,

117

Sn,

118

Sn,

119

Sn,

122

Sn,

124

(

120

Sn,

118

Sn, and

116

Sn are the most abundant.)

Radioisotopes

121

Sn,

123

Sn,

125

Sn,

126

Sn,

127

that decay by 

−

(plus others with higher mass and short half-life)

Radioisotopes that

110

Sn,

111

Sn,

113

decay by 

or by (plus others with lower mass and short half-life)

electron capture

First Thoughts

Data like these exist for every element; in general, they are most conveniently ac-

cessed on the Web or in a chemistry handbook. You cannot reason out which iso-

topes actually exist. Although tin’s location provides some guidance, you must still

look them up.

Solution

For several reasons, tin would be expected to have a number of stable isotopes. First,

tin is in the middle of the periodic table where nuclei are more stable and where a

few extra neutrons do little to upset the balance of nuclear forces. Second, it has

an atomic number of 50, one of the “magic numbers.” Finally, 8 of the 10 isotopes

have even numbers of protons and neutrons, and the even isotopes are the most

abundant—another indication of their stability.

Further Insights

Isotopic stabilities hold some surprises. For example, a radioisotope may fall be-

tween a pair of stable isotopes. We noted this above for tin. It also happens for chlo-

rine, where Cl-35 and Cl-37 are stable, but Cl-36, which has an odd number of both

protons (17) and neutrons (19), is radioactive. Again, nuclear stability (or instabil-

ity) arises from a combination of several factors and is therefore difﬁcult to predict.

PRACTICE 21.6

Predict whether each of the following isotopes might be stable or radioactive.

Br b.

101

Ru c.

136

Ba d.

180

See Problems 59–62.

21.6 Nuclear Stability and Human-made Radioactive Nuclides 919

Predicting Nuclear Decay

Type of Decay Reason for Instability Change in n/p Ratio

Alpha emission Nucleus too heavy Increase (small for heavy nuclides)

Beta emission n/p too high (*) Decrease

Positron emission n/p too low (*) Increase

Gamma Too much energy None

Nucleus energetically excited

Electron capture n/p too low (*) Increase

*n/p represents the neutron-to-proton ratio.

TABLE 21.7

144

235

139

Application

HEMICAL

ENCOUNTERS:

Nuclear Weapons

920 Chapter 21 Nuclear Chemistry

21.7

Splitting the Atom: Nuclear Fission

With the discovery of ﬁssion came the birth of new radioisotopes and of a new

consciousness that the nuclear age was upon us. Nuclear ﬁssion was discovered in

the 1930s through the work of scientists such as Enrico Fermi, Fritz Strassman,

Otto Hahn, and Lise Meitner. Work on ﬁssion continued in the early 1940s in

both Germany and the United States, culminating in the deployment by the

United States of the ﬁrst atomic bomb used in war, fueled by uranium-235, on the

town of Hiroshima, Japan, on August 6, 1945, and of a second combat-based

atomic bomb, fueled by plutonium-239, on Nagasaki, Japan, on August 9, 1945.

The war ended shortly after the second bomb was dropped, but the nuclear age

had just begun.

Why do

235

U and

239

Pu split and release energy? The answer lies in part in the

thermodynamics of nuclear stability. Refer to Figure 21.9. Uranium nuclei are not

so thermodynamically stable as iron, bromine, and other elements in the region

of greatest stability near the top of the curve. The answer also lies in considering

the precarious balance that exists in large nuclei such as uranium and plutonium.

These nuclei are very heavy and are held together by the strong force between nu-

cleons. However, opposing the strong force are the many proton–proton repul-

sions in an atom of this size. For some atoms, the injection of extra mass into the

nucleus can tip the balance in favor of the proton repulsions and send the nucleus

ﬂying apart into two or more pieces. This is what happens with

235

U and

239

Pu. A

neutron, with no charge, is an ideal particle to shoot into a nucleus. Once it slips

into the nucleus, a heavier nuclide is formed that fragments in a matter of

nanoseconds. Figure 21.11 illustrates the process for the ﬁssion of uranium-235.

