7.7
PRESCRIBED FOCUS AREA
Applications and uses of science
Radioactivity — a double-edged sword
When most people hear the word
‘radioactivity’, they immediately
conjure up mental images of
scientists in lead suits holding
ticking Geiger counters out in front
of them, or of the horrible burns
or cancers suffered by survivors of
atomic bombs or the Chernobyl
disaster. But is destruction all that
there is to radioactivity, or can
it also be a valuable lifesaver? In
order to appreciate the doubleedged nature of radioactivity, let’s
first look at what it is and where it
comes from.
Radioisotopes
So far, you have learned that all
atoms of the same element have
the same number of protons — in
other words, they all have the same
atomic number. However, while
the atoms of a particular element
have the same number of protons,
they may not always the same
number of neutrons in their nuclei.
For example, the normal version
of hydrogen has a single proton
in its nucleus but another version
of hydrogen (called deuterium)
has a proton and a neutron in its
nucleus and yet another version
(called tritium) has one proton
and two neutrons.
Atoms that have the same
number of protons, but differ in
number of neutrons they have,
are called isotopes. Most elements
have two or three different isotope
forms. The isotopes of any element
have the same chemical properties
— they just differ in atomic mass.
The protons and neutrons
of the nucleus are held tightly
together by something called
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the strong nuclear force, which overcomes the repulsive force that the
positive protons exert on each other. Without the strong nuclear force,
the repulsive force between the protons would tear the nucleus apart.
In most isotopes, the two forces balance out and the nucleus remains
intact and is said to be stable.
In some isotopes, however, the presence of extra neutrons in the
nucleus makes it much harder for the strong nuclear force to keep them
intact and the nuclei start to break apart or decay. When these unstable
nuclei break up, energy in the form of radiation is released and smaller,
more stable elements are formed. As a result, isotopes whose nuclei have
a tendency to decay are said to be radioactive.
Some examples of stable and radioactive isotopes
Element
Symbol
Number of
protons
Number of
neutrons
Stable or
radioactive
Carbon-12
C
6
6
Stable
Carbon-14
C
6
8
Radioactive
Uranium-235
U
92
235
Radioactive
Uranium-238
U
92
238
Stable
There are approximately 50 isotopes, including Uranium-235 and
Carbon-14 that are naturally radioactive. Most radioactive isotopes
(about 2000 in total) are made radioactive artificially by bombarding
their atoms with sub-atomic particles like protons and neutrons. The
radiation energy released by the decay of both naturally and artificially
radioactive nuclei can be used for a number of different purposes.
Nuclear radiation
There are three types of radiation: alpha (α), beta (β) and gamma (γ).
Alpha radiation is made up of alpha particles (α particles). Each alpha
particle is made up of two neutrons and two protons — essentially the
same as the nucleus of a Helium atom. These particles are the largest of
the radiation particles and move relatively slowly — at about 20 000 km/s.
They cannot travel easily through materials and can be stopped by a sheet
of paper or human skin. They do not pose much of a threat outside of the
body but they can cause serious damage if breathed in, eaten or injected.
Beta radiation is made up of beta particles (β particles) which are fastmoving electrons. Smaller than the alpha particles, beta particles travel at
99% of the speed of light (which travels at 300 000 km/s). Beta particles
can penetrate human skin and damage living tissue, but they cannot
penetrate thin layers of plastic, wood or aluminium.
Gamma radiation is different from the other two because it is made up
of electromagnetic waves (as are radio waves and microwaves) rather than
particles. These gamma rays, as they are known, have no mass and travel at
the speed of light. They have a lot more penetrating power than alpha or
beta particles and can be stopped only by a thick shield of lead or concrete.
As they pass through the body, they can cause serious and permanent
damage to the living tissue and the DNA of the cells themselves.
α particles — absorbed in
a few centimetres of air, or
by a piece of paper
or layer of dead skin
Paper
β rays — barely
affected by air;
absorbed in many
centimetres of lead
Wood
γ particles — absorbed in
about 100 cm of air, or a
few centimetres of wood
Lead
Concrete
The different penetrating powers of alpha (α), beta (β) and gamma (γ) radiation
Radioisotopes and
nuclear power
The radioactive properties of
uranium are used in the generation
of electricity in nuclear reactors.
Uranium is an element that occurs
naturally in most rocks, although
usually in very small amounts.
