Radiation, nuclear fusion and
nuclear fission
L.Q: Can I explain how these
processes releases masses of energy?
What you need to know
The atom
Draw a labelled diagram of the atom showing the nucleus and labelling protons,
neutrons and electrons.
nucleus
neutron
electron
proton
Where are subatomic particles found?
Protons, neutrons and electrons are NOT evenly distributed in an atom.
The protons and neutrons exist in a dense
core at the centre of the atom. This is
called the nucleus.
The electrons are spread out
around the edge of the atom. They
orbit the nucleus in layers called
shells.
What we used to think…
It was believed that atoms were:
1. Spheres of positive charge.
2. With negative charges spread
through it.
This resembled a plum-pudding, so it was called
the ‘Plum –pudding’ model.
This was wrong!
How did we discover current ideas about the
structure of the atom?
Rutherford’s team:
Ernest Rutherford and his team of scientists performed a famous experiment in
Manchester:
They fired some alpha particles at a piece of thin gold foil (only a few atoms
thick):
If the ‘Plum Pudding’ model of
the atom was correct, the alpha
particles should pass straight
through and only be slightly
deflected.
This did not
happen.
What Rutherford’s team observed……..
1. Most of the alpha particles went straight through
the foil.
2. Some alpha particles were deflected through large
angles.
3. A very few alpha particles were reflected straight
back.
Rutherford’s conclusions
Observation
Most alpha particles went
straight through the foil.
A few were deflected
through large angles.
A very few were reflected
straight back.
Conclusion
Atoms are mostly space.
The nucleus is very small
compared to the size of
the atom and it contains
most of the mass and all
the positive charge.
Does the nucleus make sense?
• How are all those protons held together when
they have the same charge?
• Strong force between protons and neutrons
that is stronger than the electrostatic
repulsion between protons
• Hold protons and neutrons together in a
dense core nucleus
• Called the strong force
Calculate the number of protons, electrons and
neutrons shown below -
12
13
C
6
14
C
6
C
6
Thesehave
They
are
all
different
theisotopes
atoms
element
numbers
of that
the
carbon,
of
same
neutrons.
what
element
is the
with
difference
different
between
numbers
them?
of
What
do
we
call
are
unstable
and
emit
radiation
to become
more
Isotopes
Radioisotopes
neutrons?
stable?
Isotope
• Element that exists in different forms –
isotopes have the same atomic number but
different mass numbers
• Different number of neutrons
• Isotopes unstable – give off radiation
Radiation
• Form of energy given off by unstable nuclei
• Ionising
• Atoms that give off radiation are said to be
radioactive
• Most objects give off some type of radiation –
even us!!
• Energy is released and some mass is lost from the
release of a particle
• So the nuclei becomes a new element
Background radiation
Background radiation is the radiation all around us.
Rocks
Air
Building materials
Outer space
Food
Physical and chemical processes
• These do not change radioactivity of a sample
• E.g. Freezing a sample or reacting it with
another chemical cannot change radioactivity
Why are some substances radioactive?
• Radioactive atoms have
an unstable nucleus
• It becomes stable by
losing a particle
• This is known as
radioactive decay
Types of radiation
•
•
•
•
What is emitted by the nucleus?
Alpha radiation (particle)
Beta radiation (particle)
Gamma radiation (EM wave)
Type of decay:
Alpha decay
What is emitted?
Alpha particle (helium nuclei)
Description of decay:
Example of decay:
Effect on M and A:
2 neutrons and 2 protons are emitted
from the nucleus.
238
92
U 
234
4
90
2
Th +  + energy
M decreases by 4, A decreases by 2
(M-4, A-2)
Type of decay:
Beta decay
What is emitted?
High energy electron
Description of decay:
Example of decay:
Effect on M and A:
A neutron in the nucleus decays into a
proton and a high energy electron which is
emitted.
14
C 
6
14
0
N +  + energy
7
-1
M stays the same, A increases by 1
(M=, A+1)
Type of decay:
Gamma decay
What is emitted?
High energy electromagnetic radiation.
Description of decay:
Nucleus changes shape into a more
stable shape. Gamma radiation emitted
as a result.
