Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 1

Radioactivity

In 1896 Henri Becquerel on developing some photographic plates he found that the uranium emitted

radiation. Becquerel had discovered radioactivity.

Models of the Atom

Ernest Rutherford, A New Zealand physicist proved in the

early 1900s a new model of the atom.

He discovered amazing facts about the nucleus: The nucleus is very small;

Most of the atom is empty space;

The repulsion of the positively charged alpha particle

showed that the nucleus is positively charged.

This discovery led to the idea of the nuclear atom. This

was developed further by Neils Bohr, a Danish physicist.

The neutron was discovered twenty years later by an

English physicist, Chadwick.

Since the nucleus is so small, the size of an atom is

governed by the size of the electron shells. Therefore big

atoms and small atoms are all roughly the same size,

about 10-10 m in diameter.

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 2

The Atom

The Basic Atom All matter is made up of atoms. The basic atom consists of

a nucleus surrounded by electrons going round the

nucleus in orbit. Electrons are negatively charged. Here

is a Lithium atom:

The nucleus consists of:

Protons which are positively charged.

Neutrons that have no charge.

The protons and neutrons have very nearly the same relative mass.

The mass of a proton or neutron in kilograms is about

1.6 × 10-27 kg. The mass of an electron is about

1/1800 the mass of a proton. The mass of an electron is

about 9.1 × 10-31 kg.

Particle Charge

Proton + 1

Neutron 0

Electron - 1

The symbol e is often called the electronic charge. Its

value is 1.6 × 10-19 C.

The protons and neutrons are the nucleons.

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 3

Atoms and Ions

Elements are often written like this:

A is the total number of nucleons. This is called the

mass number or the nucleon number.

Z is the total number of protons. This is called the

atomic number or the proton number.

The number of protons determines the element. If we change the number of protons in the nucleus from 6 to 7,

we change the element from carbon to nitrogen. This will

change the chemistry radically.

To work out the number of neutrons we take away the

number of protons from the number of nucleons:

No of neutrons = mass number - atomic number

If the number of electrons is the same as the number of

protons, the atom carries zero overall charge. It is

described as neutral.

The nucleus is very tiny, about 1/10 000 the size of an

atom.

If we change the number of electrons, the atom is

charged. It becomes an ion:

Remove an electron, the overall charge is positive.

We have a positive ion.

Add an electron, we have a negative ion.

Ions are NEVER made by adding or taking away protons.

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 4

Isotopes

Isotopes have the same number of protons, but different numbers of neutrons. If we change the number of

protons, we change the element completely. Isotopes

have the same chemical properties as the normal

element.

Examples of isotopes: (e.g. helium-3, carbon-12, iodine-131 and

uranium-238).

For example, the most common isotope of hydrogen has

no neutrons at all; there's also a hydrogen isotope called

deuterium, with one neutron, and another, tritium, with

two neutrons.

Hydrogen Deuterium Tritium

Ordinary hydrogen is written 1H1, deuterium is 2H1,

and tritium is 3H1.

Light elements tend to have about as many neutrons as

protons; heavy elements apparently need more neutrons than protons in order to stick together. Atoms with a few

too many neutrons, or not quite enough, can sometimes

exist for a while, but they're unstable.

Unstable atoms are radioactive: their nuclei change or decay by spitting out radiation, in the form of

particles or electromagnetic waves.

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 5

Radioactivity

Some isotopes of atoms can be unstable.

They may have:

a) Too much energy or

b) The wrong number of particles in the nucleus.

We call these radioisotopes.

To make themselves more stable, they throw out particles

and/or energy from the nucleus. We call this process

‘radioactive decay’. The atom is also said to

disintegrate.

The atom left behind (the daughter) is different from the

original atom (the parent). It is an atom of a new

element. For example uranium breaks down to radon

which in turn breaks down into other elements.

The particles and energy given out are what we call

‘radiation’ or ‘radioactive emissions’.

Three types exist :

Alpha decay; Beta decay ; Gamma radiation.

Alpha and beta decays result in the emission of a

particle. Gamma radiation is an electromagnetic wave

of very short wavelength .

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 6

Properties of Radiation

The table shows some properties:

Radiation Description Penetration

Ionising Power Effect of Electric or Magnetic field

Alpha

()

Helium nucleus

2p + 2n Q = + 2 e

Few cm air Thin paper

Intensely ionising

Deflection as a positive charge

Beta () High speed electron Q = -1 e

Few mm of aluminium

Less than alpha

Deflection in opposite direction to alpha.

Gamma

()

Very short wavelength em radiation

Several cm lead, couple of m of concrete

Weakly ionising

No effect.