Australia is one of several countries
that have large high-grade deposits
of uranium. Uranium is converted
to uranium dioxide and then
sealed in rods, called fuel rods.
The uranium undergoes a fission
reaction in the reactor when
neutrons are fired at the radioactive
uranium. This causes the uranium
nuclei to split and form two new
elements, releasing neutrons,
Nuclear equation:
235
U
92
+
1
n
0
radiation and heat in the process.
This heat energy is used to heat
water to produce steam, which
is used to turn the turbines that
generate the electricity.
Radiotherapy in the
treatment of cancer
Radiotherapy is the use of
radioisotopes, or other radiation
such as X-rays, to kill cancer cells
or prevent them from multiplying.
It can be targeted at a small area
so that surrounding tissue is not
damaged. Radiotherapy is often
used along with other treatments
such as surgery or chemotherapy.
Radiation can be directed at the
cancer by a machine like the one
141
Ba
56
+
92
Kr
36
+
French physicist Henri Becquerel
accidentally discovered radioactivity
while investigating the fluorescence
of uranium salts in 1896. When he
developed a photographic plate that
had been in a drawer near his bench
top, he found that it had been fogged
up by radiation from the uranium salts.
This effect of radioactivity is now
used in a protective device worn by
people who work with radioactive
materials. The ‘fogging’ of the film in
this device measures the amount of
radioactivity they have been exposed to.
Becquerel was the first scientist to
report the effects of radioactivity on
living tissue. He suffered from burns on
his skin as a result of carrying a small
quantity of the element radium in his
pocket.
below. This method is known
as external radiotherapy. The
other method, known as internal
radiotherapy or brachytherapy,
involves placing radioisotopes
inside the body at or near the site
of the cancer. In some cases both
methods are used. The type of
treatment depends on the type of
cancer, its size and location as well
as the general health of the patient.
Radioisotopes in the
diagnosis of disease
Radioactive substances may be
inserted into the body to detect
or identify the cause of disease.
The radiation produced by the
1
3 0n
Gamma rays
141
Ba
56
235
U
92
Barium nucleus
Nuclear
reaction
Slow
neutron
Uranium nucleus
An example of a nuclear fission reaction
ENERGY
92
Kr
36
Krypton nucleus
Fast neutrons
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substance while it is in the part of the body under
investigation is measured to diagnose the problem.
Some radioisotopes can be used to obtain images
of parts of the body. The gamma rays emitted by these
radioisotopes are used to produce the images. PET
(positron emission tomography) scans use cameras
surrounding the patient to detect gamma rays coming
from radioisotopes injected into the body.
spoiling. Food in sealed containers can be preserved
by exposing it to gamma radiation. The radiation
kills the micro-organisms in the food and keeps
it from spoiling. However, there has been much
controversy about the safety of food that has been
treated in this way.
Some of the radioisotopes used in the treatment and
diagnosis of disease.
Radioactive material makes its way into air, water,
soil, food, animals and human tissue. Uranium
releases radioactive radon gas into the atmosphere
when it is mined and milled. The radioactive gas
returns to Earth as rain, contaminating soil and
water. The solid radioactive wastes from mining,
called tailings, can infiltrate through the soil and
into the ground water or is dispersed into the
environment by wind.
Radioisotope
Use
Half-life
Phosphorus-32 Used to treat leukaemia
14.3 days
Cobalt-60
Used in radiotherapy for treating
cancer
5 years
Barium-137
Diagnosis of digestive illnesses
2.6 minutes
Iodine-123
Monitoring of thyroid and adrenal
glands, and assessment of damage
caused by strokes
13 hours
Iodine-131
Diagnosis and treatment of thyroid
problems
8 days
Iron-59
Measurement of blood flow and
volume
46 days
Thallium-201
Detection of damaged heart
muscles
3 days
Preserving food
If you’ve ever suffered from food poisoning you will
understand why it is necessary to keep food from
Impacts of nuclear waste
Management of nuclear waste
The management of nuclear waste depends on the
amount, type, and period of time the waste remains
hazardous. For example:
• Low-level waste generated from hospitals and
industry is buried in shallow landfill sites.
• Intermediate-level waste in resins and chemical
sludge is buried in concrete or bitumen.
• High-level waste in spent fuel rods is transported in
thick containers to prevent leaking.
radioactive iodine (iodine-131)
moves from the ground to a
cow. The radioactive iodine
enters the cow’s milk (in a
concentrated form) . . .