239

Pu +

n → [

240

Pu] →

Zn +

167

Gd +3

n +energy

235

U +

n → [

236

U] →

139

Ba +

Kr + 3

n +energy

Nuclear reactions such as these were initially of interest because the tremen-

dous energy they released could be unleashed in a weapon. Since the violent birth

of ﬁssion in 1945,these reactions have found a variety of other,morehumanitarian

Lise Meitner (1878–1968) and Otto Hahn

(1879–1968). As a woman in the male-

dominated world of the early 1900s,

Meitner worked as a physicist with Otto

Hahn. She was responsible for the dis-

covery of protactinium and was the ﬁrst

to explain nuclear ﬁssion correctly. De-

spite her contributions, Otto Hahn did

not acknowledge her work when he re-

ceived the 1944 Nobel Prize. In belated

recognition of her work on radioisotopes,

element 109 is named meitnerium.

FIGURE 21.11

A nucleus of

235

U undergoing nuclear

ﬁssion.

uses. Today, using non-weapons-grade ﬁssionable fuel, they provide the energy

in nuclear power plants worldwide, they power spacecraft and submarines, and

they are the source of many isotopes used in nuclear medicine.

Nuclear Reactors as a Vital Source of Electricity

The 442 nuclear reactors currently in operation worldwide are alternatives to the

pollution and greenhouse gas emissions caused by coal-burning and oil-burning

power generation. The world’s ﬁrst nuclear power plant went on line in 1954 in

the Russian city of Obninsk. This was immediately followed by the construction

of a nuclear facility in Sellaﬁeld, England. It wasn’t until 1957 that the ﬁrst full-

scale power plant in the United States began operation in Shippingport, Pennsyl-

vania. Municipal power generation by nuclear ﬁssion is not new. However, the

use of nuclear ﬁssion has been controversial because of safety concerns, the two

most important being the possible accidental release of radiation, and the dis-

posal and long-term (thousands, and perhaps millions, of years!) safeguarding of

radioactive wastes. Accidental releases of radiation occurred on a relatively small

scale at the Three Mile Island nuclear facility in Pennsylvania in 1979 and on a

much larger scale just outside the Ukrainian town of Chernobyl, in 1986. Still,

much of the world uses nuclear power to meet its energy needs, as shown in

Figure 21.12.

The goal of any large power plant is to generate electricity by turning a tur-

bine, which converts the mechanical energy into electricity. Steam, resulting from

21.7 Splitting the Atom: Nuclear Fission 921

Application

HEMICAL

ENCOUNTERS:

Nuclear Reactors

as a Vital Source

of Electricity

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000

United States

France

Japan

Russian Federation

Germany

South Korea

Canada

Ukraine

United Kingdom

Sweden

Spain

China

Belgium

Czech Republic

Switzerland

India

Bulgaria

Finland

Slovak Republic

Brazil

South Africa

Hungary

Mexico

Lithuania

Argentina

Slovenia

Romania

Netherlands

Pakistan

Armenia

376 MW

425 MW

449 MW

655 MW

656 MW

935 MW

1185 MW

1310 MW

1755 MW

1800 MW

1901 MW

2442 MW

2676 MW

2722 MW

3040 MW

3220 MW

3528 MW

5801 MW

6602 MW

7585 MW

8869 MW

11,852 MW

13,107 MW

12,599 MW

16,810 MW

20,339 MW

21,743 MW

46,772 MW

63,363 MW

99,210 MW

Megawatts (MW) of electricity

FIGURE 21.12

Nuclear power plants are located in many countries of the world. The United States, France, Japan,

and the Russian Federation possess the majority of the plants, with the capability to produce over

230,000 MW of energy annually.

the heating of water, supplies the energy to turn the turbine. The essential differ-

ence among the different types of power plants is the fuel source that creates the

steam from water. In conventional nuclear reactors, this energy is supplied by the

controlled ﬁssion of

235

U (we say that the ﬁssion “goes critical”).

