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Radioactivity
The Discovery of Radioactivity
In 1896 Henri Becquerel was using naturally fluorescent
minerals to study the properties of x-rays, which had been
discovered in 1895 by Wilhelm Roentgen. He exposed
potassium uranyl sulfate to sunlight and then placed it on
photographic plates wrapped in black paper, believing that
the uranium absorbed the sun’s energy and then emitted it
as x-rays. On developing his photographic plates the
images were strong and clear, proving that the uranium
emitted radiation without an external source of energy such
as the sun. Becquerel had discovered radioactivity.
The term radioactivity was actually coined by Marie Curie,
andher husband Pierre. The Curies extracted uranium from
ore and to their surprise, found that the leftover ore showed
more activity than the pure uranium. They concluded that
the ore contained other radioactive elements. This led to the
discoveries of the elements polonium and radium.
Ernest Rutherford, who did many experiments studying
the properties of radioactive decay, named these alpha,
beta, and gamma particles, and classified them by their
ability to penetrate matter.
Models of the Atom
J J Thompson proposed the plum pudding model, in which
electrons were placed like cherries in a matrix of positive
charge. The neutron had not yet been discovered.
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This was the accepted model. Nobody had any reason to
believe otherwise until Ernest Rutherford, A New Zealand
physicist proved otherwise in the early 1900s.
Rutherford bombarded a thin layer of gold atoms with
alpha particles. He was using the alpha particles like
bullets, expecting to see the atoms burst like watermelons.
He described his experiment as firing artillery shells at tissue
paper.
To his surprise he found that some of the alpha particles
bounced back in the direction they came from. Other
particles went straight though, while other particles were
deflected.
This alpha scattering showed some 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 (and
goalkeeper of the Danish Olympic football team). It is the
model of the atom shown at the start of this topic. 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.
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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 neutron has slightly more mass than
the proton, but at this level we are going to say that the
relative mass of both the proton and the neutron is 1. 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
Proton
Neutron
Electron
Charge
+1e
0
-1e
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The symbol e is often called the electronic charge. Its
value is 1.6 × 10-19 C.
The protons and neutrons are the nucleons.
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. It is the equivalent to the size of a pea on the floor of
your school dining hall.
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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.
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).
Atoms of the same element can have different numbers of
neutrons; the different possible versions of each element are
called isotopes. 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.
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There are "preferred" combinations of neutrons and protons,
at which the forces holding nuclei together seem to balance
best. 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.
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’.
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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 .
Properties of Radiation
The table shows some properties:
Radiation
Description
Penetratio
n
Ionising Power
Effect of Electric or
Magnetic field
Alpha
()
Helium
nucleus
2p + 2n
Q=+2e
High speed
electron
Q = -1 e
Few cm
air
Thin
paper
Few mm
of
aluminium
Intensely
ionising
Deflection as a
positive charge
Less than
alpha
Deflection in opposite
direction to alpha.
Very short
wavelength
em
radiation
Several
cm lead,
couple of
m of
concrete
Weakly
ionising
No effect.
Beta ()
Gamma
()
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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. 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.
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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.
Radioactivity
It is found that nuclei with mass numbers greater than about
100 spontaneously decay into other types of nuclei. Such
nuclei are said to be radioactive.


alpha decay, which occurs by emission of an alpha
( 42 He) nuclei. An example of this is the decay of
Uranium:
238
92
U
234
90
9
Th +
4
2
He

beta decay, which occurs by emission of a beta
particle (electron or positron). An example of this is the
decay of Nitrogen:
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7
N
12
6
C+
0
1
e
 The notation 0 1e denotes an electron/positron - a
 positron is identical except it has a positive charge .