Fallout enters the
food chain when solid
radioactive contaminants
fall into bodies of water
and onto the soil.
The contaminants are
taken in by plants and
animals, becoming
more concentrated as
and travels to the thyroid
they move up the food
gland of a human, where
chain. In humans, the
it increases the risk of
contaminants move
cancer.
to target organs,
delivering large,
close-range
doses of
radiation. For
example . . .
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The diagram below shows
parts of the body targeted by
specific radioactive isotopes
likely to be present in the
fallout from Chernobyl. The
risk, in all cases, is that of
cancer in the targeted organ
or in related tissue.
Thyroid: iodine-131*
Lungs: krypton 85
Bone: strontium 90
and yttrium 90
Kidneys: ruthenium 106
Ovaries: iodine-131*,
ruthenium 106
and caesium 137*
Muscle: caesium 137*
*These isotopes were
detected by Swedish
monitors early in the week
after the Chernobyl disaster.
Other methods of managing waste include
vitrification, where liquid radioactive waste is mixed
with glass and poured into steel drums. The steel
drums are then dug deep into the ground or under the
sea floor. Another possible option is to send nuclear
waste into space.
With 150 000 tonnes of used nuclear fuel in the
world, which is increasing daily, individuals, groups
and governments are concerned about the lack of a
satisfactory disposal system.
Activities
14 The use of barium-137 in the diagnosis of digestive
illnesses involves the patient drinking it in a syrup. What
property of barium-137 makes its use quite safe?
REMEMBER
15 Is cobalt-60, used in the treatment of cancer, more
likely to be used in external radiotherapy or internal
radiotherapy? Use the information in the table to explain
your answer.
1 Recall how isotopes of the same element are different
from each other.
2 Explain why the isotopes of some elements are
radioactive.
3 Identify the type of nuclear radiation described by the
following statements.
(a) A radioactive particle that has the same size and
mass as an electron
(b) A radioactive particle that is made up of two protons
and two neutrons
(c) The type of radiation that can penetrate the human
body and can be stopped only by a thick shield of
lead or concrete
(d) A radioactive particle that can travel almost at the
speed of light
4 Recall the name of the nuclear reaction that takes place
in nuclear power stations.
16 Discuss the positive and negative impacts of uranium on
people and the environment.
USING DATA
17 A scientist wished to determine the type of radiation
emitted by a radioisotope. She had three materials
(paper, plastic and lead) and an instrument called a
Geiger counter, which detects nuclear radiation. She
covered the radioisotope with each of the three materials
and measured the radiation that passed through each
material. The results of her experiment are shown in the
table below.
Results of radioactivity experiment
5 Describe three uses of radioactive elements.
Material
6 Describe what radiotherapy is and how it prevents the
spread of cancer through the body.
Paper
No effect on readings
Plastic
Readings fell by two-thirds
Lead
Large fall in readings
7 Distinguish between internal radiotherapy and external
radiotherapy.
8 Explain how radioisotopes used in food preservation stop
food from spoiling.
Effect on Geiger counter readings
What type of nuclear radiation does this radioisotope
emit? Explain your answer.
9 Describe the nature of nuclear waste.
10 Explain how radioactive waste can affect people via its
effect on the environment
11 Describe some technological sloutions to the disposal of
nuclear wastes
THINK
12 About 0.01 per cent of the potassium in your body is the
40
radioisotope 19K .
(a) How many protons and neutrons are in each atom of
this radioisotope?
(b) The stable nuclei of potassium atoms have one less
neutron than the nuclei of potassium’s unstable
radioisotope. Write down the complete symbol for the
stable isotope of potassium.
13 Is iodine-131 a more stable radioisotope than barium-137?
Explain.
INVESTIGATE
18 On a cold spring morning in April 1986, a nuclear reactor
at Chernobyl, in what is now Ukraine, exploded and
released radioactive gases into the atmosphere. Create a
report on:
(a) how the disaster happened
(b) how it affected the workers at the power plant and
the surrounding towns and villages
(c) the attempts to reduce or control the damage caused
by the radiation
(d) the long-term effects of the explosion.
19 Describe radioactive fallout?
20 Radiotherapy is an effective method of treating cancer.
However, it has a number of side effects. Research what
the side effects are.
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