Conventional nuclear reactors have three essential parts, as shown in Fig-

ure 21.13. The nuclear reactor part comprises 100 to 200 fuel rods that contain

the ﬁssionable uranium. A series of movable control rods, typically made of

boron or cadmium, absorb neutrons as a way of controlling the rate of ﬁssion.

The rods are located at the bottom of a pool of water, which acts as a moderator

to slow these neutrons so that they can be captured by the uranium in the fuel

rods. The other two parts of the system, the steam generator and the turbine and

condenser, are common to many types of conventional power plants. Other types

of nuclear reactors also exist. Breeder reactors, for example, are so named because

they produce more ﬁssionable fuel than they consume. In these reactors, rela-

tively abundant

238

U is bombarded with neutrons to “breed”

239

Pu, along with the

emission of -particles.

238

U +

n →

239



−→

239



−→

239

Prototype large breeder reactors either have been built or are being built in

China, France, Scotland, the United States, India, Japan, and the former Soviet

Union. Breeder reactors are not currently used in the commercial production of

energy because of the exceptionally long (24,400 years) half-life of

239

Pu. In these

reactors, sodium metal is used as a coolant because it can transfer heat away from

the reactor core much better than water and has a much higher boiling point,

allowing it to remain liquid without being pressurized. Because of the high oper-

ating temperature of the liquid sodium, along with sodium’s capacity to absorb

neutrons (becoming radioactive after it travels by the core), sodium is viewed

as particularly hazardous. The future of breeder reactors is, at the very least,

uncertain.

922 Chapter 21 Nuclear Chemistry

High-

pressure

water

Pump

Steam

turbine

Electrical

output

Cooling pond

27°C38°C

Steam

generator

Reactor

Containment

shell

Steam

Control

rods

Condenser

FIGURE 21.13

The essential parts of a nuclear power

plant. The nuclear reactor, submersed in

a pool of water, contains the uranium-

based fuel rods and a series of control

rods to slow the ﬁssion process. The

high-pressure water, heated by the ﬁs-

sion, travels to a steam generator where

the heat vaporizes water and creates

high-pressure steam. The steam is used

to turn a turbine and generate electricity.

The steam is then condensed by passing

large amounts of water in a cooling pond

through the condenser.

Here are the details of ﬁssion as we understand

it today:

■

Fission reactions release energy. The masses of the

product nuclei are less than those of the starting

nuclei, and the source of the energy is this mass dif-

ference. The energy released is orders of magnitude

higher than that of “ordinary” chemical reactions.

■

With rare exception, ﬁssion is not a naturally

occurring process. We initiate it in some nuclei

by brute force—that is, by smashing them with a

high-energy neutron. The impact simply drives

the nucleus apart. A few nuclei can be induced to

undergo ﬁssion after they capture lower-energy

neutrons. Furthermore, the only naturally occurring ﬁssionable nucleus is

235

U, which is present in only 0.72% of all uranium atoms. The low availabil-

ity of ﬁssionable fuels slowed the development of ﬁssion. It also spurred the

breeding of human-made ﬁssionable fuels such as

233

U and

239

Pu.

■

Once induced, ﬁssion releases more neutrons, typically 2 to 3 neutrons, per

event. These daughter neutrons usually are traveling fast and may escape with-

out further interaction with a ﬁssionable nucleus. In this instance, we have a

condition known as a

subcritical mass. But if enough ﬁssionable nuclei are

nearby (a

critical mass) or if the neutrons are slowed, these neutrons can con-

tinue the ﬁssion process in the absence of a neutron source. A self-sustaining

chain reaction is possible. If too much ﬁssionable material (a supercritical mass)

is present, the reaction goes out of control.

■

Nuclei can split in more than one way, forming a whole host of ﬁssion prod-

ucts. The split is usually into two or sometimes three fragments. This means

that ﬁssion is “messy” because of the many products.

■

Fission is also messy because the products are usually radioactive. They tend

to be neutron-rich and beta emitters. In the case of nuclear reactors, the ra-

dioactive products end up in high-level and low-level nuclear waste—in other

words, storage. In the case of weapons testing above ground, they result in

nuclear fallout.