Effect on A and Z:
A stays the same, Z stays the same
(A=, Z=)
The penetration power of the three types
of radiation.



Thin mica
Skin or paper
stops ALPHA
Thin aluminium
stops BETA
Thick lead
reduces GAMMA
STRONG FORCE!!
• Electrostatic forces between protons cause
them to be repelled....but....
• A stronger force keeps the protons and
neutrons concentrated in the nucleus
• This is a called the strong force
• This force only works when the protons and
neutrons are very close together.
• If they move too far apart the strong force
disappears
Nuclear fuel
• Fuel which releases energy through changes
in the nucleus
• The energy released from a nuclear material is
much greater than a chemical reaction
containing a similar mass of material
Nuclear fusion
• Two nuclei collide
• Two nuclei fuse to form a larger nuclei, releasing
lots of energy
• Two H nuclei form He
• Must get close enough to overcome the repulsion
of electrostatic attraction
• And allow the strong force to hold them together
• Need energy for that – carried out at high
temperatures/pressure
E = mc2
•
•
•
•
•
Mass can be converted into energy
E = energy
M = mass
C = speed of light in a vacuum (3x108 m/s)
Mass is lost when fission/fusion occurs
Nuclear Power - fission
When a nucleus decays it gives out heat energy.
In a nuclear power station, the uranium-235 atoms decay and give out energy
and neutrons.
Each time a uranium atom splits it produces 2 or 3 neutrons (depending on
the reaction). These go on to hit other uranium atoms, which causes them to
decay. A chain reaction is set up where more and more energy is released.
In a nuclear reactor the process is carefully controlled so that neutrons are
absorbed harmlessly and the energy released is controlled.
In a nuclear bomb the reaction is not controlled, and the bomb explodes!
Nuclear fission
• Uranium/plutonium
• Neutron splits a large and unstable nucleus into two
smaller parts, roughly equal in size
• This will release more neutrons and cause more
neighbouring nuclei to break down
• Generates an uncontrolled chain reaction of nuclear
fission
• In nuclear power stations the reaction is controlled
• Neutron release is controlled so reaction occurs at a
controlled pace
• If not could cause a nuclear explosion
Nuclear Power - fission
Fast neutron from previous decay cause the Uranium nucleus to split.
Nuclear Power
Kr
n
More
n
n
Uranium
n
In the reaction above a
neutron from a previous
decay can lead to more
and more decays.
This is called a chain reaction.
Ba
decays
How is this controlled
• Uranium is very radioactive and therefore the fission
reaction needs to be controlled to prevent an explosion
• Nuclear fission of uranium occurs when the nucleus
absorbs a neutron
• The nucleus splits into two smaller nuclei and releases
some neutrons
• Each of these neutrons can then be absorbed by another
uranium nucleus
• This is an example of a chain reaction where each fission
causes more fissions to occur
• Each fission releases lots of energy
• Fuel rods become very hot so can heat coolant which then
turns into steam which turn turbines
• Control rods containing Boron are put in
between the uranium rods, to absorb some of
the neutrons
• This keeps the rate of nuclear fission constant
• So there is a steady release of energy, not
uncontrolled
• Coolant is used to take away the heat
produced by nuclear fission
Nuclear fission
•
•
•
•
•
•
Splitting of an atom called “chain
reaction” (controlled)
A slow moving neutron gets
absorbed by uranium (U-235) or
plutonium (Pu - 239
When one atom of U-235 splits 2 or
3 neutrons are given off
Each neutron goes on to split
another 3 atoms, so another 9
neutrons are released, then 27 etc
If there isn’t a large enough
amount of U-235 no reaction will
occur
The minimum amount of uranium
needed is called the critical mass
Fission vs fusion
•
•
•
•
•
•
•
•
•
Fission:
Nucleus absorbs neutron
Unstable
Splits large nucleus into
smaller nuclei
Releases more neutrons
That split more large nuclei
Starts a chain reaction
Uranium/plutonium
Mass lost as energy (E=mc2)
•
•
•
•
•
•
•
•
Fusion:
Small nuclei collide
High temp/pressure
Two nuclei fuse together
Larger nuclei formed
E.g. Hydrogen – helium
Mass lost as energy (E=mc2)
More energy released
compared with fission