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.