EXERCISE 21.7 Fission: A Chain Reaction

Draw a sketch to show why the ﬁssion of

235

U can be called a chain reaction. What

factors do you think will inﬂuence whether the ﬁssion chain reaction will merely

sustain itself or will be explosive?

First Thoughts

Fission of

235

U is initiated by neutrons and produces neutrons. This can result in a

chain reaction if enough of the neutrons released are able to interact with more

235

Solution

As shown here, the ﬁssion events

quickly multiply, and the reaction

becomes explosive. But if each ﬁssion

event simply produced one more

ﬁssion (instead of three), the chain

reaction would simply sustain itself.

21.7 Splitting the Atom: Nuclear Fission 923

Fissionable

nucleus

235

Unstable

intermediate

Fission

products

Energ

236

139

Fission of

235

Nucleus

Supercritical massSubcritical mass

Most radioactive

particles escape

the reaction.

Few radioactive

particles escape

the reaction.

The conditions needed to sustain a chain reaction.

Further Insights

For a chain reaction to be explosive, two conditions must be met: (1) The ﬁssion re-

action must produce more than one neutron, and (2) these neutrons must efﬁ-

ciently produce more ﬁssion events.

235

U produces 2 to 3 neutrons per event, so the

ﬁrst condition is met. The second is a bit tricky. First, you need a critical mass of

235

U if you want each ﬁssion event to induce one additional event. For

235

U, this is a

fairly large (on the order of kilograms) amount. To be explosive, you need to have

a mass of

235

U that is even greater. Second, you need to minimize the presence of

substances such as

238

U that absorb neutrons and stop the reaction. Finally, you

need to keep the

235

U in one place, which is difﬁcult because the energy released

tends to blow it apart. What are the implications? Although the principles of build-

ing a nuclear weapon are fairly simple, engineering one can be very difﬁcult.

PRACTICE 21.7

Draw a sketch that would illustrate the ﬁssion reaction of

235

U that is just sustain-

able. (Hint: A sustainable reaction is not a supercritical chain reaction.)

See Problems 67 and 68.

21.8 Medical Uses of Radioisotopes

Now used almost routinely for diagnosis, radioisotopes enable us to image inter-

nal organs, bone, and tissue structures. We can watch biological processes such as

oxygen uptake and brain activity. Radioisotopes also make possible the treatment

of tumors without anesthesia or invasive surgery. There are more than 40,000

such procedures are carried out in the United States each day.

Tracer Isotopes for Diagnosis

We have already mentioned that you can image certain internal structures such as

bones (or bullets or swallowed coins) using X-rays. The bones, by absorbing

the incoming radiation, show up as shadows on the X-ray ﬁlms or detectors.

However, the heart, liver, and thyroid gland do not show up very well on X-ray

ﬁlms because they absorb so little of the X-rays.

Imaging these organs is the job of radioactive tracers, more technically

called diagnostic

radiopharmaceuticals. Radiopharmaceuticals are used in small

amounts, so they release only low amounts of radiation into the body. They are

considered safe to use for diagnostic purposes. Through introduction into the

body by mouth, inhalation, or injection, the radiopharmaceutical can be used to

outline the organs of interest. This imaging occurs by measuring the emitted ra-

diation outside the patient. Either a “hot spot” (a tumor that preferentially ab-

sorbs the radioisotope) or a “cold spot” (a place where the surrounding tissues

preferentially absorb the radioisotope) will produce the desired result. Using an

alpha or beta emitter as a radioactive source would not work, because these parti-

cles would be absorbed by the tissues inside the body before they could be detected

outside the body.