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 7

Beta Particle

Alpha particle This consists

of a helium

nucleus. If

we send

alpha

particles through the

poles of a magnet (a magnetic field), we find that they

are deflected. This means that they are charged. If we

pass them between a positively charged plate and a

negatively charged plate (an electric field), we find that

they are attracted to the negatively charged plate. This

means they are positively charged.

Alpha particles are stopped by a few cm of air. This means that an alpha source can be used safely with

minimal shielding. Your skin will stop alpha particles.

Alpha particles are intensely ionising. Being quite big

and moving fast, they collide frequently with other atoms,

knocking off electrons, causing ionisation. They rapidly

lose their energy. Eventually they stop and then pick up

two stray electrons to become helium atoms. All the

Earth's helium atoms are thought to come from alpha

decay.

Beta particle This

consists

of a fast

moving

electron . If we send a beta particles through the poles of

a magnet (a magnetic field), we find that they are

deflected in the opposite direction to alpha particles. This

means that they are charged.

Alpha Particle

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 8

If we pass them between a positively charged plate and a

negatively charged plate (an electric field), we find that

they are attracted to the positively charged plate. This

means they are negatively charged.

Gamma Radiation

Gamma rays are very

short wavelength and

highly energetic

electromagnetic radiation. They are given

off by very energetic or

excited nuclei when some other decay has occurred.

Cobalt-60 is a common source of gamma rays.

Gamma radiation does not in itself alter the nucleon and

proton numbers. Gamma rays are not affected by electric

or magnetic fields.

Because alpha particles carry more electric charge, are

more massive, and move slowly compared to beta and

gamma particles, they interact much more easily with

matter. Beta particles are much less massive and move

faster, but are still electrically charged. A sheet of

aluminum one millimeter thick or several meters of air will

stop these electrons and positrons. Because gamma rays

carry no electric charge, they can penetrate large

distances through materials before interacting–several

centimeters of lead or a meter of concrete is needed to stop most gamma rays.

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 9

Measuring Radiation

In the old days, radiation was detected by

exposing a sheet of photographic film to

the radioactive source. Each decay caused the deposit of a grain of silver, and it was

possible measure the density of the

deposits when the film was developed.

This method is still used today with film

badges that people wear if they are

working with radioactive materials.

To get a real-time measurement, we measure the

radiation from a radioactive sample using a radiation

detector called a Geiger-Müller tube. This is connected to a counter.

The radioactive decay is measured by the number of

counts per second. When we take readings it is important that we measure the background count.

There is radioactivity all around us; it's a natural part of

the environment. So we find out what the background

count is, then we take that away from the count we get

with the source.

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 10

cloud chamber, device used to detect elementary

particles and other ionizing radiation. A cloud chamber

consists essentially of a closed container filled with a

supersaturated vapor, e.g., water in air. When ionizing

radiation passes through the vapor, it leaves a trail of charged particles (ions) that serve as condensation

centers for the vapor, which condenses around them.

ALPHA PARTICLES PRODUCE STRAIGHT LONG LINES

BETA PARTICLES PRODUCE STRAIGHT WEAK LINES

GAMMA RAYS LOOK LIKE TINY CURLY STRANDS OF HAIR

Half-Life

Radioactive decay is a random process. If you look at a nucleus, it might decay within ten seconds, or twenty

two million years. Since there are many billions of nuclei,

a random decay pattern is seen.

What is half-life?

Radioactive substances will give out radiation all the

time, regardless of what happens to them physically or

chemically. As they decay the atoms change to daughter

atoms, until eventually there won’t be any of the original atoms left.

Different substances decay at different rates and so will

last for different lengths of time. We use the half-life of a

substance to tell us which substances decay the quickest.

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 11

Half-life – is the time it takes for half of the

radioactive particles to decay.

It is also the time it takes for the count-rate of a

substance to reduce to half of the original value.

We cannot predict exactly which atom will decay at a

certain time but we can estimate, using the half-life, how

many will decay over a period of time.

The half-life of a substance can be found by measuring

the count-rate of the substance with a Geiger-Muller tube

over a period of time. By plotting a graph of count-rate

against time the half-life can be seen on the graph.

This would also work if you plotted the number of parent

atoms against time.

The longer the half-life of a substance the slower the

substance will decay and the less radiation it will emit in a

certain length of time.

Each radioactive isotope decays in its own way and has its

own half-life which is defined as:

the time taken for half the original number of atoms

to decay.

This is shown on the graph:

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 12

.

If it takes 4 days for half the atoms to decay:

after 4 days, 1/2 are left over;

after 8 days, 1/4 are left over;

after 12 days, 1/8 are left over. This is called exponential decay.

Some half lives are extremely short, much less than 1

second. Some are very long, about 4500 million years.