gamma decay, which occurs by emission of gamma
particles (photons, or quanta of light). An example of
this is the decay of a Carbon atom in an excited state:
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
6
C
*
12
6
C+
where denotes the photon.
Alpha Decay
When a nucleus decays by alpha decay, it ejects a helium
nucleus (NOT atom). The nucleus recoils, just like a canon
firing a canon ball.
Alpha Particle
A helium nucleus consist of 2 PROTONS AND 2 NEUTRONS .
So when an alpha particle is emitted the atomic number
goes down by 2, because 2 protons are lost from the
nucleus. The mass number goes down by 4 because 4
nucleons are lost.
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Beta Decay
A beta particle is a high speed electron which is ejected
from the nucleus. A neutron turns into a proton and the
electron is ejected. It has nothing to do with the electrons
surrounding the atom.
Beta Particle
The atomic number goes up by 1, so a new element is
formed, but the mass number stays the same. The
electron comes out of the nucleus, NOT the electron shells.
Gamma Decay
When a nucleus decays by gamma
decay, a nucleus changes from a
higher energy state to a lower
energy state through the emission of
electromagnetic radiation
(photons). The number of protons
(and neutrons) in the nucleus does
not change in this process, so the
parent and daughter atoms are the same chemical
element. In the gamma decay of a nucleus, the emitted
photon and recoiling nucleus each have a well-defined
energy after the decay.
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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 ratemeter.
The radioactive decay is measured by the number of counts
per second. A computer can act as a ratemeter and store
the results. It will also plot a graph.
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.
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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
The Gold Leaf Electroscope
Dry air is normally a good
insulator, thus a charged
electroscope will stay that way,
as the charge cannot escape.
When an electroscope is
charged, the gold leaf sticks
out, because the charges on the
gold repel the charges on the
metal stalk.
When a radioactive source comes near, the air is ionised,
and starts to conduct electricity. This means that the charge
can "leak" away, the electroscope discharges and the gold
leaf falls.
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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.
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.
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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:
.
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.
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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!)
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!
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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
(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
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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
Alpha ()
Use
Used in smoke detectors
Beta ()
Checking the thickness of
paper sheet in
manufacture.
Radioactive tracers in
medical research and
diagnosis
Medical research.
Non-destructive testing of
castings.
Gamma ()
Hazard
If taken in to the body
(ingested), alpha emitters
can do immense damage to
living tissues
Some risk of tissue damage,
although nowhere near as
dangerous as alpha.
Can cause genetic damage
and cancer.
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
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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 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.
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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.
An older unit is the curie, named after Pierre and Marie
Curie. One curie is approximately the activity of 1 gram of
radium and equals (exactly) 3.7 x 1010 becquerel. The
activity depends only on the number of decays per second,
not on the type of decay, the energy of the decay products,
or the biological effects of the radiation .
Nuclear Energy
Sixty years ago the power of nuclear energy was
demonstrated to the world. Little Boy, a free-fall atomic
bomb was dropped on Hiroshima in August 1945. A few
days later an aerial mine, Fat Man, was detonated over
Nagasaki. The destruction and carnage caused by these
bombs is well known. The energy released was due to the
conversion of 20 grams of nuclear material to heat.
The conversion of nuclear energy to heat is at the heart of
nuclear energy. Scientists have learned to control the
process so that instead of an explosion, a steady heat
source is achieved. A nuclear reactor can boil water to
steam to turn a steam turbine. In the early days, nuclear
energy was greeted with unbounded optimism. All sorts of
nuclear powered devices were conceived, including railway
engines and aeroplanes. It was hoped that nuclear power
would be so cheap that it would not be necessary to meter
electricity. However this proved not to be the case.
We will look at the processes that release nuclear energy.
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Fission
Very large nuclei tend to be rather unstable. This means
that they are radioactive. Some nuclei, for example,
Uranium-235 and Plutonium-239, can be made so unstable
that they split into two or more nuclei of more stable
elements. This is called fission. The nuclei are called
fissile.
These fissile nuclei are isotopes of more stable elements
(e.g. Uranium-238). If left alone, they decay radioactively
by emitting alpha particles.
Fission is not a spontaneous process. It has to be started by
injecting a neutron into the nucleus.
The neutron has to be injected at the right speed:


too fast, the neutron will pass right through, or knock
out another neutron.
too slow, the neutron will bounce off the nucleus.
Many pictures show the neutron smashing the nucleus like a
bullet. This is wrong. It's more like that the neutron
"tickles" the nucleus.
The nucleus is not a neat array of protons and neutrons. It
is very active , changing shape all the time. It's like a
"wobbly drop". When the extra neutron is taken into the
nucleus, the wobbly drop goes dumbbell-shaped like this:
The weak spot at the neck makes the nucleus fly apart to
form two or more new nuclei. A lot of energy is released.
Nuclear energy gives off far more heat energy than
chemical reactions.
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Also two or three (or more) neutrons are released. These
can go on to be absorbed by other nuclei to cause a chain
reaction, which is shown in the picture below.
If the chain reaction is not controlled, a nuclear explosion
will occur. In a nuclear reactor, only one neutron is allowed
to pass on to be incorporated into one nucleus.
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.
Fission has NOTHING whatever to do with radioactivity.
Alpha and beta particles are NOT emitted during fission.
However many of the new daughter nuclei are radioactive.
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Fusion
This involves light nuclei, two isotopes of hydrogen,
deuterium and tritium.
If two helium nuclei are forced together, they join together
or fuse to form a helium nucleus, giving off lots of energy,
more than in fission.
The two nuclei have to be slammed together by heating
them to temperatures of millions of degrees Celsius
before they fuse.
The vast amounts of energy can be released in a massive
explosion. The amount of hydrogen involved in a hydrogen
bomb explosion is tiny; it would fill a party balloon.
Achieving controlled fusion has proved more challenging,
and commercial fusion power stations remain a distant
prospect.
Fusion is the process that fuels stars. In the Sun, four
million tonnes of hydrogen fuel is consumed every second.
This sounds a lot, but the Sun has enough fuel to keep
burning for another 4500 million years, by which time we
will all be long gone and forgotten.
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