To understand imaging, we no longer can ignore the chemical form of the nu-

clide, as we have done throughout most of this chapter. The art of getting a good

image lies in understanding the chemical behavior of the nuclide in the body. For

example, although you can make elemental iodine,

131

, from radioactive

131

you cannot feed this chemical to a patient to image the thyroid gland, because io-

dine is chemically reactive and would damage the mouth and stomach. Similarly,

an organic fat-soluble compound containing iodine would tend to concentrate in

924 Chapter 21 Nuclear Chemistry

Application

HEMICAL

ENCOUNTERS:

Tracer Isotopes

for Diagnosis

Video Lesson: Applications

of Nuclear Chemistry

the lymph system rather than travel via the bloodstream to the thyroid. The

chemical form of choice for thyroid studies is the iodide ion,

131

–

, in the water-

soluble form of sodium iodide,Na

131

I,which can be swallowed in a salty“cocktail.”

Each radioactive substance must be prepared in a carefully tailored chemical

form, because each organ in the body has a different chemical proﬁle.

EXERCISE 21.8

123

I and

131

I : Cousins But Not Twins

Radioactive

131

I is used for both treatment and diagnostic scans, whereas radioac-

tive

123

I is used only for diagnosis. Account for this difference after looking up the

decay mode for each nuclide.

Solution

Both of these isotopes of iodine behave the same chemically in the body (they both

are taken up by a normal thyroid gland), so the difference must lie in their nuclear

properties. A table of radioactive decay shows that both

131

I and

123

I release gamma

rays. Because gamma rays easily penetrate matter, they can be detected outside the

body. Therefore, both can be used for diagnosis. For destruction of a tumor, you

need alpha or beta particles that damage tissue at short distances.

131

I is a beta emit-

ter;

123

I is not.

PRACTICE 21.8

Understanding the biological action of salicylic acid can be enhanced by imaging a

patient fed with a radiopharmaceutical. Which of the following salicylic acid deriv-

atives would not be a suitable choice for such a study?

See Problems 73–76.

In many cases, technetium-99m is the nuclide of choice. The existence of

technetium was predicted by Mendeleev as “eka-manganese,” and it is the only

non-rare-Earth element he foresaw that was not discovered in his lifetime.

Although it was ﬁrst produced in 1939 by Glen Seaborg and Emilio Segrè, it was

not used in nuclear medicine until the 1960s. Technetium has a versatile chem-

istry and is dispensed in dozens of chemical compounds for imaging different

parts of the body. Furthermore, technetium-99m emits only gamma rays, which

are little absorbed by the body and therefore expose the patient to only a small ra-

diation dose. Because it has such a short half-life, Tc-99m cannot be stored in a

ﬂask on a shelf. Rather, it usually is generated in the hospital from a source of

molybdenum called a “molybdenum (or sometimes technetium) cow”:

Mo n

99m

Tc +

−1

 +



99m

Tc n

Tc +



The molybdenum-99 isotope used in the cow is not naturally occurring on Earth.

It is generated as a ﬁssion product of uranium in nuclear reactors and shipped to

hospitals worldwide. The

99m

Tc nuclide is separated from the molybdenum as it

is needed in a process called “milking the cow.” Figure 21.14 shows the earliest

chromatographic column used to do a hands-on separation of

99m

Tc from

Mo.

131

21.8 Medical Uses of Radioisotopes 925

FIGURE 21.14

This photograph shows the early column

chromatography apparatus used to sepa-

rate Tc-99m from the parent isotope

(Mo-99). The molybdenum remains on

the column, and the technetium

passes through. The early work on

technetium-99m was done at

Brookhaven National Lab.

926 Chapter 21 Nuclear Chemistry

Why does nuclear medicine employ such

exotic nuclei as technetium and palla-

dium yet seemingly ignore the elements that make up

most of our body, including hydrogen, oxygen, carbon,

nitrogen, sulfur, and potassium? All of these elements

play biochemical roles in the development of cells and

tissues, yet none has been mentioned as a diagnostic or

therapeutic tool. Why the omission?

Think back on what is needed to do imaging. First is

a source of radiation that penetrates well enough to be

detected outside the body. Gamma emitters usually are

the nuclides of choice. Second, you need availability of

the nuclide in sufﬁcient quantity to do a study. Third,

you need a half-life that is reasonably short, and if it is

very short, the nuclide must be generated on site. Finally,

you need to create a chemical form of the nuclide that

will give either a “hot spot” or a “cold spot” in the area of

medical interest.