Using radioactivity

Different radioactive substances can be used for different purposes. The type of radiation they emit and the half-life

are the two things that help us decide what jobs a

substance will be best for. Here are the main uses you will

be expected to know about:

1. Uses in medicine to kill cancer –

radiation damages or kills cells, which can cause cancer,

but it can also be used to kill cancerous cells inside the

body. Sources of radiation that are put in the body need

to have a high count-rate and a short half life so that they are effective, but only stay in the body for a short

period of time. If the radiation source is outside of the

body it must be able to penetrate to the required depth in

the body. (Alpha radiation can’t travel through the

skin remember!)

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 13

2. Uses in industry –

one of the main uses for radioactivity in industry is to

detect the thickness of materials. The thicker a

material is the less the amount of radiation that will be

able to pass. Alpha particles would not be able to go

through metal at all, gamma waves would go straight

through regardless of the thickness. Beta particles

should be used, as any change in thickness would change

the amount of particles that could go through the metal.

They can even use this idea to detect when

toothpaste tubes are full of toothpaste!

3. Photographic radiation detectors –

these make use of the fact that radiation can change the

colour of photographic film. The more radiation that is

absorbed by the film the darker the colour it will go when

it is developed. This is useful for people working with

radiation, they wear radiation badges to show them how

much radiation they are being exposed to.

4. Dating materials –

The older a radioactive substance is the less radiation it

will release. This can be used to find out how old things

are. The half-life of the radioactive substance can be used

to find the age of an object containing that substance.

There are three main examples of this:

i) Carbon dating – many natural substances contain two

isotopes of Carbon. Carbon-12 is stable and doesn’t

disintegrate. Carbon-14 is radioactive. Over time Carbon-

14 will slowly decay. As the half-life is very long for

Carbon-14, objects that are thousands of years old can be

compared to new substances and the change in the

amount of Carbon-14 can date the object.

ii)Uranium decays by a series of disintegrations that

eventually produces a stable isotope of lead. Types of rock

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 14

(igneous) contain this type of uranium so can be dated, by

comparing the amount of uranium and lead in the rock

sample.

iii) Igneous rocks also contain potassium-40, which

decays to a stable form of Argon. Argon is a gas but if it

can’t escape from the rock then the amount of trapped

argon can be used to date the rock.

5. Smoke Detectors and Americium-241

6. Agricultural Applications - radioactive tracers

Radioisotopes can be used to help understand chemical

and biological processes in plants.

7. Food Irradiation

Food irradiation is a method of treating food in order to

make it safer to eat and have a longer shelf life.

Uses and Hazard of Radiation

Radiation Use Hazard

Alpha () Used in smoke detectors If taken in to the body (ingested), alpha emitters can do immense damage to living tissues

Beta () Checking the thickness of paper sheet in manufacture. Radioactive tracers in medical research and diagnosis

Some risk of tissue damage, although nowhere near as dangerous as alpha.

Gamma () Medical research. Non-destructive testing of castings.

Can cause genetic damage and cancer.

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 15

Background radiation

There is a certain amount of radiation around us (and

even inside us) all the time. There always has been –

since the beginning of the Earth. It is called Background

radiation.

Background radiation comes from a huge number of

sources.

Cosmic radiation Radiation from rocks

Radioactive waste

In most areas, Background radiation is safe. It is at such a

low level that it doesn’t harm you. You need to be

exposed to many times the normal background level

before you notice any symptoms.

Dangers of handling radioactive substances

Each type of radiation that can be emitted can be

absorbed by different materials and ionises different

amounts. They are equally dangerous but for different

reasons.

Alpha particles:

Although alpha particles cannot penetrate the skin, if it

gets into the body it can ionise many atoms in a short

distance. This makes it potentially extremely dangerous. A

radioactive substance that emits just alpha particles can

therefore be handled with rubber gloves, but it must not

be inhaled, eaten, or allowed near open cuts or the eyes.

Beta particles:

Beta particles are much more penetrating and can travel

easily through skin. Sources that emit beta particles must

be held with long handled tongs and pointed away from

the body. Inside of the body beta particles do not ionise

Form 5 – Unit 3– Theme 7: Radiation and its Uses Page 16

as much as alpha particles but it is much harder to

prevent them entering the body.

Gamma waves:

These waves are very penetrating and it is almost

impossible to absorb them completely. Sources of gamma

waves must also be held with long handled tongs and

pointed away from the body. Lead lined clothing can

reduce the amount of waves reaching the body. Gamma

waves are the least ionising of the three types of radiation

but it is extremely difficult to prevent them entering the

body.

Units of Radioactivity

The number of decays per second, or activity, from a

sample of radioactive nuclei is measured in

becquerel (Bq), after Henri Becquerel. One decay

per second equals one becquerel.

Nuclear energy gives off far more heat energy than

chemical reactions.

Reactors in nuclear power station do the same job as the

boiler; they boil water to steam. They also can be used

to make radioactive isotopes for medical purposes.