Carbon-14, though biologically active, has too long

a half-life and is a pure beta-minus emitter (see Fig-

ure 21.15). However,

C, with a n/p that is lower than

those of the stable isotopes, is a positron emitter. Other

How do we know?

Imaging with Positron Emission Tomography (PET)

These thyroid scans were taken using

radioactive iodine (I-123). The normal

scan on the left shows uniform iodine

uptake; the two thyroid lobes are simi-

lar in size. The lobe marked with an

arrow in the photo on the right is not

functioning properly, as is typical in thy-

roid cancer. A biopsy would follow to

conﬁrm the presence of cancer.

positron emitters include

N, and

S. Positrons

themselves do not penetrate very far. But when a positron

encounters an electron, which happens almost imme-

diately, an annihilation occurs whereby the particle

(positron) and antiparticle (electron) are converted into

energy:

 + e

−

n 2



The photons from the two gamma rays are emitted in

exactly opposite directions. When a gamma detector is

positioned both above and below the patient, if each one

simultaneously records an event, then a positron was

annihilated. By feeding the data from the detectors into a

computer, it is possible to reconstruct an image of where

the positron emission took place.

Positron emission imaging is better known as a PET

scan, short for positron emission tomography. It is a

Today such processes are automated. The half-life of

Mo is a brief 67 hours, so

it is shipped to medical suppliers for immediate distribution to hospitals.

One of the more widely used technetium compounds is sodium pertechne-

tate, NaTcO

. The pertechnetate ion, TcO

–

, has properties similar to those of the

chloride ion, Cl

–

, and concentrates in brain tumors, in the thyroid and salivary

glands, and in areas of the body where blood is pooling (as happens in internal

bleeding). Similarly, technetium pyrophosphate, TcP

, can be used to image

the heart to see the extent of damage to heart muscle after a heart attack.

Although it is well developed, nuclear medicine is still a relatively young

ﬁeld; radioisotope tracers were developed in the 1930s and put into widespread

Tc image of heart muscle.

–

A burst of energy is

released as a

positron and an

electron collide.

21.8 Medical Uses of Radioisotopes 927

FIGURE 21.15

The isotopes of carbon.

Type of decay

Half-life

n/p ratio too low

0.127 s 19.3 s 2.45 s 0.75 s 0.19 s

20.3

min

5715

years

+

or EC

Stable Stable –– – –

n/p ratio too high

use in the 1950s and 1960s. In the 1980s, the ready availability of computers

to help process images led to explosive growth in the ﬁeld that continues

today. There are announcements of new techniques, new types of images, or

methods that require lower amounts of radiation almost daily. (There are also

cautions that costly scans are being used too routinely to warrant either the

risks or the costs involved.) The odds are that you know somebody who has

waged battle with cancer. Nuclear medicine is undoubtedly a part of that per-

son’s health history.

much trickier procedure than other types of nuclear

imaging, largely because positron emitters tend to have

half-lives on the order of minutes. To have enough

radioactive material present, they must be produced

nearby or on site at a hospital. In either case, tech-

nicians are needed to maintain the production

equipment. PET scans also require the injection of

radioactive material, which is not the case for MRI

or CT scans.

The payoff with PET, though, is impressive. It

produces “functional imaging”—that is, images of

chemical processes in action. For example, it can

record the brain in action during a seizure or when

the patient is hearing music, thus decoding the

neural pathways. Using glucose labeled with

C or

with

F (see Figure 21.16), PET can reveal brain

metabolism. Similarly,

F-labeled estrogen can be

used within a patient to show how tumors grow.

The color-enhanced real-time images are far more

dramatic than the black-and-white slices produced

by other means.

FIGURE 21.16

F-labeled 2-deoxyglucose, or FDG,

is used to study glucose metabolism

in the body. It can be used to differ-

entiate benign tumors from cancer-

ous ones, because the latter use glu-

cose at a higher rate. After a patient

consumes this radiopharmaceutical,

tumors within her or his body show

up as white spots where glucose

metabolism is higher.